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[[File:Big-O-notation.png|400px|thumb|Example of Big O notation: ''f''(''x'') ∈ O(''g''(''x'')) as there exists ''c''&nbsp;>&nbsp;0 (e.g., ''c''&nbsp;=&nbsp;1) and ''x''<sub>0</sub> (e.g., ''x''<sub>0</sub>&nbsp;=&nbsp;5) such that ''f''(''x'')&nbsp;<&nbsp;''cg''(''x'') whenever ''x''&nbsp;>&nbsp;''x''<sub>0</sub>.]]
In [[mathematics]], the '''limit inferior''' and '''limit superior''' of a [[sequence]] can be thought of as limiting (i.e., eventual and extreme) bounds on the sequence. They can be thought of in a similar fashion for a [[function (mathematics)|function]] (see [[limit of a function]]). For a set, they are the [[infimum]] and [[supremum]] of the set's [[limit point]]s, respectively. In general, when there are multiple objects around which a sequence, function, or set accumulates, the inferior and superior limits extract the smallest and largest of them; the type of object and the measure of size is context-dependent, but the notion of extreme limits is invariant.
Limit inferior is also called '''infimum limit''', '''liminf''', '''inferior limit''', '''lower limit''', or '''inner limit'''; limit superior is also known as '''supremum limit''', '''limit supremum''', '''limsup''', '''superior limit''', '''upper limit''', or '''outer limit'''.  


In [[mathematics]], '''big O notation''' describes the [[asymptotic analysis|limiting behavior]] of a [[function (mathematics)|function]] when the argument tends towards a particular value or infinity, usually in terms of simpler functions. It is a member of a larger family of notations that is called '''Landau notation''', '''Bachmann–Landau notation''' (after [[Edmund Landau]] and [[Paul Gustav Heinrich Bachmann|Paul Bachmann]]),<ref name="Beigi2011">{{cite book|author=Homayoon Beigi|title=Fundamentals of Speaker Recognition|url=http://books.google.com/books?id=qIMDvu3gJCQC&pg=PA777|date=9 December 2011|publisher=Springer|isbn=978-0-387-77592-0|pages=777}}</ref><ref name="Holmes2012">{{cite book|author=Mark H. Holmes|title=Introduction to Perturbation Methods|url=http://books.google.com/books?id=EX5iNglbqB4C&pg=PA4|date=5 December 2012|publisher=Springer|isbn=978-1-4614-5477-9|pages=4–}}</ref> or '''asymptotic notation'''.  In [[computer science]], big O notation is used to [[Computational complexity theory|classify algorithms]]<ref name="mitlecture">{{cite web|title=Big O Notation (MIT Lecture)|url=http://web.mit.edu/16.070/www/lecture/big_o.pdf|accessdate=7 June 2014|format=Lecture|quote=Big O notation (with a capital letter O, not a zero), also called Landau's symbol, is a  symbolism used in complexity theory}}</ref><ref name=quantumcomplexity>{{cite web|last1=Mohr|first1=Austin|title=Quantum Computing in Complexity Theory and Theory of Computation|url=http://www.austinmohr.com/Work_files/complexity.pdf|accessdate=7 June 2014|page=2}}</ref> by how they respond (''e.g.,'' in their processing time or working space requirements) to changes in input size. In [[analytic number theory]], it is used to estimate the "error committed"  while replacing the asymptotic size, or asymptotic mean size, of an [[arithmetic function|arithmetical function]], by the value, or mean value, it takes at a large finite argument. A famous example is the problem of estimating the remainder term in the [[prime number theorem]].
[[Image:Lim sup example 5.png|right|thumb|300px|An illustration of limit superior and limit inferior. The sequence ''x''<sub>''n''</sub> is shown in blue. The two red curves approach the limit superior and limit inferior of ''x''<sub>''n''</sub>, shown as dashed black lines. In this case, the sequence ''accumulates'' around the two limits. The superior limit is the larger of the two, and the inferior limit is the smaller of the two. The inferior and superior limits agree if and only if the sequence is convergent (i.e., when there is a single limit).]]


Big O notation characterizes functions according to their growth rates: different functions with the same growth rate may be represented using the same O notation. The letter O is used because the growth rate of a function is also referred to as order of the function.  A description of a function in terms of big O notation usually only provides an [[upper bound]] on the growth rate of the function. Associated with big O notation are several related notations, using the symbols [[#Related asymptotic notations|''o'', Ω, ω, and Θ]], to describe other kinds of bounds on asymptotic growth rates.
==Definition for sequences==
The limit inferior of a sequence (''x''<sub>''n''</sub>) is defined by


Big O notation is also used in many other fields to provide similar estimates.
:<math>\liminf_{n\to\infty}x_n := \lim_{n\to\infty}\Big(\inf_{m\geq n}x_m\Big)</math>


==Formal definition==
or
Let ''f'' and ''g'' be two functions defined on some subset of the [[real number]]s. One writes


:<math>f(x)=O(g(x))\text{ as }x\to\infty\,</math>
:<math>\liminf_{n\to\infty}x_n := \sup_{n\geq 0}\,\inf_{m\geq n}x_m=\sup\{\,\inf\{\,x_m:m\geq n\,\}:n\geq 0\,\}.</math>


[[if and only if]] there is a positive constant M such that for all sufficiently large values of ''x'', the absolute value of ''f''(''x'') is at most M  multiplied by the absolute value of ''g''(''x''). That is, ''f''(''x'')&nbsp;=&nbsp;''O''(''g''(''x'')) if and only if there exists a positive real number ''M'' and a real number ''x''<sub>0</sub> such that
Similarly, the limit superior of (''x''<sub>''n''</sub>) is defined by


:<math>|f(x)| \le \; M |g(x)|\text{ for all }x \ge x_0.</math>
:<math>\limsup_{n\to\infty}x_n := \lim_{n\to\infty}\Big(\sup_{m\geq n}x_m\Big)</math>


In many contexts, the assumption that we are interested in the growth rate as the variable ''x'' goes to infinity is left unstated, and one writes more simply that ''f''(''x'')&nbsp;=&nbsp;''O''(''g''(''x'')).
or
The notation can also be used to describe the behavior of ''f'' near some real number ''a'' (often, ''a''&nbsp;=&nbsp;0): we say


:<math>f(x)=O(g(x))\text{ as }x\to a\,</math>
:<math>\limsup_{n\to\infty}x_n := \inf_{n\geq 0}\,\sup_{m\geq n}x_m=\inf\{\,\sup\{\,x_m:m\geq n\,\}:n\geq 0\,\}.</math>


if and only if there exist positive numbers ''δ'' and ''M'' such that
Alternatively, the notations <math>\varliminf_{n\to\infty}x_n:=\liminf_{n\to\infty}x_n</math> and <math>\varlimsup_{n\to\infty}x_n:=\limsup_{n\to\infty}x_n</math> are sometimes used.


:<math>|f(x)| \le \; M |g(x)|\text{ for }|x - a| < \delta.</math>
If the terms in the sequence are real numbers, the limit superior and limit inferior always exist, as real numbers or ±∞ (i.e., on the [[extended real number line]]). More generally, these definitions make sense in any [[partially ordered set]], provided the [[supremum|suprema]] and [[infimum|infima]] exist, such as in a [[complete lattice]].


If ''g''(''x'') is non-zero for values of ''x'' [[Mathematical jargon#sufficiently large|sufficiently close]] to ''a'', both of these definitions can be unified using the [[limit superior]]:
Whenever the ordinary limit exists, the limit inferior and limit superior are both equal to it; therefore, each can be considered a generalization of the ordinary limit which is primarily interesting in cases where the limit does ''not'' exist. Whenever lim inf ''x''<sub>''n''</sub> and lim sup ''x''<sub>''n''</sub> both exist, we have


:<math>f(x)=O(g(x))\text{ as }x \to a\,</math>
:<math>\liminf_{n\to\infty}x_n\leq\limsup_{n\to\infty}x_n.</math>


if and only if
Limits inferior/superior are related to [[big-O notation]] in that they bound a sequence only "in the limit"; the sequence may exceed the bound. However, with big-O notation the sequence can only exceed the bound in a finite prefix of the sequence, whereas the limit superior of a sequence like e<sup>−''n''</sup> may actually be less than all elements of the sequence. The only promise made is that some tail of the sequence can be bounded by the limit superior (inferior) plus (minus) an arbitrarily small positive constant.


:<math>\limsup_{x\to a} \left|\frac{f(x)}{g(x)}\right| < \infty.</math><!-- These absolute value bars may be left out. -->
The limit superior and limit inferior of a sequence are a special case of those of a function (see below).


==Example==
== The case of sequences of real numbers ==
In typical usage, the formal definition of ''O'' notation is not used directly; rather, the ''O'' notation for a function ''f'' is derived by the following simplification rules:
*If ''f''(''x'') is a sum of several terms, the one with the largest growth rate is kept, and all others omitted.
*If ''f''(''x'') is a product of several factors, any constants (terms in the product that do not depend on ''x'') are omitted.
For example, let <math>f(x) = 6x^4 - 2x^3 +5</math>, and suppose we wish to simplify this function, using ''O'' notation, to describe its growth rate as ''x'' approaches infinity. This function is the sum of three terms: 6''x''<sup>4</sup>, −2''x''<sup>3</sup>, and 5. Of these three terms, the one with the highest growth rate is the one with the largest exponent as a function of ''x'', namely 6''x''<sup>4</sup>. Now one may apply the second rule: 6''x''<sup>4</sup> is a product of 6 and ''x''<sup>4</sup> in which the first factor does not depend on ''x''. Omitting this factor results in the simplified form ''x''<sup>4</sup>. Thus, we say that ''f''(''x'') is a "big-oh" of (''x''<sup>4</sup>). Mathematically, we can write ''f''(''x'')&nbsp;=&nbsp;''O''(''x''<sup>4</sup>).
One may confirm this calculation using the formal definition: let ''f''(''x'')&nbsp;=&nbsp;6''x''<sup>4</sup>&nbsp;−&nbsp;2''x''<sup>3</sup>&nbsp;+&nbsp;5 and ''g''(''x'')&nbsp;=&nbsp;''x''<sup>4</sup>. Applying the [[#Formal definition|formal definition]] from above, the statement that ''f''(''x'')&nbsp;=&nbsp;''O''(''x''<sup>4</sup>) is equivalent to its expansion,
:<math>|f(x)| \le \; M |g(x)|</math>
for some suitable choice of ''x''<sub>0</sub> and ''M'' and for all ''x''&nbsp;&gt;&nbsp;''x''<sub>0</sub>. To prove this, let ''x''<sub>0</sub>&nbsp;=&nbsp;1 and ''M''&nbsp;=&nbsp;13. Then, for all ''x''&nbsp;&gt;&nbsp;''x''<sub>0</sub>:
:<math>\begin{align}|6x^4 - 2x^3 + 5| &\le 6x^4 + |2x^3| + 5\\
                                      &\le 6x^4 + 2x^4 + 5x^4\\                                   
                                      &= 13x^4\end{align}</math>
so
:<math> |6x^4 - 2x^3 + 5| \le 13 \,x^4 .</math>


==Usage==
In [[mathematical analysis]], limit superior and limit inferior are important tools for studying sequences of [[real number]]s. Since the supremum and infimum of an unbounded set of real numbers may not exist (the reals are not a complete lattice), it is convenient to consider sequences in the [[affinely extended real number system]]: we add the positive and negative infinities to the real line to give the complete [[totally ordered set]] (−∞,∞), which is a complete lattice.
Big O notation has two main areas of application. In [[mathematics]], it is commonly used to describe how closely a finite series approximates a given function, especially in the case of a truncated [[Taylor series]] or [[asymptotic expansion]]. In [[computer science]], it is useful in the [[analysis of algorithms]]. In both applications, the function ''g''(''x'') appearing within the ''O''(...) is typically chosen to be as simple as possible, omitting constant factors and lower order terms.
There are two formally close, but noticeably different, usages of this notation: [[Infinity|infinite]] asymptotics and [[infinitesimal]] asymptotics. This distinction is only in application and not in principle, however—the formal definition for the "big O" is the same for both cases, only with different limits for the function argument.


