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{{Calculus |Differential}}
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In [[mathematics]], an '''implicit equation''' is a [[relation (mathematics)|relation]]  of the form ''R''(''x''<sub>1</sub>,..., ''x''<sub>''n''</sub>) = 0, where ''R'' is a [[function (mathematics)|function]] of several variables (often a [[polynomial]]). The set of the values of the variables that satisfy this relation is a curve if ''n'' = 2 and a surface if ''n''=3. The terms '''implicit curve''' and '''implicit surface''' are usual to denote curves and surfaces defined in this way. The implicit equations are the basis of [[algebraic geometry]], whose basic subjects of study are the simultaneous solutions of several implicit equations whose left-hand sides are polynomials. These sets of simultaneous solutions are called [[affine algebraic set]]s.
 
For example, the implicit equation of the [[unit circle]] is <math> x^2 +y^2-1 = 0.</math>
 
An '''implicit function''' is a [[function (mathematics)|function]] that is defined implicitly by an implicit equation, by associating one of the variables (the [[value (mathematics)|value]]) to the others (the [[argument of a function|argument]]s).  
 
For most implicit functions, there is no formula which define them explicitly. Even when such a formula may exist, one must often restrict the domain of definition and the target to have a well defined function. For the example, the implicit equation of the unit circle defines ''y'' as a function of ''x'' only, if -1&nbsp;≤&nbsp;''x''&nbsp;≤&nbsp;1 and one considers only non-negative (or non-positive) values for the values of the function.
 
The [[implicit function theorem]] provides a condition under which a relation defines an implicit function. It states that if the left hand side of the equation  ''R''(''x'', ''y'') = 0 is [[differentiable function|differentiable]] and satisfies some mild condition on its [[partial derivative]]s at some point (''a'', ''b'') such that ''R''(''a'', ''b'') = 0, then it defines a function ''y'' = ''f''(''x'') over some [[interval (mathematics)|interval]] containing ''a''. Geometrically, the graph defined by  ''R''(''x'',''y'') = 0 will overlap [[local property|locally]] with the graph of some equation ''y'' = ''f''(''x'').
 
==Examples==
===Inverse functions===
A common type of implicit function is an [[inverse function]]. If  ''f'' is a function, then the inverse function of ''f'', called ''f''<sup>−1</sup>, is the function giving a solution of the equation
 
:''x'' = ''f''(''y'')
 
for ''y'' in terms of ''x''. This solution is
 
:<math> y = f^{-1}(x).</math>
 
Intuitively, an inverse function is obtained from ''f'' by interchanging the roles of the dependent and independent variables. Stated another way, the inverse function gives the solution for  ''y'' of the equation
 
:<math>R(x,y) = x-f(y) = 0. \, </math>
 
'''Examples.'''
# The [[natural logarithm]] ln(''x'') gives the solution ''y'' = ln(''x'') of the equation ''x'' − ''e''<sup>''y''</sup> = 0 or equivalently of ''x'' = ''e''<sup>''y''</sup>. Here ''f''(''y'') = ''e''<sup>''y''</sup> and ''f''<sup>−1</sup>(''x'') = ln(''x'').
# The [[product log]] is an implicit function giving the solution for ''y'' of the equation ''x'' − ''y'' ''e''<sup>''y''</sup>&nbsp;=&nbsp;0.
 
===Algebraic functions===
{{main|Algebraic function}}
An '''algebraic function''' is a function that satisfies a polynomial equation whose coefficients are themselves polynomials. For example, an algebraic function in one variable x gives a solution for ''y'' of an equation
 
:<math>a_n(x)y^n+a_{n-1}(x)y^{n-1}+\cdots+a_0(x)=0 \, </math>
 
where the coefficients ''a<sub>i</sub>''(''x'') are polynomial functions of ''x''.  Algebraic functions play an important role in [[mathematical analysis]] and [[algebraic geometry]].  A simple example of an algebraic function is given by the unit circle equation:
 
:<math>x^2+y^2-1=0. \, </math>
 
Solving for ''y'' gives an explicit solution:
 
:<math>y=\pm\sqrt{1-x^2}. \, </math>
 
But even without specifying this explicit solution, it is possible to refer to the implicit solution of the unit circle equation.
 
While explicit solutions can be found for equations that are [[quadratic equations|quadratic]], [[cubic equation|cubic]], and [[quartic equation|quartic]] in ''y'', the same is not in general true for [[quintic equation|quintic]] and higher degree equations, such as
 
:<math> y^5 + 2y^4 -7y^3 + 3y^2 -6y - x = 0. \, </math>
 
Nevertheless, one can still refer to the implicit solution ''y'' = ''g''(''x'') involving the multi-valued implicit function ''g''.
 