===Infinite asymptotics===
=== Interpretation ===
Big O notation is useful when [[analysis of algorithms|analyzing algorithms]] for efficiency.  For example, the time (or the number of steps) it takes to complete a problem of size ''n'' might be found to be ''T''(''n'') = 4''n''<sup>2</sup> − 2''n'' + 2.
As ''n'' grows large, the ''n''<sup>2</sup> [[term (mathematics)|term]] will come to dominate, so that all other terms can be neglected—for instance when ''n'' = 500, the term 4''n''<sup>2</sup> is 1000 times as large as the 2''n'' term. Ignoring the latter would have negligible effect on the expression's value for most purposes.
Further, the [[coefficient]]s become irrelevant if we compare to any other [[Orders of approximation|order]] of expression, such as an expression containing a term n<sup>3</sup> or n<sup>4</sup>.  Even if ''T''(''n'') = 1,000,000''n''<sup>2</sup>, if ''U''(''n'') = ''n''<sup>3</sup>, the latter will always exceed the former once ''n'' grows larger than 1,000,000 (''T''(1,000,000) = 1,000,000<sup>3</sup>= ''U''(1,000,000)). Additionally, the number of steps depends on the details of the machine model on which the algorithm runs, but different types of machines typically vary by only a constant factor in the number of steps needed to execute an algorithm.
So the big O notation captures what remains: we write either
:<math>\ T(n)= O(n^2) \, </math>
or
:<math>T(n)\in O(n^2) \, </math>
and say that the algorithm has ''order of n<sup>2</sup>'' time complexity.
Note that "=" is not meant to express "is equal to" in its normal mathematical sense, but rather a more colloquial "is", so the second expression is technically accurate (see the "[[#Equals sign|Equals sign]]" discussion below) while the first is a common [[Abuse of notation#Big O notation|abuse of notation]].<ref name="Introduction to Algorithms">Thomas H. Cormen et al., 2001, [http://highered.mcgraw-hill.com/sites/0070131511/ Introduction to Algorithms, Second Edition]</ref>


===Infinitesimal asymptotics===
Consider a sequence <math>(x_n)</math> consisting of real numbers. Assume that the limit superior and limit inferior are real numbers (so, not infinite).
Big O can also be used to describe the [[error bound|error term]] in an approximation to a mathematical function. The most significant terms are written explicitly, and then the least-significant terms are summarized in a single big O term.  For example, in the exponential series,
* The limit superior of <math>x_n</math> is the smallest real number <math>b</math> such that, for any positive real number <math>\varepsilon</math>, there exists a [[natural number]] <math>N</math> such that <math>x_n<b+\varepsilon</math> for all <math>n>N</math>. In other words, any number larger than the limit superior is an eventual upper bound for the sequence. Only a finite number of elements of the sequence are greater than <math>b+\varepsilon</math>.
:<math>e^x=1+x+\frac{x^2}{2}+O(x^3) \,, \qquad\text{as } x\to 0  \,, </math>
* The limit inferior of <math>x_n</math> is the largest real number <math>b</math> that, for any positive real number <math>\varepsilon</math>, there exists a natural number <math>N</math> such that <math>x_n>b-\varepsilon</math> for all <math>n>N</math>. In other words, any number below the limit inferior is an eventual lower bound for the sequence. Only a finite number of elements of the sequence are less than <math>b-\varepsilon</math>.
expresses the fact that the error, the difference <math>\ e^x - (1 + x + x^2/2)</math>, is smaller in [[absolute value]] than some constant times <math>|x^3|</math> when <math>x</math> is close enough to&nbsp;0.


==Properties==
=== Properties ===
If the function ''f'' can be written as a finite sum of other functions, then the fastest growing one determines the order of
''f''(''n''). For example
:<math>f(n) = 9 \log n + 5 (\log n)^3 + 3n^2 + 2n^3 = O(n^3) \,, \qquad\text{as } n\to\infty  \,\!.</math> <!-- note: "\,\!" forces TeX rendering -->
In particular, if a function may be bounded by a polynomial in ''n'', then as ''n'' tends to ''infinity'', one may disregard ''lower-order'' terms of the polynomial.
''O''(''n''<sup>''c''</sup>) and ''O''(''c''<sup>''n''</sup>) are very different. If ''c'' is greater than one, then the latter grows much faster. A function that grows faster than ''n''<sup>''c''</sup> for any ''c''  is called ''superpolynomial''.  One that grows more slowly than any exponential function of the form <math>c^n</math> is called ''subexponential''. An algorithm can require time that is both superpolynomial and subexponential; examples of this include the fastest known algorithms for [[integer factorization]].
''O''(log ''n'') is exactly the same as ''O''(log(''n''<sup>''c''</sup>)). The logarithms differ only by a constant factor (since
<math>\log(n^c)=c \log n</math>) and thus the big O notation ignores that. Similarly, logs with different constant bases are equivalent.


Exponentials with different bases, on the other hand, are not of the same order. For example, <math>2^n</math> and <math>3^n</math> are not of the same order.
The relationship of limit inferior and limit superior for sequences of real numbers is as follows
Changing units may or may not affect the order of the resulting algorithm. Changing units is equivalent to multiplying the appropriate variable by a constant wherever it appears. For example, if an algorithm runs in the order of ''n''<sup>2</sup>, replacing ''n'' by ''cn'' means the algorithm runs in the order of <math>c^2n^2</math>, and the big O notation ignores the constant <math>c^2</math>. This can be written as <math> c^2n^2 \in O(n^2) </math>. If, however, an algorithm runs in the order of <math>2^n</math>, replacing n with cn gives <math>2^{cn} = (2^c)^n</math>. This is not equivalent to <math>2^n</math> in general.
Changing of variable may affect the order of the resulting algorithm. For example, if an algorithm's running time is ''O''(''n'') when measured in terms of the number ''n'' of ''digits'' of an input number&nbsp;''x'', then its running time is ''O''(log&nbsp;''x'') when measured as a function of the input number ''x'' itself, because ''n''&nbsp;=&nbsp;Θ(log&nbsp;''x'').


===Product===
:<math>\limsup_{n\to\infty} (-x_n) = -\liminf_{n\to\infty} x_n</math>
:<math> f_1 \in O(g_1) \text{ and } f_2\in O(g_2)\, \Rightarrow f_1  f_2\in O(g_1  g_2)\,</math>
:<math>f\cdot O(g) \subset O(f g)</math>


===Sum===
As mentioned earlier, it is convenient to extend <math>\mathbb{R}</math> to [−∞,]. Then, (''x''<sub>''n''</sub>) in [−∞,∞] [[limit of a sequence|converges]] [[if and only if]]
:<math> f_1 \in O(g_1) \text{ and }
  f_2\in O(g_2)\, \Rightarrow f_1 + f_2\in O(|g_1| + |g_2|)\,</math>
::This implies <math> f_1 \in O(g) \text{ and } f_2 \in O(g) \Rightarrow f_1+f_2 \in O(g) </math>, which means that <math>O(g)</math> is a [[convex cone]].
:If ''f'' and ''g'' are positive functions, <math>f + O(g) \in O(f + g)</math>


===Multiplication by a constant===
:<math>\liminf_{n\to\infty} x_n = \limsup_{n\to\infty} x_n</math>
:Let ''k'' be a constant. Then:
:<math>\ O(k g) = O(g)</math> if ''k'' is nonzero.
:<math>f\in O(g) \Rightarrow kf\in O(g). </math>


==Multiple variables==
in which case <math>\lim_{n\to\infty} x_n</math> is equal to their common value. (Note that when working just in <math>\mathbb{R}</math>, convergence to −∞ or ∞ would not be considered as convergence.) Since the limit inferior is at most the limit superior, the condition
Big ''O'' (and little o, and Ω...) can also be used with multiple variables.
To define Big ''O'' formally for multiple variables, suppose <math>f(\vec{x})</math> and <math>g(\vec{x})</math> are two functions defined on some subset of <math>\mathbb{R}^n</math>. We say
:<math>f(\vec{x})\text{ is }O(g(\vec{x}))\text{ as }\vec{x}\to\infty</math>
if and only if
:<math>\exists M\,\exists C>0\text{ such that } |f(\vec{x})| \le C |g(\vec{x})|\text{ for all }\vec{x} \text{ with } x_i \ge M \text{ for all }i.</math>
For example, the statement
:<math>f(n,m) = n^2 + m^3 + O(n+m) \text{ as } n,m\to\infty\,</math>
asserts that there exist constants ''C'' and ''M'' such that
:<math>\forall n, m \ge M\colon |g(n,m)| \le C(n+m),</math>
where ''g''(''n'',''m'') is defined by
:<math>f(n,m) = n^2 + m^3 + g(n,m).\,</math>
Note that this definition allows all of the coordinates of <math>\vec{x}</math> to increase to infinity. In particular, the statement
:<math>f(n,m) = O(n^m) \text{ as } n,m\to\infty\,</math>
(i.e., <math>\exists C\,\exists M\,\forall n\,\forall m\dots</math>) is quite different from
:<math>\forall m\colon f(n,m) = O(n^m) \text{ as } n\to\infty</math>
(i.e., <math>\forall m\,\exists C\,\exists M\,\forall n\dots</math>).


==Matters of notation==
:<math>\liminf_{n\to\infty} x_n = \infty \;\;\Rightarrow\;\; \lim_{n\to\infty} x_n = \infty</math>


===Equals sign===
and the condition
The statement "''f''(''x'') is ''O''(''g''(''x''))" as defined above is usually written as ''f''(''x'')&nbsp;=&nbsp;''O''(''g''(''x'')). Some consider this to be an [[abuse of notation]], since the use of the equals sign could be misleading as it suggests a symmetry that this statement does not have. As [[Nicolaas Govert de Bruijn|de Bruijn]] says, ''O''(''x'')&nbsp;=&nbsp;''O''(''x''<sup>2</sup>) is true but ''O''(''x''<sup>2</sup>)&nbsp;=&nbsp;''O''(''x'') is not.<ref>{{Cite book| author = [[N. G. de Bruijn]] | title=Asymptotic Methods in Analysis | place=Amsterdam |publisher=North-Holland | year=1958 | pages=5–7 | url=http://books.google.com/?id=_tnwmvHmVwMC&pg=PA5&vq=%22The+trouble+is%22 | isbn=978-0-486-64221-5}}</ref> [[Donald Knuth|Knuth]] describes such statements as "one-way equalities", since if the sides could be reversed, "we could deduce ridiculous things like ''n''&nbsp;=&nbsp;''n''<sup>2</sup> from the identities ''n''&nbsp;=&nbsp;''O''(''n''<sup>2</sup>) and ''n''<sup>2</sup>&nbsp;=&nbsp;''O''(''n''<sup>2</sup>)."<ref name="Concrete Mathematics">
{{Cite book|
author=[[Ronald Graham]], Donald Knuth, and [[Oren Patashnik]]|
title=Concrete Mathematics|
place=Reading, Massachusetts| publisher=Addison–Wesley|
edition=2| year=1994 | page=446 |
url=http://books.google.com/?id=pntQAAAAMAAJ&dq=editions:ISBN 0-201-55802-5 | isbn=978-0-201-55802-9}}
</ref>
For these reasons, it would be more precise to use [[set notation]] and write ''f''(''x'')&nbsp;∈&nbsp;''O''(''g''(''x'')), thinking of ''O''(''g''(''x'')) as the class of all functions ''h''(''x'') such that |''h''(''x'')|&nbsp;≤&nbsp;''C''|''g''(''x'')| for some constant ''C''.<ref name="Concrete Mathematics"/> However, the use of the equals sign is customary.  Knuth pointed out that "mathematicians customarily use the = sign as they use the word 'is' in English: Aristotle is a man, but a man isn't necessarily Aristotle."<ref>{{Cite journal| author=Donald Knuth | title=Teach Calculus with Big O | date=June–July 1998 | journal=[[Notices of the American Mathematical Society]] | volume=45 | issue=6 | page=687 | url=http://www.ams.org/notices/199806/commentary.pdf}} ([http://www-cs-staff.stanford.edu/~knuth/ocalc.tex Unabridged version])</ref>


===Other arithmetic operators===
:<math>\limsup_{n\to\infty} x_n = - \infty \;\;\Rightarrow\;\; \lim_{n\to\infty} x_n = - \infty.</math>
Big O notation can also be used in conjunction with other arithmetic operators in more complicated equations.  For example, ''h''(''x'') + ''O''(''f''(''x'')) denotes the collection of functions having the growth of ''h''(''x'') plus a part whose growth is limited to that of ''f''(''x''). Thus,
:<math>g(x) = h(x) + O(f(x))\,</math>
expresses the same as
:<math>g(x) - h(x) \in O(f(x))\,.</math>


====Example {{anchor|Example (Matters of notation)}}====
If <math>I = \liminf_{n\to\infty} x_n</math> and <math>S = \limsup_{n\to\infty} x_n</math>, then the interval [''I'', ''S''] need not contain any of the numbers ''x''<sub>''n''</sub>, but every slight enlargement [''I''&nbsp;&minus;&nbsp;ε, ''S''&nbsp;+&nbsp;ε] (for arbitrarily small ε > 0) will contain ''x''<sub>''n''</sub> for all but finitely many indices ''n''. In fact, the interval [''I'', ''S''] is the smallest closed interval with this property. We can formalize this property like this: there exist [[subsequence]]s <math>x_{k_n}</math> and <math>x_{h_n}</math> of <math>x_n</math> (where <math>k_n</math> and <math>h_n</math> are monotonous) for which we have
Suppose an [[algorithm]] is being developed to operate on a set of ''n'' elements. Its developers are interested in finding a function ''T''(''n'')  that will express how long the algorithm will take to run (in some arbitrary measurement of time) in terms of the number of elements in the input set. The algorithm works by first calling a subroutine to sort the elements in the set and then perform its own operations. The sort has a known time complexity of ''O''(''n''<sup>2</sup>), and after the subroutine runs the algorithm must take an additional <math> 55n^3+2n+10</math> time before it terminates.  Thus the overall time complexity of the algorithm can be expressed as
:<math>T(n)=55n^3+O(n^2).\ </math>
Here the terms 2''n''+10 are subsumed within the faster-growing ''O''(''n''<sup>2</sup>).  Again, this usage disregards some of the formal meaning of the "=" symbol, but it does allow one to use the big O notation as a kind of convenient placeholder.


===Declaration of variables===
:<math>\liminf_{n\to\infty} x_n+\epsilon>x_{h_n} \;\;\;\;\;\;\;\;\; x_{k_n} > \limsup_{n\to\infty} x_n-\epsilon</math>
Another feature of the notation, although less exceptional, is that function arguments may need to be inferred from the context when several variables are involved. The following two right-hand side big O notations have dramatically different meanings:
:<math>f(m) = O(m^n)\,,</math>
:<math>g(n)\,\, = O(m^n)\,.</math>
The first case states that ''f''(''m'') exhibits polynomial growth, while the second, assuming ''m'' > 1, states that ''g''(''n'') exhibits exponential growth.
To avoid confusion, some authors{{who|date=January 2015}} use the notation
:<math>g(x) \in O(f(x))\,.</math>
rather than the less explicit
:<math>g \in O(f)\,,</math>


===Multiple usages===
On the other hand, there exists a <math>n_0\in\mathbb{N}</math> so that for all <math>n\geq n_0</math>
In more complicated usage, ''O''(...) can appear in different places in an equation, even several times on each side. For example, the following are true for <math>n\to\infty</math>
:<math>(n+1)^2 = n^2 + O(n)\ </math>
:<math>(n+O(n^{1/2}))(n + O(\log n))^2 = n^3 + O(n^{5/2})\ </math>
:<math>n^{O(1)} = O(e^n).\ </math>
The meaning of such statements is as follows: for ''any'' functions which satisfy each ''O''(...) on the left side, there are ''some'' functions satisfying each ''O''(...) on the right side, such that substituting all these functions into the equation makes the two sides equal. For example, the third equation above means: "For any function <math>f(n)=O(1)\,</math>, there is some function <math>g(n)=O(e^n)\,</math> such that <math>n^{f(n)}=g(n)</math>." In terms of the "set notation" above, the meaning is that the class of functions represented by the left side is a subset of the class of functions represented by the right side. In this use the "=" is a formal symbol that unlike the usual use of "=" is not a [[symmetric relation]]. Thus for example <math>n^{O(1)} = O(e^n)\, </math> does not imply the false statement <math>O(e^n) = n^{O(1)}\, </math>.


==Orders of common functions==
:<math> \liminf_{n\to\infty} x_n-\epsilon < x_n < \limsup_{n\to\infty} x_n+\epsilon</math>
{{Further|Time complexity#Table of common time complexities}}


Here is a list of classes of functions that are commonly encountered when analyzing the running time of an algorithm.  In each case, ''c'' is a constant and ''n'' increases without bound. The slower-growing functions are generally listed first.
To recapitulate:


{| class="wikitable"
* If <math>\Lambda</math> is greater than the limit superior, there are at most finitely many <math>x_n</math> greater than <math>\Lambda</math>; if it is less, there are infinitely many.
|-
* If <math>\lambda</math> is less than the limit inferior, there are at most finitely many <math>x_n</math> less than <math>\lambda</math>; if it is greater, there are infinitely many.
!Notation !! Name !! Example
|-
|<math>O(1)\,</math> || [[Constant time|constant]] || Determining if a binary number is even or odd; Calculating <math>(-1)^n</math>; Using a constant-size [[lookup table]]
|-
|<math>O(\log \log n)\,</math> || double logarithmic || Number of comparisons spent finding an item using [[interpolation search]] in a sorted array of uniformly distributed values
|-
|<math>O(\log n)\,</math> || [[Logarithmic time|logarithmic]] || Finding an item in a sorted array with a [[Binary search algorithm|binary search]] or a balanced search [[Tree data structure|tree]] as well as all operations in a [[Binomial heap]]
|-
|<math>O(\log^c n),\;c>1\,</math> || [[Polylogarithmic time|polylogarithmic]] || Matrix chain ordering can be solved in polylogarithmic time on a [[Parallel Random Access Machine]].
|-
|<math>O(n^c),\;0<c<1\,</math> || fractional power || Searching in a [[kd-tree]]
|-
|<math>O(n)\,</math> || [[linear time|linear]] || Finding an item in an unsorted list or a malformed tree (worst case) or in an unsorted array; adding two ''n''-bit integers by [[Ripple carry adder|ripple carry]]
|-
|<math>O(n\log^* n)\,</math> || n [[log-star]] n || Performing [[Polygon triangulation|triangulation]] of a simple polygon using Seidel's algorithm, or the [[Proof of O(log*n) time complexity of union–find|union–find algorithm]]. Note that <math>\log^*(n) =
\begin{cases}
0, & \text{if }n \leq 1 \\
1 + \log^*(\log n), & \text{if }n>1
\end{cases}</math>
|-
|<math>O(n\log n)=O(\log n!)\,</math> || [[Linearithmic time|linearithmic]], loglinear, or quasilinear || Performing a [[fast Fourier transform]]; [[heapsort]], [[quicksort]] (best and average case), or [[merge sort]]
|-
|<math>O(n^2)\,</math> || [[quadratic time|quadratic]] || Multiplying two ''n''-digit numbers by a simple algorithm; [[bubble sort]] (worst case or naive implementation), [[Shell sort]], quicksort (worst case), [[selection sort]] or [[insertion sort]]
|-
|<math>O(n^c),\;c>1</math> || [[polynomial time|polynomial]] or algebraic || [[Tree-adjoining grammar]] parsing; maximum [[Matching (graph theory)|matching]] for [[bipartite graph]]s
|-
|<math>L_n[\alpha,c],\;0 < \alpha < 1=\,</math><br><math>e^{(c+o(1)) (\ln n)^\alpha (\ln \ln n)^{1-\alpha}}</math> || [[L-notation]] or [[sub-exponential time|sub-exponential]] || Factoring a number using the [[quadratic sieve]] or [[number field sieve]]
|-
|<math>O(c^n),\;c>1</math> || [[exponential time|exponential]] || Finding the (exact) solution to the [[travelling salesman problem]] using [[dynamic programming]]; determining if two logical statements are equivalent using [[brute-force search]]
|-
|<math>O(n!)\,</math> || [[factorial]] || Solving the traveling salesman problem via brute-force search; generating all unrestricted permutations of a [[Partially ordered set|poset]]; finding the [[determinant]] with [[expansion by minors]]; enumerating [[Bell number|all partitions of a set]]
|-
|<math>O(n * n!)\,</math> || n × n factorial || Attempting to sort a list of elements using the incredibly inefficient [[bogosort]] algorithm.
|}
The statement <math>f(n)=O(n!)\,</math> is sometimes weakened to <math>f(n)=O\left(n^n\right)</math> to derive simpler formulas for asymptotic complexity.
For any <math>k>0</math> and <math>c>0</math>, <math>O(n^c(\log n)^k)</math> is a subset of <math>O(n^{c+\varepsilon })</math> for any <math> \varepsilon >0</math>, so may be considered as a polynomial with some bigger order.


==Related asymptotic notations==
In general we have that
Big ''O'' is the most commonly used asymptotic notation for comparing functions, although in many cases Big ''O'' may be replaced with Big Theta Θ for asymptotically tighter bounds.  Here, we define some related notations in terms of Big ''O'', progressing up to the family of Bachmann–Landau notations to which Big ''O'' notation belongs.
===Little-o notation=== <!-- [[Little-o notation]] redirects here -->
{{redirect|Little o|the baseball player|Omar Vizquel}}
The relation <math>f(x) \in  o(g(x))</math> is read as "<math>f(x)</math> is little-o of <math>g(x)</math>". Intuitively, it means that <math>g(x)</math> grows much faster than <math>f(x)</math>, or similarly, the growth of <math>f(x)</math> is nothing compared to that of <math>g(x)</math>.  It assumes that ''f'' and ''g'' are both functions of one variable. Formally, ''f''(''n'')&nbsp;=&nbsp;''o''(''g''(''n'')) as {{math|''n'' → ∞}} means that for every positive constant <math>\epsilon</math> there exists a constant ''N'' such that
:<math>|f(n)|\leq\epsilon|g(n)|\qquad\text{for all }n\geq N~.</math><ref name="Concrete Mathematics"/>


Note the difference between the earlier [[#Formal definition|formal definition]] for the big-O notation, and the present definition of little-o: while the former has to be true for ''at least one'' constant ''M'' the latter must hold for ''every'' positive constant <math>\epsilon</math>, however small.<ref name="Introduction to Algorithms"/> In this way little-o notation makes a stronger statement than the corresponding big-O notation: every function that is little-o of ''g'' is also big-O of ''g'', but not every function that is big-O ''g'' is also little-o of ''g'' (for instance ''g'' itself is not, unless it is identically zero near ∞).
:<math>\inf_n x_n \leq \liminf_{n \to \infty} x_n \leq \limsup_{n \to \infty} x_n \leq \sup_n x_n</math>


If ''g''(''x'') is nonzero, or at least becomes nonzero beyond a certain point, the relation ''f''(''x'')&nbsp;=&nbsp;''o''(''g''(''x'')) is equivalent to
The liminf and limsup of a sequence are respectively the smallest and greatest [[Limit point|cluster points]].
:<math>\lim_{x \to \infty}\frac{f(x)}{g(x)}=0.</math>
For example,
* <math>2x  \in o(x^2) \,\!</math>
* <math>2x^2 \not \in  o(x^2)</math>
* <math>1/x \in o(1)</math>
Little-o  notation is common in mathematics but rarer in computer science. In computer science the variable (and function value) is most often a natural number. In mathematics, the variable and function values are often  real numbers. The following properties can be useful:
* <math>o(f) + o(f) \subseteq o(f)</math>
* <math>o(f)o(g) \subseteq o(fg)</math>
* <math>o(o(f)) \subseteq o(f)</math>
* <math>o(f) \subset O(f)</math> (and thus the above properties apply with most combinations of o and O).
As with big O notation, the statement "<math>f(x)</math> is <math>o(g(x))</math>" is usually written as <math> f(x) = o(g(x))</math>, which is a slight [[abuse of notation]].


=== Big Omega notation ===
* For any two sequences of real numbers <math>\{a_n\}, \{b_n\}</math>, the limit superior satisfies [[subadditivity]] whenever the right side of the inequality is defined (i.e., not <math>\infty - \infty</math> or <math>-\infty + \infty</math>):
:<math>\limsup_{n \to \infty} (a_n + b_n) \leq \limsup_{n \to \infty}(a_n) + \limsup_{n \to \infty}(b_n).</math>.


There are two very widespread and incompatible definitions of the statement
Analogously, the limit inferior satisfies [[superadditivity]]:
:<math>\liminf_{n \to \infty} (a_n + b_n) \geq \liminf_{n \to \infty}(a_n) + \liminf_{n \to \infty}(b_n).</math>


:<math>f(x)=\Omega(g(x))\ (x\rightarrow a),</math>
In the particular case that one of the sequences actually converges, say <math>a_n \to a </math>, then the inequalities above become equalities (with <math>\limsup_{n \to \infty}a_n</math> or <math>\liminf_{n \to \infty}a_n</math> being replaced by <math>a</math>).


where <math>a</math> is some real number, <math>\infty</math>,  or <math>-\infty</math>, where <math>f</math> and <math>g</math> are real functions defined in a neighbourhood of <math>a</math>, and where <math>g</math> is positive in this neighbourhood.
==== Examples ====


The first one (chronologically) is used in [[analytic number theory]], and the other one in [[computational complexity theory]]. When the two subjects meet, this situation is bound to generate confusion.
* As an example, consider the sequence given by ''x''<sub>''n''</sub> = [[trigonometric function|sin]](''n''). Using the fact that [[pi]] is [[irrational number|irrational]], one can show that
:<math>\liminf_{n\to\infty} x_n = -1</math>


==== The Hardy–Littlewood definition ====
and


In 1914 [[Godfrey Harold Hardy|G.H. Hardy]] and [[John Edensor Littlewood|J.E. Littlewood]] introduced the new symbol <math>\Omega</math>,<ref name="HL">G. H. Hardy and J. E. Littlewood, "Some problems of Diophantine approximation", Acta Mathematica 37 (1914), p. 225</ref> which is defined as follows:
:<math>\limsup_{n\to\infty} x_n = +1.</math>


:<math>f(x)=\Omega(g(x))\ (x\rightarrow\infty)\;\Leftrightarrow\;\limsup_{x \to \infty} \left|\frac{f(x)}{g(x)}\right|> 0</math>.
(This is because the sequence {1,2,3,...} is [[Equidistributed mod 1|equidistributed mod 2&pi;]], a consequence of the [[Equidistribution theorem]].)


Thus <math>f(x)=\Omega(g(x))</math> is the negation of <math>f(x)=o(g(x))</math>.
* An example from [[number theory]] is


In 1918 the same authors  introduced the two new symbols  <math>\Omega_R</math> and <math>\Omega_L</math>,<ref name="HL2">G. H. Hardy and J. E. Littlewood, « Contribution to the theory of the Riemann zeta-function and the theory of the distribution of primes », ''[[Acta Mathematica]]'', vol. 41, 1918.</ref> thus defined:
:<math>\liminf_{n\to\infty}(p_{n+1}-p_n),</math>


:<math>f(x)=\Omega_R(g(x))\ (x\rightarrow\infty)\;\Leftrightarrow\;\limsup_{x \to \infty} \frac{f(x)}{g(x)}> 0</math>;
where ''p''<sub>''n''</sub> is the ''n''-th [[prime number]].
The value of this limit inferior is conjectured to be 2 – this is the [[twin prime conjecture]] – but {{as of|2014|4|lc=y}} has only been proven to be less than or equal to 246.<ref>{{cite web|title=Bounded gaps between primes|url=http://michaelnielsen.org/polymath1/index.php?title=Bounded_gaps_between_primes|website=Polymath wiki|accessdate=14 May 2014}}</ref> The corresponding limit superior is <math>+\infty</math>, because there are arbitrary [[Gaps between prime numbers|gaps between consecutive primes]].


:<math>f(x)=\Omega_L(g(x))\ (x\rightarrow\infty)\;\Leftrightarrow\;\liminf_{x \to \infty} \frac{f(x)}{g(x)}< 0</math>.
== Real-valued functions ==


Hence <math>f(x)=\Omega_R(g(x))</math> is the negation of <math>f(x)<o(g(x))</math>, and <math>f(x)=\Omega_L(g(x))</math> the negation of <math>f(x)>o(g(x))</math>.
Assume that a function is defined from a subset of the real numbers to the real numbers. As in the case for sequences, the limit inferior and limit superior are always well-defined if we allow the values +∞ and −∞; in fact, if both agree then the limit exists and is equal to their common value (again possibly including the infinities). For example, given ''f''(''x'') = sin(1/''x''), we have lim sup<sub>''x''→''0''</sub> ''f''(''x'') = 1 and lim inf<sub>''x''→''0''</sub> ''f''(''x'') = −1. The difference between the two is a rough measure of how "wildly" the function oscillates, and in observation of this fact, it is called the [[Oscillation (mathematics)|oscillation]] of ''f'' at ''a''. This idea of oscillation is sufficient to, for example, characterize [[Riemann integral|Riemann-integrable]] functions as  continuous except on a set of [[measure zero]] [http://tt.lamf.uwindsor.ca/314folder/analbookfiles/RintexistLebesgue.pdf].  Note that points of nonzero oscillation (i.e., points at which ''f'' is "[[pathological (mathematics)|badly behaved]]") are discontinuities which, unless they make up a set of zero, are confined to a negligible set.


Contrary to a later assertion of  [[Donald Knuth|D.E. Knuth]],<ref name="knuth">Donald Knuth. "Big Omicron and big Omega and big Theta", SIGACT News, Apr.-June 1976, 18-24. [http://www.phil.uu.nl/datastructuren/10-11/knuth_big_omicron.pdf]</ref> [[Edmund Landau]] did use these three symbols, with the same meanings, in 1924.<ref name="landau">E. Landau, "Über die Anzahl der Gitterpunkte in gewissen Bereichen. IV." Nachr. Gesell. Wiss. Gött. Math-phys. Kl. 1924, 137–150.</ref>
== Functions from metric spaces to metric spaces ==


These Hardy-Littlewood symbols are prototypes, which after Landau were never used again exactly thus.
There is a notion of lim sup and lim inf for functions defined on a [[metric space]] whose relationship to limits of real-valued functions mirrors that of the relation between the lim sup, lim inf, and the limit of a real sequence. Take metric spaces ''X'' and ''Y'', a subspace ''E'' contained in ''X'', and a function ''f''&nbsp;:&nbsp;''E''&nbsp;→&nbsp;''Y''.  The space ''Y'' should also be an [[Totally ordered set|ordered set]], so that the notions of supremum and infimum make sense. Define, for any [[limit point]] ''a'' of ''E'',


:<math>\Omega_R</math> became <math>\Omega_+</math>, and <math>\Omega_L</math> became <math>\Omega_-</math>.
:<math>\limsup_{x \to a} f(x) = \lim_{\varepsilon \to 0} ( \sup \{ f(x) : x \in E \cap B(a;\varepsilon)\setminus\{a\} \} ) </math>
and


These three symbols <math>\Omega, \Omega_+, \Omega_-</math>, as well as <math>f(x)=\Omega_\pm(g(x))</math> (meaning that <math>f(x)=\Omega_+(g(x))</math> and <math>f(x)=\Omega_-(g(x))</math> are both satisfied), are now currently used in [[analytic number theory]].
:<math>\liminf_{x \to a} f(x) = \lim_{\varepsilon \to 0} ( \inf \{ f(x) : x \in E \cap B(a;\varepsilon)\setminus\{a\} \} ) </math>


==== Simple examples ====
where ''B''(''a'';ε) denotes the [[Ball (mathematics)|metric ball]] of radius ε about ''a''.


We have
Note that as ε shrinks, the supremum of the function over the ball is monotone decreasing, so we have


:<math>\sin x=\Omega(1)\ (x\rightarrow\infty),</math>
:<math>\limsup_{x\to a} f(x)  = \inf_{\varepsilon > 0} (\sup \{ f(x) : x \in E \cap B(a;\varepsilon)\setminus\{a\} \}) </math>
and similarly
:<math>\liminf_{x\to a} f(x) = \sup_{\varepsilon > 0}(\inf \{ f(x) : x \in E \cap B(a;\varepsilon)\setminus\{a\} \}).</math>


and more precisely
This finally motivates the definitions for general topological spaces. Take ''X'', ''Y'', ''E'' and ''a'' as before, but now let ''X'' and ''Y'' both be topological spaces. In this case, we replace metric balls with neighborhoods:


:<math>\sin x=\Omega_\pm(1)\ (x\rightarrow\infty).</math>
:<math>\limsup_{x\to a} f(x) = \inf \{ \sup \{ f(x) : x \in E \cap U\setminus\{a\} \} :  U\ \mathrm{open}, a \in U, E \cap U\setminus\{a\} \neq \emptyset  \}</math>
:<math>\liminf_{x\to a} f(x) = \sup \{ \inf \{ f(x) : x \in E \cap U\setminus\{a\} \} :  U\ \mathrm{open}, a \in U, E \cap U\setminus\{a\} \neq \emptyset  \}</math>


We have
(there is a way to write the formula using a ''lim'' using nets and the neighborhood filter). This version is often useful in discussions of [[semi-continuity]] which crop up in analysis quite often. An interesting note is that this version subsumes the sequential version by considering sequences as functions from the natural numbers as a topological subspace of the extended real line, into the space (the closure of '''N''' in (−∞,∞) is '''N''' ∪ {∞}.)


:<math>\sin x+1=\Omega(1)\ (x\rightarrow\infty),</math>
== Sequences of sets ==


and more precisely
The [[power set]] ℘(''X'') of a [[Set (mathematics)|set]] ''X'' is a [[complete lattice]] that is ordered by [[inclusion (set theory)|set inclusion]], and so the supremum and infimum of any set of subsets (in terms of set inclusion) always exist. In particular, every subset ''Y'' of ''X'' is bounded above by ''X'' and below by the empty set ∅ because ∅ ⊆ ''Y'' ⊆ ''X''. Hence, it is possible (and sometimes useful) to consider superior and inferior limits of sequences in ℘(''X'') (i.e., sequences of subsets of ''X'').


:<math>\sin x+1=\Omega_+(1)\ (x\rightarrow\infty);</math>
There are two common ways to define the limit of sequences of sets. In both cases:
* The sequence ''accumulates'' around sets of points rather than single points themselves. That is, because each element of the sequence is itself a set, there exist accumulation ''sets'' that are somehow nearby to infinitely many elements of the sequence.
* The supremum/superior/outer limit is a set that [[join (mathematics)|join]]s these accumulation sets together. That is, it is the union of all of the accumulation sets. When ordering by set inclusion, the supremum limit is the least upper bound on the set of accumulation points because it ''contains'' each of them. Hence, it is the supremum of the limit points.
* The infimum/inferior/inner limit is a set where all of these accumulation sets [[meet (mathematics)|meet]]. That is, it is the intersection of all of the accumulation sets. When ordering by set inclusion, the infimum limit is the greatest lower bound on the set of accumulation points because it is ''contained in'' each of them. Hence, it is the infimum of the limit points.
* Because ordering is by set inclusion, then the outer limit will always contain the inner limit (i.e., lim&nbsp;inf&nbsp;''X''<sub>''n''</sub> ⊆ lim&nbsp;sup&nbsp;''X''<sub>''n''</sub>). Hence, when considering the convergence of a sequence of sets, it generally suffices to consider the convergence of the outer limit of that sequence.
The difference between the two definitions involves how the [[topology]] (i.e., how to quantify separation) is defined. In fact, the second definition is identical to the first when the [[discrete metric]] is used to induce the topology on ''X''.


however
===General set convergence===


:<math>\sin x+1\not=\Omega_-(1)\ (x\rightarrow\infty).</math>
In this case, a sequence of sets approaches a limiting set when the elements of each member of the sequence approach the elements of the limiting set. In particular, if {''X''<sub>''n''</sub>} is a sequence of subsets of ''X'', then:
* lim&nbsp;sup&nbsp;''X''<sub>''n''</sub>, which is also called the '''outer limit''', consists of those elements which are limits of points in ''X''<sub>''n''</sub> taken from [[countably infinite|(countably) infinite]]ly many ''n''. That is, ''x'' ∈ lim&nbsp;sup&nbsp;''X''<sub>''n''</sub> if and only if there exists a sequence of points ''x''<sub>''k''</sub> and a ''subsequence'' {''X''<sub>''n''<sub>''k''</sub></sub>} of {''X''<sub>''n''</sub>} such that ''x''<sub>''k''</sub> ∈ ''X''<sub>''n''<sub>''k''</sub></sub> and ''x''<sub>''k''</sub> → ''x'' as ''k'' → ∞.
* lim&nbsp;inf&nbsp;''X''<sub>''n''</sub>, which is also called the '''inner limit''', consists of those elements which are limits of points in ''X''<sub>''n''</sub> for all but finitely many ''n'' (i.e., [[cofinitely]] many ''n''). That is, ''x'' ∈ lim&nbsp;inf&nbsp;''X''<sub>''n''</sub> if and only if there exists a ''sequence'' of points {''x''<sub>''k''</sub>} such that ''x''<sub>''k''</sub> ∈ ''X''<sub>''k''</sub> and ''x''<sub>''k''</sub> → ''x'' as ''k'' → ∞.
The limit lim&nbsp;''X''<sub>''n''</sub> exists if and only if lim&nbsp;inf ''X''<sub>''n''</sub> and lim&nbsp;sup ''X''<sub>''n''</sub> agree, in which case lim&nbsp;''X''<sub>''n''</sub> = lim&nbsp;sup ''X''<sub>''n''</sub> = lim&nbsp;inf ''X''<sub>''n''</sub>.<ref name="GSTeel09">{{Cite journal
    |last1=Goebel
    |first1=Rafal
    |last2=Sanfelice
    |first2=Ricardo G.
    |last3=Teel
    |first3=Andrew R.
    |title=Hybrid dynamical systems
    |journal=IEEE Control Systems Magazine
    |year=2009
    |volume=29
    |issue=2
    |pages=28&ndash;93
    |doi=10.1109/MCS.2008.931718}}</ref>


==== The Knuth definition ====
===Special case: discrete metric===


In 1976 [[Donald Knuth|D.E. Knuth]] published a paper<ref name="knuth"/> to justify his use of the  <math>\Omega</math>-symbol to describe a stronger property. Knuth wrote: "For all the applications I have seen so far in computer science, a stronger requirement […] is much more appropriate". He defined
In this case, which is frequently used in [[measure theory]], a sequence of sets approaches a limiting set when the limiting set includes elements from each of the members of the sequence. That is, this case specializes the first case when the topology on set ''X'' is induced from the [[discrete metric]]. For points ''x'' ∈ ''X'' and ''y'' ∈ ''X'', the discrete metric is defined by
:<math>d(x,y) := \begin{cases} 0 &\text{if } x = y,\\ 1 &\text{if } x \neq y. \end{cases}</math>
So a sequence of points {''x''<sub>''k''</sub>} converges to point ''x'' ∈ ''X'' if and only if ''x''<sub>''k''</sub> = ''x'' for all but finitely many ''k''. The following definition is the result of applying this metric to the general definition above.


:<math>f(x)=\Omega(g(x))\Leftrightarrow g(x)=O(f(x))</math>
If {''X''<sub>''n''</sub>} is a sequence of subsets of ''X'', then:
* lim&nbsp;sup&nbsp;''X''<sub>''n''</sub> consists of elements of ''X'' which belong to ''X''<sub>''n''</sub> for '''infinitely many''' ''n'' (see [[countably infinite]]). That is, ''x'' ∈ lim&nbsp;sup&nbsp;''X''<sub>''n''</sub> if and only if there exists a subsequence {''X''<sub>''n''<sub>''k''</sub></sub>} of {''X''<sub>''n''</sub>} such that ''x'' ∈ ''X''<sub>''n''<sub>''k''</sub></sub> for all ''k''.
* lim&nbsp;inf&nbsp;''X''<sub>''n''</sub> consists of elements of ''X'' which belong to ''X''<sub>''n''</sub> for '''all but finitely many''' ''n'' (i.e., for [[cofinitely]] many ''n''). That is, ''x'' ∈ lim&nbsp;inf&nbsp;''X''<sub>''n''</sub> if and only if there exists some ''m''>0 such that ''x'' ∈ ''X''<sub>''n''</sub> for all ''n''>''m''.
The limit lim&nbsp;''X'' exists if and only if lim&nbsp;inf ''X'' and lim&nbsp;sup ''X'' agree, in which case lim&nbsp;''X'' = lim&nbsp;sup ''X'' = lim&nbsp;inf ''X''.<ref name="Halmos50">{{Cite book
|title=Measure Theory
|last=Halmos
|first=Paul R.
|year=1950
|location=Princeton, NJ
|publisher=D. Van Nostrand Company, Inc.}}</ref> This definition of the inferior and superior limits is relatively strong because it requires that the elements of the extreme limits also be elements of each of the sets of the sequence.


with the comment: "Although I have changed Hardy and Littlewood's definition of <math>\Omega</math>, I feel justified in doing so because their definition is by no means in wide use, and because there are other ways to say what they want to say in the comparatively rare cases when their definition applies". However, the Hardy–Littlewood definition had been used for at least 25 years.<ref name="titchmarsh">E. C. Titchmarsh, The Theory of the Riemann Zeta-Function (Oxford; Clarendon Press, 1951)</ref>
Using the standard parlance of set theory, consider the infimum of a sequence of sets. The infimum is a greatest lower ''bound'' or [[meet (mathematics)|meet]] of a set. In the case of a sequence of sets, the sequence constituents meet at a set that is somehow smaller than each constituent set. [[inclusion (set theory)|Set inclusion]] provides an ordering that allows set intersection to generate a greatest lower bound ∩''X''<sub>''n''</sub> of sets in the sequence {''X''<sub>''n''</sub>}. Similarly, the supremum, which is the least upper bound or [[join (mathematics)|join]], of a sequence of sets is the union ∪''X''<sub>''n''</sub> of sets in sequence {''X''<sub>''n''</sub>}.
In this context, the inner limit lim&nbsp;inf&nbsp;''X''<sub>''n''</sub> is the largest meeting of tails of the sequence, and the outer limit lim&nbsp;sup&nbsp;''X''<sub>''n''</sub> is the smallest joining of tails of the sequence.


===Family of Bachmann–Landau notations===
*Let ''I''<sub>''n''</sub> be the meet of the ''n''<sup>th</sup> tail of the sequence. That is,
{| class="wikitable"
::<math>I_n := \inf \{ X_m : m \in \{n, n+1, n+2, \ldots\}\} = \bigcap_{m=n}^{\infty} X_m = X_n \cap X_{n+1} \cap X_{n+2} \cap \cdots.</math>
|-
:Then ''I''<sub>''k''</sub> &sube; ''I''<sub>''k''+1</sub> &sube; ''I''<sub>''k''+2</sub> because ''I''<sub>''k''+1</sub> is the intersection of fewer sets than ''I''<sub>''k''</sub>. In particular, the sequence {''I''<sub>''k''</sub>} is non-decreasing. So the inner/inferior limit is the least upper bound on this sequence of '''meets of tails'''. In particular,
! Notation
::<math>\begin{align}
! Name
\liminf_{n\to\infty}X_n &:= \lim_{n\to\infty} \inf\{X_m: m \in \{n, n+1, \ldots\}\}\\
! Intuition
&= \sup\{\inf\{X_m: m \in \{n, n+1, \ldots\}\}: n \in \{1,2,\dots\}\}\\
! Informal definition: for sufficiently large <math>n</math>...
&= {\bigcup_{n=1}^\infty}\left({\bigcap_{m=n}^\infty}X_m\right).
! Formal Definition
\end{align}</math>
<!-- !Alternative definition -->
:So the inferior limit acts like a version of the standard infimum that is unaffected by the set of elements that occur only finitely many times. That is, the infimum limit is a subset (i.e., a lower bound) for all but finitely many elements.
|-
|<math>f(n) \in O(g(n))</math>


or
*Similarly, let ''J''<sub>''m''</sub> be the join of the ''m''<sup>th</sup> tail of the sequence. That is,
::<math>J_m := \sup \{ X_m : m \in \{n, n+1, n+2, \ldots\}\} = \bigcup_{m=n}^{\infty} X_m = X_n \cup X_{n+1} \cup X_{n+2} \cup \cdots.</math>
:Then ''J''<sub>''k''</sub> &supe; ''J''<sub>''k''+1</sub> &supe; ''J''<sub>''k''+2</sub> because ''J''<sub>''k''+1</sub> is the union of fewer sets than ''J''<sub>''k''</sub>. In particular, the sequence {''J''<sub>''k''</sub>} is non-increasing. So the outer/superior limit is the greatest lower bound on this sequence of '''joins of tails'''. In particular,
::<math>\begin{align}
\limsup_{n\to\infty}X_n &:= \lim_{n\to\infty} \sup\{X_m: m \in \{n, n+1, \ldots\}\}\\
&= \inf\{\sup\{X_m: m \in \{n, n+1, \ldots\}\}: n \in \{1,2,\dots\}\}\\
&= {\bigcap_{n=1}^\infty}\left({\bigcup_{m=n}^\infty}X_m\right).
\end{align}</math>
:So the superior limit acts like a version of the standard supremum that is unaffected by the set of elements that occur only finitely many times. That is, the supremum limit is a superset (i.e., an upper bound) for all but finitely many elements.


<math>f(n) = O(g(n))</math>
The limit lim&nbsp;''X''<sub>''n''</sub> exists if and only if lim&nbsp;sup&nbsp;''X''<sub>''n''</sub>=lim&nbsp;inf&nbsp;''X''<sub>''n''</sub>, and in that case, lim&nbsp;''X''<sub>''n''</sub>=lim&nbsp;inf&nbsp;''X''<sub>''n''</sub>=lim&nbsp;sup&nbsp;''X''<sub>''n''</sub>. In this sense, the sequence has a limit so long as all but finitely many of its elements are equal to the limit.
| Big O; Big Oh; Big Omicron<ref name="knuth"/>  
| <math>f</math> is bounded above by <math>g</math> (up to constant factor) asymptotically
<!-- |<math>\limsup_{n \to \infty} \left|\frac{f(n)}{g(n)}\right| < \infty</math> -->
|<math>|f(n)|  \leq  k\cdot g(n)</math> for some positive ''k''
|<math>\exists k>0 \; \exists n_0 \; \forall n>n_0 \; |f(n)| \leq k\cdot |g(n)|</math> <br> or <br> <math>  \exists k>0 \; \exists n_0 \; \forall n>n_0 \; f(n) \leq k\cdot g(n)</math>
|-
|<math>f(n) \in \Omega(g(n))</math>


or
===Examples===


<math>f(n) = \Omega(g(n))</math>
The following are several set convergence examples. They have been broken into sections with respect to the metric used to induce the topology on set ''X''.
| Big Omega
|'''Two definitions : '''


Number theory:
; Using the [[discrete metric]]


<math>f</math> is not dominated by <math>g</math> asymptotically
* The [[Borel–Cantelli lemma]] is an example application of these constructs.


Complexity theory:
; Using either the discrete metric or the [[Euclidean metric]]


<math>f</math> is bounded below by <math>g</math> asymptotically
* Consider the set ''X'' = {0,1} and the sequence of subsets:
|Number theory:
::<math>\{X_n\} = \{ \{0\},\{1\},\{0\},\{1\},\{0\},\{1\},\dots \}.</math>
:The "odd" and "even" elements of this sequence form two subsequences, <nowiki>{{</nowiki>0},{0},{0},...} and <nowiki>{{</nowiki>1},{1},{1},...}, which have limit points 0 and 1, respectively, and so the outer or superior limit is the set {0,1} of these two points. However, there are no limit points that can be taken from the {''X''<sub>''n''</sub>} sequence as a whole, and so the interior or inferior limit is the empty set {}. That is,
:* lim&nbsp;sup&nbsp;''X''<sub>''n''</sub> = {0,1}
:* lim&nbsp;inf&nbsp;''X''<sub>''n''</sub> = {}
:However, for {''Y''<sub>''n''</sub>} = <nowiki>{{</nowiki>0},{0},{0},...} and {''Z''<sub>''n''</sub>} = <nowiki>{{</nowiki>1},{1},{1},...}:
:* lim&nbsp;sup&nbsp;''Y''<sub>''n''</sub> = lim&nbsp;inf&nbsp;''Y''<sub>''n''</sub> = lim&nbsp;''Y''<sub>''n''</sub> = {0}
:* lim&nbsp;sup&nbsp;''Z''<sub>''n''</sub> = lim&nbsp;inf&nbsp;''Z''<sub>''n''</sub> = lim&nbsp;''Z''<sub>''n''</sub> = {1}


<math>f(n)  \geq  k\cdot g(n)</math> for infinitely many values of ''n'' and for some positive ''k''
* Consider the set ''X'' = {50, 20, -100, -25, 0, 1} and the sequence of subsets:
::<math>\{X_n\} = \{ \{50\}, \{20\}, \{-100\}, \{-25\}, \{0\},\{1\},\{0\},\{1\},\{0\},\{1\},\dots \}.</math>
:As in the previous two examples,
:* lim&nbsp;sup&nbsp;''X''<sub>''n''</sub> = {0,1}
:* lim&nbsp;inf&nbsp;''X''<sub>''n''</sub> = {}
:That is, the four elements that do not match the pattern do not affect the lim&nbsp;inf and lim&nbsp;sup because there are only finitely many of them. In fact, these elements could be placed anywhere in the sequence (e.g., at positions 100, 150, 275, and 55000). So long as the ''tails'' of the sequence are maintained, the outer and inner limits will be unchanged. The related concepts of ''essential'' inner and outer limits, which use the [[essential supremum]] and [[essential infimum]], provide an important modification that "squashes" countably many (rather than just finitely many) interstitial additions.


Complexity theory:
; Using the Euclidean metric


<math>f(n)  \geq  k\cdot g(n)</math> for some positive ''k''
* Consider the sequence of subsets of [[rational number]]s:
<!--|<math>\liminf_{n \to \infty} \left|\frac{f(n)}{g(n)}\right| > 0 </math> -->
::<math>\{X_n\} = \{ \{0\},\{1\},\{1/2\},\{1/2\},\{2/3\},\{1/3\}, \{3/4\}, \{1/4\}, \dots \}.</math>
|Number theory:
:The "odd" and "even" elements of this sequence form two subsequences, <nowiki>{{</nowiki>0},{1/2},{2/3},{3/4},...} and <nowiki>{{</nowiki>1},{1/2},{1/3},{1/4},...}, which have limit points 1 and 0, respectively, and so the outer or superior limit is the set {0,1} of these two points. However, there are no limit points that can be taken from the {''X''<sub>''n''</sub>} sequence as a whole, and so the interior or inferior limit is the empty set {}. So, as in the previous example,
:* lim&nbsp;sup&nbsp;''X''<sub>''n''</sub> = {0,1}
:* lim&nbsp;inf&nbsp;''X''<sub>''n''</sub> = {}
:However, for {''Y''<sub>''n''</sub>} = <nowiki>{{</nowiki>0},{1/2},{2/3},{3/4},...} and {''Z''<sub>''n''</sub>} = <nowiki>{{</nowiki>1},{1/2},{1/3},{1/4},...}:
:* lim&nbsp;sup&nbsp;''Y''<sub>''n''</sub> = lim&nbsp;inf&nbsp;''Y''<sub>''n''</sub> = lim&nbsp;''Y''<sub>''n''</sub> = {1}
:* lim&nbsp;sup&nbsp;''Z''<sub>''n''</sub> = lim&nbsp;inf&nbsp;''Z''<sub>''n''</sub> = lim&nbsp;''Z''<sub>''n''</sub> = {0}
:In each of these four cases, the elements of the limiting sets are not elements of any of the sets from the original sequence.


<math>\exists k>0 \; \forall n_0 \; \exists n>n_0 \; f(n) \geq k\cdot g(n)</math>
* The Ω limit (i.e., [[limit set]]) of a solution to a [[dynamic system]] is the outer limit of solution trajectories of the system.<ref name="GSTeel09"/>{{rp|50&ndash;51}} Because trajectories become closer and closer to this limit set, the tails of these trajectories ''converge'' to the limit set.
:* For example, an LTI system that is the [[cascade connection]] of several [[stability theory|stable]] systems with an undamped second-order [[LTI system]] (i.e., zero [[damping ratio]]) will oscillate endlessly after being perturbed (e.g., an ideal bell after being struck). Hence, if the position and velocity of this system are plotted against each other, trajectories will approach a circle in the [[state space (controls)|state space]]. This circle, which is the &Omega; limit set of the system, is the outer limit of solution trajectories of the system. The circle represents the locus of a trajectory corresponding to a pure sinusoidal tone output; that is, the system output approaches/approximates a pure tone.


Complexity theory:
==Generalized definitions==


<math>\exists k>0 \; \exists n_0 \; \forall n>n_0 \; f(n) \geq k\cdot g(n)</math>
The above definitions are inadequate for many technical applications. In fact, the definitions above are specializations of the following definitions.
|-
|<math>f(n) \in \Theta(g(n))</math>
| Big Theta
| <math>f</math> is bounded both above and below by <math>g</math> asymptotically
|<math>k_1\cdot g(n) \leq f(n) \leq k_2\cdot g(n)</math> for some positive ''k''<sub>1</sub>, ''k''<sub>2</sub>
<!-- |<math>0 < \liminf_{n \to \infty} \left|\frac{f(n)}{g(n)}\right| \leq \limsup_{n \to \infty} \left|\frac{f(n)}{g(n)}\right|< \infty</math> -->
<!--|<math>f(n) \in O(g(n)) \cap \Omega(g(n))</math> -->
|<math>\exists k_1>0 \; \exists k_2>0 \; \exists n_0 \; \forall n>n_0</math>
<math>k_1\cdot g(n) \leq f(n) \leq k_2\cdot g(n)</math>
|-
|<math>f(n) \in o(g(n))</math>


or
===Definition for a set===


<math>f(n) = o(g(n))</math>
The limit inferior of a set ''X'' ⊆ ''Y'' is the [[infimum]] of all of the [[limit point]]s of the set. That is,
| Small O; Small Oh
:<math>\liminf X := \inf \{ x \in Y : x \text{ is a limit point of } X \}\,</math>
| <math>f</math> is dominated by <math>g</math> asymptotically
Similarly, the limit superior of a set ''X'' is the [[supremum]] of all of the limit points of the set. That is,
| <math>|f(n)| \le k\cdot|g(n)|</math>, for every fixed positive number <math>k</math> 
:<math>\limsup X := \sup \{ x \in Y : x \text{ is a limit point of } X \}\,</math>
<!-- |<math>\lim_{n \to \infty} \left|\frac{f(n)}{g(n)}\right| = 0</math> -->
Note that the set ''X'' needs to be defined as a subset of a [[partially ordered set]] ''Y'' that is also a [[topological space]] in order for these definitions to make sense. Moreover, it has to be a [[complete lattice]] so that the suprema and infima always exist. In that case every set has a limit superior and a limit inferior. Also note that the limit inferior and the limit superior of a set do not have to be elements of the set.
<!-- |<math>\forall \;k>0,\exists \;n(k) : |f(n)| < \; k |g(n)|, \forall n>n(k).</math> -->
|<math>\forall k>0 \; \exists n_0 \; \forall n>n_0 \; |f(n)| \le k\cdot |g(n)|</math>
|-
|<math>f(n) \in \omega(g(n))</math>
| Small Omega
| <math>f</math> dominates <math>g</math> asymptotically
| <math>|f(n)| \ge k\cdot|g(n)|</math>, for every fixed positive number <math>k</math>
<!-- |<math>\lim_{n \to \infty} \left|\frac{f(n)}{g(n)}\right| = \infty</math> -->
|<math>\forall k>0 \; \exists n_0 \; \forall n>n_0 \  |f(n)| \ge k\cdot |g(n)|</math>
<!-- Sometimes this is expressed as "\forall M<\infty" - any preference here? -->
|-
|<math>f(n)\sim g(n)\!</math><!-- force PNG; ~ is often ugly -->
| On the order of
| <math>f</math> is equal to <math>g</math> asymptotically
|<math>f(n)/g(n) \to 1</math>
|<math>\forall \varepsilon>0\;\exists n_0\;\forall n>n_0\;\left|{f(n) \over g(n)}-1\right|<\varepsilon</math>
|}


===Definition for filter bases===
Aside from the Big ''O'' notation, the Big Theta Θ and Big Omega Ω notations are the two most often used in computer science; the small omega ω notation is occasionally used in computer science.


Aside from the Big ''O'' notation, the small ''o'', Big Omega Ω and <math>\sim</math> notations are the three most often used in number theory; the small omega ω notation is never used in number theory.
Take a [[topological space]] ''X'' and a [[filter base]] ''B'' in that space. The set of all [[cluster point]]s for that filter base is given by
:<math>\bigcap \{ \overline{B}_0 : B_0 \in B \}</math>
where <math>\overline{B}_0</math> is the [[closure (topology)|closure]] of <math>B_0</math>. This is clearly a [[closed set]] and is similar to the set of limit points of a set. Assume that ''X'' is also a [[partially ordered set]]. The limit superior of the filter base ''B'' is defined as
:<math>\limsup B := \sup \bigcap \{ \overline{B}_0 : B_0 \in B \}</math>
when that supremum exists.  When ''X'' has a [[total order]], is a [[complete lattice]] and has the [[order topology]],
:<math>\limsup B = \inf\{ \sup B_0 : B_0 \in B \}</math>
Proof:
Similarly, the limit inferior of the filter base ''B'' is defined as
:<math>\liminf B := \inf \bigcap \{ \overline{B}_0 : B_0 \in B \}</math>
when that infimum exists; if ''X'' is totally ordered, is a complete lattice, and has the order topology, then
:<math>\liminf B = \sup\{ \inf B_0 : B_0 \in B \}</math>


=== Use in computer science ===
If the limit inferior and limit superior agree, then there must be exactly one cluster point and the limit of the filter base is equal to this unique cluster point.
{{details|Analysis of algorithms}}
Informally, especially in computer science, the Big ''O'' notation often is permitted to be somewhat abused to describe an asymptotic tight bound where using Big Theta Θ notation might be more factually appropriate in a given context. For example, when considering a function <math>T(n) = 73n^3 + 22n^2 + 58</math>, all of the following are generally acceptable, but tightnesses of bound (i.e., numbers 2 and 3 below) are usually strongly preferred over laxness of bound (i.e., number 1 below).
#''T''(''n'')&nbsp;=&nbsp;''O''(''n''<sup>100</sup>), which is identical to ''T''(''n'')&nbsp;∈&nbsp;''O''(''n''<sup>100</sup>)
#''T''(''n'')&nbsp;=&nbsp;''O''(''n''<sup>3</sup>), which is identical to ''T''(''n'')&nbsp;∈&nbsp;''O''(''n''<sup>3</sup>)
#''T''(''n'')&nbsp;=&nbsp;Θ(''n''<sup>3</sup>), which is identical to ''T''(''n'')&nbsp;∈&nbsp;Θ(''n''<sup>3</sup>).
The equivalent English statements are respectively:
#''T''(''n'') grows asymptotically no faster than ''n''<sup>100</sup>
#''T''(''n'') grows asymptotically no faster than ''n''<sup>3</sup>
#''T''(''n'') grows asymptotically as fast as ''n''<sup>3</sup>.
So while all three statements are true, progressively more information is contained in each. In some fields, however, the Big O notation (number 2 in the lists above) would be used more commonly than the Big Theta notation (bullets number 3 in the lists above) because functions that grow more slowly are more desirable. For example, if <math>T(n)</math> represents the running time of a newly developed algorithm for input size <math>n</math>, the inventors and users of the algorithm might be more inclined to put an upper asymptotic bound on how long it will take to run without making an explicit statement about the lower asymptotic bound.


===Extensions to the Bachmann–Landau notations===
====Specialization for sequences and nets====
Another notation sometimes used in computer science is Õ (read ''soft-O''): ''f''(''n'')&nbsp;=&nbsp;''Õ''(''g''(''n'')) is shorthand
Note that filter bases are generalizations of [[net (mathematics)|nets]], which are generalizations of [[sequence]]s. Therefore, these definitions give the limit inferior and [[Net (mathematics)#Limit superior|limit superior]] of any net (and thus any sequence) as well. For example, take topological space <math>X</math> and the net <math>(x_\alpha)_{\alpha \in A}</math>, where <math>(A,{\leq})</math> is a [[directed set]] and <math>x_\alpha \in X</math> for all <math>\alpha \in A</math>. The filter base ("of tails") generated by this net is <math>B</math> defined by
for ''f''(''n'')&nbsp;=&nbsp;''O''(''g''(''n'')&nbsp;log<sup>''k''</sup>&nbsp;''g''(''n'')) for some ''k''. Essentially, it is Big O notation, ignoring logarithmic factors because the growth-rate effects of some other super-logarithmic function indicate a growth-rate explosion for large-sized input parameters that is more important to predicting bad run-time performance than the finer-point effects contributed by the logarithmic-growth factor(s). This notation is often used to obviate the "nitpicking" within growth-rates that are stated as too tightly bounded for the matters at hand (since log<sup>''k''</sup>&nbsp;''n'' is always ''o''(''n''<sup>ε</sup>) for any constant ''k'' and any ε&nbsp;>&nbsp;0).
:<math>B := \{ \{ x_\alpha : \alpha_0 \leq \alpha \} : \alpha_0 \in A \}.\,</math>
Therefore, the limit inferior and limit superior of the net are equal to the limit superior and limit inferior of <math>B</math> respectively. Similarly, for topological space <math>X</math>, take the sequence <math>(x_n)</math> where <math>x_n \in X</math> for any <math>n \in \mathbb{N}</math> with <math>\mathbb{N}</math> being the set of [[natural number]]s. The filter base ("of tails") generated by this sequence is <math>C</math> defined by
:<math>C := \{ \{ x_n : n_0 \leq n \} : n_0 \in \mathbb{N} \}.\,</math>
Therefore, the limit inferior and limit superior of the sequence are equal to the limit superior and limit inferior of <math>C</math> respectively.


Also the [[L-notation|L notation]], defined as
==See also==
:<math>L_n[\alpha,c]=O\left(e^{(c+o(1))(\ln n)^\alpha(\ln\ln n)^{1-\alpha}}\right),</math>
* [[Essential supremum and essential infimum]]
is convenient for functions that are between polynomial and exponential.
* [[Envelope (waves)]]


==Generalizations and related usages==
==References==
The generalization to functions taking values in any [[normed vector space]] is straightforward (replacing absolute values by norms), where ''f'' and ''g'' need not take their values in the same space. A generalization to functions ''g'' taking values in any [[topological group]] is also possible.
The "limiting process" ''x→x<sub>o</sub>'' can also be generalized by introducing an arbitrary [[filter base]], i.e. to directed [[net (mathematics)|net]]s ''f'' and ''g''.
The ''o'' notation can be used to define [[derivative]]s and [[differentiability]] in quite general spaces, and also (asymptotical) equivalence of functions,
:<math> f\sim g \iff (f-g) \in o(g) </math>
which is an [[equivalence relation]] and a more restrictive notion than the relationship "''f'' is Θ(''g'')" from above. (It reduces to <math>\lim f/g = 1</math> if ''f'' and ''g'' are positive real valued functions.)  For example, 2''x'' is Θ(''x''), but 2''x''&nbsp;−&nbsp;''x'' is not ''o''(''x'').
 
==History (Bachmann–Landau, Hardy, and Vinogradov notations)==
The symbol O was first introduced by number theorist [[Paul Bachmann]] in 1894, in the second volume of his book ''Analytische Zahlentheorie'' ("[[analytic number theory]]"), the first volume of which (not yet containing big O notation) was published in 1892.<ref>[[Nicholas J. Higham]], ''Handbook of writing for the mathematical sciences'', SIAM. ISBN 0-89871-420-6, p. 25</ref> The number theorist [[Edmund Landau]] adopted it, and was thus inspired to introduce in 1909 the notation o;<ref>Edmund Landau. Handbuch der Lehre von der Verteilung der Primzahlen, Teubner, Leipzig 1909, p.883.</ref> hence both are now called Landau symbols. These notations were used in applied mathematics during the 1950s for asymptotic analysis.<ref>{{cite book
| title = Asymptotic Expansions
| last = Erdelyi
| first = A.
| year = 1956
| isbn = 978-0486603186
}}</ref> The big O was popularized in computer science by [[Donald Knuth]], who re-introduced the related Omega and Theta notations.<ref name="knuth"/> Knuth also noted that the Omega notation had been introduced by Hardy and Littlewood<ref name="HL" /> under a different meaning "≠''o''" (i.e. "is not an ''o'' of"), and proposed the above definition. Hardy and Littlewood's original definition (which was also used in one paper by Landau<ref name="landau" />) is still used in number theory (where Knuth's definition is never used). In fact, Landau also used in 1924, in the paper just mentioned, the symbols <math>\Omega_R</math> ("right") and <math>\Omega_L</math> ("left"), which were introduced in 1918 by Hardy and Littlewood,<ref name="HL2" /> and which were precursors for the modern symbols <math>\Omega_+</math> ("is not smaller than a small o of") and <math>\Omega_-</math> ("is not larger than a small o of"). Thus the Omega symbols (with their original meanings) are sometimes also referred to as "Landau symbols".
Also, Landau never used the Big Theta and small omega symbols.
 
Hardy's symbols were (in terms of the modern ''O'' notation)
:<math> f \preceq g\iff f \in O(g) </math> &nbsp; and &nbsp; <math> f\prec g\iff f\in o(g); </math>
(Hardy however never defined or used the notation <math>\prec\!\!\prec</math>, nor <math>\ll</math>, as it has been sometimes reported).
It should also be noted that Hardy introduces  the symbols <math>\preceq </math> and <math>\prec </math> (as well as some other symbols) in his 1910 tract "Orders of Infinity", and makes use of it only in three papers (1910–1913). In his nearly 400 remaining papers and books he consistently uses the Landau symbols O and o.
 
Hardy's notation is not used anymore. On the other hand, in the 1930s,<ref>See for instance "A new estimate for ''G(n)'' in Waring's problem" (Russian). Doklady Akademii Nauk SSSR 5, No 5-6 (1934), 249-253. Translated in English in: Selected works / Ivan Matveevič Vinogradov ; prepared by the Steklov Mathematical Institute of the Academy of Sciences of the USSR on the occasion of his 90th birthday. Springer-Verlag, 1985.</ref> the Russian number theorist  [[Ivan Matveyevich Vinogradov]]  introduced his notation
<math>\ll</math>, which  has been increasingly used in number theory instead of  the <math>O</math> notation. We have
 
:<math> f\ll g \iff f \in O(g), </math>
 
and frequently both notations are used in the same paper.
 
The big-O originally stands for "order of" ("Ordnung", Bachmann 1894), and is thus a roman letter. Neither Bachmann nor Landau ever call it "Omicron". The symbol was much later on (1976) viewed by Knuth as a capital [[omicron]],<ref name="knuth"/> probably in reference to his definition of the symbol [[Omega]]. The digit [[0 (number)|zero]] should not be used.
 
==See also==
* [[Asymptotic expansion]]: Approximation of functions generalizing Taylor's formula
* [[Asymptotically optimal]]: A phrase frequently used to describe an algorithm that has an upper bound asymptotically within a constant of a lower bound for the problem
* [[Big O in probability notation]]: ''O<sub>p</sub>'',''o<sub>p</sub>''
* [[Limit superior and limit inferior]]: An explanation of some of the limit notation used in this article
* [[Nachbin's theorem]]: A precise method of bounding [[complex analytic]] functions so that the domain of convergence of [[integral transform]]s can be stated


==References and Notes==
{{reflist}}
{{Reflist}}


==Further reading==
{{refbegin}}
* [[Paul Bachmann]]. ''Die Analytische Zahlentheorie. Zahlentheorie''. pt. 2 Leipzig: B. G. Teubner, 1894.
*{{cite book
* [[Edmund Landau]]. ''Handbuch der Lehre von der Verteilung der Primzahlen''. 2 vols. Leipzig: B. G. Teubner, 1909.
| last      = Amann
* [[G. H. Hardy]]. ''Orders of Infinity: The 'Infinitärcalcül' of Paul du Bois-Reymond'', 1910.
| first      = H.
* [[Donald Knuth]]. ''The Art of Computer Programming'', Volume 1: ''Fundamental Algorithms'', Third Edition. Addison–Wesley, 1997. ISBN 0-201-89683-4. Section 1.2.11: Asymptotic Representations, pp.&nbsp;107–123.
|author2=Escher, Joachim
* [[Paul Vitanyi]], [[Lambert Meertens]], Big Omega versus the wild functions, Bull. European Association Theoret. Comput. Sci. (EATCS) 22(1984), 14-19. Also: ACM-SIGACT News, 16:4(1984) 56-59.
  | title      = Analysis
* [[Thomas H. Cormen]], [[Charles E. Leiserson]], [[Ronald L. Rivest]], and [[Clifford Stein]]. ''[[Introduction to Algorithms]]'', Second Edition. MIT Press and McGraw–Hill, 2001. ISBN 0-262-03293-7. Section 3.1: Asymptotic notation, pp.&nbsp;41–50.
| publisher  = Basel; Boston: Birkhäuser
* {{cite book|author = [[Michael Sipser]] | year = 1997 | title = Introduction to the Theory of Computation | publisher = PWS Publishing | isbn = 0-534-94728-X}} Pages 226–228 of section 7.1: Measuring complexity.
| year      = 2005
* Jeremy Avigad, Kevin Donnelly. ''[http://www.andrew.cmu.edu/~avigad/Papers/bigo.pdf Formalizing O notation in Isabelle/HOL]''
| pages      =
* Paul E. Black,  [http://www.nist.gov/dads/HTML/bigOnotation.html "big-O notation"], in ''Dictionary of Algorithms and Data Structures'' [online], Paul E. Black, ed., U.S. National Institute of Standards and Technology. 11 March 2005. Retrieved December 16, 2006.
| isbn      = 0-8176-7153-6
* Paul E. Black, [http://www.nist.gov/dads/HTML/littleOnotation.html "little-o notation"], in ''Dictionary of Algorithms and Data Structures'' [online], Paul E. Black, ed., U.S. National Institute of Standards and Technology. 17 December 2004. Retrieved December 16, 2006.
}}
* Paul E. Black, [http://www.nist.gov/dads/HTML/omegaCapital.html "Ω"], in ''Dictionary of Algorithms and Data Structures'' [online], Paul E. Black, ed., U.S. National Institute of Standards and Technology. 17 December 2004. Retrieved December 16, 2006.
*{{cite book
* Paul E. Black, [http://www.nist.gov/dads/HTML/omega.html "ω"], in ''Dictionary of Algorithms and Data Structures'' [online], Paul E. Black, ed., U.S. National Institute of Standards and Technology. 29 November 2004. Retrieved December 16, 2006.
| last      = González
* Paul E. Black, [http://www.nist.gov/dads/HTML/theta.html "Θ"], in ''Dictionary of Algorithms and Data Structures'' [online], Paul E. Black, ed., U.S. National Institute of Standards and Technology. 17 December 2004. Retrieved December 16, 2006.
| first      = Mario O
| title     = Classical complex analysis
| publisher = New York: M. Dekker
| year      = 1991
| pages      =
| isbn       = 0-8247-8415-4
}}
{{refend}}


==External links==
==External links==
{{wikibooks|Data Structures|Asymptotic Notation#Big-O Notation|Big-O Notation}}
* {{springer|title=Upper and lower limits|id=p/u095830}}
* [http://oeis.org/wiki/Growth_of_sequences Online Encyclopedia of Integer Sequences]
* [http://www.soe.ucsc.edu/classes/cmps102/Spring04/TantaloAsymp.pdf Introduction to Asymptotic Notations]
* [http://mathworld.wolfram.com/LandauSymbols.html Landau Symbols]
* [http://www.perlmonks.org/?node_id=573138 Big-O Notation – What is it good for]


{{DEFAULTSORT:Big O Notation}}
[[Category:Limits (mathematics)]]
[[Category:Mathematical notation]]
[[Category:Asymptotic analysis]]
[[Category:Analysis of algorithms]]

Revision as of 12:09, 9 August 2014

In mathematics, the limit inferior and limit superior of a sequence can be thought of as limiting (i.e., eventual and extreme) bounds on the sequence. They can be thought of in a similar fashion for a function (see limit of a function). For a set, they are the infimum and supremum of the set's limit points, respectively. In general, when there are multiple objects around which a sequence, function, or set accumulates, the inferior and superior limits extract the smallest and largest of them; the type of object and the measure of size is context-dependent, but the notion of extreme limits is invariant. Limit inferior is also called infimum limit, liminf, inferior limit, lower limit, or inner limit; limit superior is also known as supremum limit, limit supremum, limsup, superior limit, upper limit, or outer limit.

An illustration of limit superior and limit inferior. The sequence xn is shown in blue. The two red curves approach the limit superior and limit inferior of xn, shown as dashed black lines. In this case, the sequence accumulates around the two limits. The superior limit is the larger of the two, and the inferior limit is the smaller of the two. The inferior and superior limits agree if and only if the sequence is convergent (i.e., when there is a single limit).

Definition for sequences

The limit inferior of a sequence (xn) is defined by

or

Similarly, the limit superior of (xn) is defined by

or

Alternatively, the notations and are sometimes used.

If the terms in the sequence are real numbers, the limit superior and limit inferior always exist, as real numbers or ±∞ (i.e., on the extended real number line). More generally, these definitions make sense in any partially ordered set, provided the suprema and infima exist, such as in a complete lattice.

Whenever the ordinary limit exists, the limit inferior and limit superior are both equal to it; therefore, each can be considered a generalization of the ordinary limit which is primarily interesting in cases where the limit does not exist. Whenever lim inf xn and lim sup xn both exist, we have

Limits inferior/superior are related to big-O notation in that they bound a sequence only "in the limit"; the sequence may exceed the bound. However, with big-O notation the sequence can only exceed the bound in a finite prefix of the sequence, whereas the limit superior of a sequence like en may actually be less than all elements of the sequence. The only promise made is that some tail of the sequence can be bounded by the limit superior (inferior) plus (minus) an arbitrarily small positive constant.

The limit superior and limit inferior of a sequence are a special case of those of a function (see below).

The case of sequences of real numbers

In mathematical analysis, limit superior and limit inferior are important tools for studying sequences of real numbers. Since the supremum and infimum of an unbounded set of real numbers may not exist (the reals are not a complete lattice), it is convenient to consider sequences in the affinely extended real number system: we add the positive and negative infinities to the real line to give the complete totally ordered set (−∞,∞), which is a complete lattice.

Interpretation

Consider a sequence consisting of real numbers. Assume that the limit superior and limit inferior are real numbers (so, not infinite).

Properties

The relationship of limit inferior and limit superior for sequences of real numbers is as follows

As mentioned earlier, it is convenient to extend to [−∞,∞]. Then, (xn) in [−∞,∞] converges if and only if

in which case is equal to their common value. (Note that when working just in , convergence to −∞ or ∞ would not be considered as convergence.) Since the limit inferior is at most the limit superior, the condition

and the condition

If and , then the interval [I, S] need not contain any of the numbers xn, but every slight enlargement [I − ε, S + ε] (for arbitrarily small ε > 0) will contain xn for all but finitely many indices n. In fact, the interval [I, S] is the smallest closed interval with this property. We can formalize this property like this: there exist subsequences and of (where and are monotonous) for which we have

On the other hand, there exists a so that for all

To recapitulate:

In general we have that

The liminf and limsup of a sequence are respectively the smallest and greatest cluster points.

.

Analogously, the limit inferior satisfies superadditivity:

In the particular case that one of the sequences actually converges, say , then the inequalities above become equalities (with or being replaced by ).

Examples

  • As an example, consider the sequence given by xn = sin(n). Using the fact that pi is irrational, one can show that

and

(This is because the sequence {1,2,3,...} is equidistributed mod 2π, a consequence of the Equidistribution theorem.)

where pn is the n-th prime number. The value of this limit inferior is conjectured to be 2 – this is the twin prime conjecture – but Template:As of has only been proven to be less than or equal to 246.[1] The corresponding limit superior is , because there are arbitrary gaps between consecutive primes.

Real-valued functions

Assume that a function is defined from a subset of the real numbers to the real numbers. As in the case for sequences, the limit inferior and limit superior are always well-defined if we allow the values +∞ and −∞; in fact, if both agree then the limit exists and is equal to their common value (again possibly including the infinities). For example, given f(x) = sin(1/x), we have lim supx0 f(x) = 1 and lim infx0 f(x) = −1. The difference between the two is a rough measure of how "wildly" the function oscillates, and in observation of this fact, it is called the oscillation of f at a. This idea of oscillation is sufficient to, for example, characterize Riemann-integrable functions as continuous except on a set of measure zero [1]. Note that points of nonzero oscillation (i.e., points at which f is "badly behaved") are discontinuities which, unless they make up a set of zero, are confined to a negligible set.

Functions from metric spaces to metric spaces

There is a notion of lim sup and lim inf for functions defined on a metric space whose relationship to limits of real-valued functions mirrors that of the relation between the lim sup, lim inf, and the limit of a real sequence. Take metric spaces X and Y, a subspace E contained in X, and a function f : E → Y. The space Y should also be an ordered set, so that the notions of supremum and infimum make sense. Define, for any limit point a of E,

and

where B(a;ε) denotes the metric ball of radius ε about a.

Note that as ε shrinks, the supremum of the function over the ball is monotone decreasing, so we have

and similarly

This finally motivates the definitions for general topological spaces. Take X, Y, E and a as before, but now let X and Y both be topological spaces. In this case, we replace metric balls with neighborhoods:

(there is a way to write the formula using a lim using nets and the neighborhood filter). This version is often useful in discussions of semi-continuity which crop up in analysis quite often. An interesting note is that this version subsumes the sequential version by considering sequences as functions from the natural numbers as a topological subspace of the extended real line, into the space (the closure of N in (−∞,∞) is N ∪ {∞}.)

Sequences of sets

The power set ℘(X) of a set X is a complete lattice that is ordered by set inclusion, and so the supremum and infimum of any set of subsets (in terms of set inclusion) always exist. In particular, every subset Y of X is bounded above by X and below by the empty set ∅ because ∅ ⊆ YX. Hence, it is possible (and sometimes useful) to consider superior and inferior limits of sequences in ℘(X) (i.e., sequences of subsets of X).

There are two common ways to define the limit of sequences of sets. In both cases:

  • The sequence accumulates around sets of points rather than single points themselves. That is, because each element of the sequence is itself a set, there exist accumulation sets that are somehow nearby to infinitely many elements of the sequence.
  • The supremum/superior/outer limit is a set that joins these accumulation sets together. That is, it is the union of all of the accumulation sets. When ordering by set inclusion, the supremum limit is the least upper bound on the set of accumulation points because it contains each of them. Hence, it is the supremum of the limit points.
  • The infimum/inferior/inner limit is a set where all of these accumulation sets meet. That is, it is the intersection of all of the accumulation sets. When ordering by set inclusion, the infimum limit is the greatest lower bound on the set of accumulation points because it is contained in each of them. Hence, it is the infimum of the limit points.
  • Because ordering is by set inclusion, then the outer limit will always contain the inner limit (i.e., lim inf Xn ⊆ lim sup Xn). Hence, when considering the convergence of a sequence of sets, it generally suffices to consider the convergence of the outer limit of that sequence.

The difference between the two definitions involves how the topology (i.e., how to quantify separation) is defined. In fact, the second definition is identical to the first when the discrete metric is used to induce the topology on X.

General set convergence

In this case, a sequence of sets approaches a limiting set when the elements of each member of the sequence approach the elements of the limiting set. In particular, if {Xn} is a sequence of subsets of X, then:

  • lim sup Xn, which is also called the outer limit, consists of those elements which are limits of points in Xn taken from (countably) infinitely many n. That is, x ∈ lim sup Xn if and only if there exists a sequence of points xk and a subsequence {Xnk} of {Xn} such that xkXnk and xkx as k → ∞.
  • lim inf Xn, which is also called the inner limit, consists of those elements which are limits of points in Xn for all but finitely many n (i.e., cofinitely many n). That is, x ∈ lim inf Xn if and only if there exists a sequence of points {xk} such that xkXk and xkx as k → ∞.

The limit lim Xn exists if and only if lim inf Xn and lim sup Xn agree, in which case lim Xn = lim sup Xn = lim inf Xn.[2]

Special case: discrete metric

In this case, which is frequently used in measure theory, a sequence of sets approaches a limiting set when the limiting set includes elements from each of the members of the sequence. That is, this case specializes the first case when the topology on set X is induced from the discrete metric. For points xX and yX, the discrete metric is defined by

So a sequence of points {xk} converges to point xX if and only if xk = x for all but finitely many k. The following definition is the result of applying this metric to the general definition above.

If {Xn} is a sequence of subsets of X, then:

  • lim sup Xn consists of elements of X which belong to Xn for infinitely many n (see countably infinite). That is, x ∈ lim sup Xn if and only if there exists a subsequence {Xnk} of {Xn} such that xXnk for all k.
  • lim inf Xn consists of elements of X which belong to Xn for all but finitely many n (i.e., for cofinitely many n). That is, x ∈ lim inf Xn if and only if there exists some m>0 such that xXn for all n>m.

The limit lim X exists if and only if lim inf X and lim sup X agree, in which case lim X = lim sup X = lim inf X.[3] This definition of the inferior and superior limits is relatively strong because it requires that the elements of the extreme limits also be elements of each of the sets of the sequence.

Using the standard parlance of set theory, consider the infimum of a sequence of sets. The infimum is a greatest lower bound or meet of a set. In the case of a sequence of sets, the sequence constituents meet at a set that is somehow smaller than each constituent set. Set inclusion provides an ordering that allows set intersection to generate a greatest lower bound ∩Xn of sets in the sequence {Xn}. Similarly, the supremum, which is the least upper bound or join, of a sequence of sets is the union ∪Xn of sets in sequence {Xn}. In this context, the inner limit lim inf Xn is the largest meeting of tails of the sequence, and the outer limit lim sup Xn is the smallest joining of tails of the sequence.

  • Let In be the meet of the nth tail of the sequence. That is,
Then IkIk+1Ik+2 because Ik+1 is the intersection of fewer sets than Ik. In particular, the sequence {Ik} is non-decreasing. So the inner/inferior limit is the least upper bound on this sequence of meets of tails. In particular,
So the inferior limit acts like a version of the standard infimum that is unaffected by the set of elements that occur only finitely many times. That is, the infimum limit is a subset (i.e., a lower bound) for all but finitely many elements.
  • Similarly, let Jm be the join of the mth tail of the sequence. That is,
Then JkJk+1Jk+2 because Jk+1 is the union of fewer sets than Jk. In particular, the sequence {Jk} is non-increasing. So the outer/superior limit is the greatest lower bound on this sequence of joins of tails. In particular,
So the superior limit acts like a version of the standard supremum that is unaffected by the set of elements that occur only finitely many times. That is, the supremum limit is a superset (i.e., an upper bound) for all but finitely many elements.

The limit lim Xn exists if and only if lim sup Xn=lim inf Xn, and in that case, lim Xn=lim inf Xn=lim sup Xn. In this sense, the sequence has a limit so long as all but finitely many of its elements are equal to the limit.

Examples

The following are several set convergence examples. They have been broken into sections with respect to the metric used to induce the topology on set X.

Using the discrete metric
Using either the discrete metric or the Euclidean metric
  • Consider the set X = {0,1} and the sequence of subsets:
The "odd" and "even" elements of this sequence form two subsequences, {{0},{0},{0},...} and {{1},{1},{1},...}, which have limit points 0 and 1, respectively, and so the outer or superior limit is the set {0,1} of these two points. However, there are no limit points that can be taken from the {Xn} sequence as a whole, and so the interior or inferior limit is the empty set {}. That is,
  • lim sup Xn = {0,1}
  • lim inf Xn = {}
However, for {Yn} = {{0},{0},{0},...} and {Zn} = {{1},{1},{1},...}:
  • lim sup Yn = lim inf Yn = lim Yn = {0}
  • lim sup Zn = lim inf Zn = lim Zn = {1}
  • Consider the set X = {50, 20, -100, -25, 0, 1} and the sequence of subsets:
As in the previous two examples,
  • lim sup Xn = {0,1}
  • lim inf Xn = {}
That is, the four elements that do not match the pattern do not affect the lim inf and lim sup because there are only finitely many of them. In fact, these elements could be placed anywhere in the sequence (e.g., at positions 100, 150, 275, and 55000). So long as the tails of the sequence are maintained, the outer and inner limits will be unchanged. The related concepts of essential inner and outer limits, which use the essential supremum and essential infimum, provide an important modification that "squashes" countably many (rather than just finitely many) interstitial additions.
Using the Euclidean metric
The "odd" and "even" elements of this sequence form two subsequences, {{0},{1/2},{2/3},{3/4},...} and {{1},{1/2},{1/3},{1/4},...}, which have limit points 1 and 0, respectively, and so the outer or superior limit is the set {0,1} of these two points. However, there are no limit points that can be taken from the {Xn} sequence as a whole, and so the interior or inferior limit is the empty set {}. So, as in the previous example,
  • lim sup Xn = {0,1}
  • lim inf Xn = {}
However, for {Yn} = {{0},{1/2},{2/3},{3/4},...} and {Zn} = {{1},{1/2},{1/3},{1/4},...}:
  • lim sup Yn = lim inf Yn = lim Yn = {1}
  • lim sup Zn = lim inf Zn = lim Zn = {0}
In each of these four cases, the elements of the limiting sets are not elements of any of the sets from the original sequence.
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  • For example, an LTI system that is the cascade connection of several stable systems with an undamped second-order LTI system (i.e., zero damping ratio) will oscillate endlessly after being perturbed (e.g., an ideal bell after being struck). Hence, if the position and velocity of this system are plotted against each other, trajectories will approach a circle in the state space. This circle, which is the Ω limit set of the system, is the outer limit of solution trajectories of the system. The circle represents the locus of a trajectory corresponding to a pure sinusoidal tone output; that is, the system output approaches/approximates a pure tone.

Generalized definitions

The above definitions are inadequate for many technical applications. In fact, the definitions above are specializations of the following definitions.

Definition for a set

The limit inferior of a set XY is the infimum of all of the limit points of the set. That is,

Similarly, the limit superior of a set X is the supremum of all of the limit points of the set. That is,

Note that the set X needs to be defined as a subset of a partially ordered set Y that is also a topological space in order for these definitions to make sense. Moreover, it has to be a complete lattice so that the suprema and infima always exist. In that case every set has a limit superior and a limit inferior. Also note that the limit inferior and the limit superior of a set do not have to be elements of the set.

Definition for filter bases

Take a topological space X and a filter base B in that space. The set of all cluster points for that filter base is given by

where is the closure of . This is clearly a closed set and is similar to the set of limit points of a set. Assume that X is also a partially ordered set. The limit superior of the filter base B is defined as

when that supremum exists. When X has a total order, is a complete lattice and has the order topology,

Proof: Similarly, the limit inferior of the filter base B is defined as

when that infimum exists; if X is totally ordered, is a complete lattice, and has the order topology, then

If the limit inferior and limit superior agree, then there must be exactly one cluster point and the limit of the filter base is equal to this unique cluster point.

Specialization for sequences and nets

Note that filter bases are generalizations of nets, which are generalizations of sequences. Therefore, these definitions give the limit inferior and limit superior of any net (and thus any sequence) as well. For example, take topological space and the net , where is a directed set and for all . The filter base ("of tails") generated by this net is defined by

Therefore, the limit inferior and limit superior of the net are equal to the limit superior and limit inferior of respectively. Similarly, for topological space , take the sequence where for any with being the set of natural numbers. The filter base ("of tails") generated by this sequence is defined by

Therefore, the limit inferior and limit superior of the sequence are equal to the limit superior and limit inferior of respectively.

See also

References

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External links

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    Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

    Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

    A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

    The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

    There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
  3. 20 year-old Real Estate Agent Rusty from Saint-Paul, has hobbies and interests which includes monopoly, property developers in singapore and poker. Will soon undertake a contiki trip that may include going to the Lower Valley of the Omo.

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