==Caveats==
Not every equation ''R''(''x'', ''y'') = 0 implies a graph of a single-valued function, the circle equation being one prominent example. Another example is an implicit function given by  ''x'' − ''C''(''y'') = 0 where ''C'' is a [[cubic polynomial]] having a "hump" in its graph. Thus, for an implicit function to be a  ''true'' (single-valued) function it might be necessary to use just part of the graph. An implicit function can sometimes be successfully defined as a true function only after "zooming in" on some part of the  ''x''-axis and "cutting away" some unwanted function branches. Then an equation expressing  ''y'' as an implicit function of the other variable(s) can be written.
 
The defining equation ''R''(''x'', ''y'') = 0 can also have other pathologies. For example, the equation  ''x'' = 0 does not imply a function ''f''(''x'') giving solutions for ''y'' at all; it is a vertical line.  In order to avoid a problem like this, various constraints are frequently imposed on the allowable sorts of equations or on the [[function domain|domain]]. The implicit function theorem provides a uniform way of handling these sorts of pathologies.
 
==Implicit differentiation==
In [[calculus]], a method called '''implicit differentiation''' makes use of the [[chain rule]] to differentiate implicitly defined functions.  
 
As explained in the introduction, ''y'' can be given as a function of ''x'' implicitly rather than explicitly. When we have an equation  ''R''(''x'', ''y'') = 0, we may be able to solve it for ''y'' and then  
differentiate. However, sometimes it is simpler to differentiate  ''R''(''x'', ''y'') with respect to ''x'' and ''y'' and then solve for ''dy''/''dx''.
 
===Examples===
'''1.'''  Consider for example
 
:<math>y + x + 5 = 0 \,</math>
 
This function normally can be manipulated by using [[algebra]] to change this [[equation]] to one expressing ''y'' in terms of an [[function (mathematics)|explicit function]]:  
 
:<math>y = -x - 5 \, ,</math>
 
where the right side is the explicit function whose output value is ''y''. Differentiation then gives ''dy''/''dx'' = −1. Alternatively, one can [[total differentiation|totally differentiate]] the original equation:
 
:<math>\frac{dy}{dx} + \frac{dx}{dx} + \frac{d}{dx}(5) = 0;</math>
:<math>\frac{dy}{dx} + 1 = 0.</math>
 
Solving for ''dy''/''dx'' gives:
 
:<math>\frac{dy}{dx} = -1,</math>
 
the same answer as obtained previously.
 
'''2.'''  An example of an implicit function, for which implicit differentiation might be easier than attempting to use explicit differentiation, is  
 
:<math> x^4 + 2y^2 = 8 \, </math>
 
In order to differentiate this explicitly with respect to ''x'', one would have to obtain (via algebra)
 
:<math>y = f(x) =  \pm\sqrt{\frac{8 - x^4}{2}},</math>
 
and then differentiate this function. This creates two derivatives: one for ''y'' > 0 and another for ''y'' < 0.
 
One might find it substantially easier to implicitly differentiate the original function:
 
:<math>4x^3 + 4y\frac{dy}{dx} = 0,</math>
 
giving,
 
:<math>\frac{dy}{dx} = \frac{-4x^3}{4y} = \frac{-x^3}{y}</math>
 
'''3.'''  Sometimes standard explicit differentiation cannot be used and, in order to obtain the derivative, implicit differentiation must be employed. An example of such a case is the equation ''y''<sup>5</sup> − ''y'' = x. It is impossible to express ''y'' explicitly as a function of ''x'' and therefore ''dy''/''dx'' cannot be found by explicit differentiation. Using the implicit method, ''dy''/''dx'' can be expressed:
 
:<math>5y^4\frac{dy}{dx} - \frac{dy}{dx} = \frac{dx}{dx}</math>
 
where ''dx''/''dx'' = 1. Factoring out ''dy''/''dx'' shows that
 
:<math>\frac{dy}{dx}(5y^4 - 1) = 1</math>
 
which yields the final answer
 
:<math>\frac{dy}{dx}=\frac{1}{5y^{4}-1},</math>
 
which is defined for <math>y \ne \pm\frac{1}{\sqrt[4]{5}}.</math>
 
===Formula for two variables===
"The Implicit Function Theorem states that if ''F'' is defined on an open disk containing (''a'', ''b''), where ''F''(''a'', ''b'') = 0, ''F<sub>y</sub>''(''a'', ''b'') ≠ 0, and ''F<sub>x</sub>'' and ''F<sub>y</sub>'' are continuous on the disk, then the equation ''F''(''x'', ''y'') = 0 defines ''y'' as a function of ''x'' near the point (''a'', ''b'') and the derivative of this function is given by"<ref name="Stewart1998">{{cite book
  | last = Stewart
  | first = James
  | title = Calculus Concepts And Contexts
  | publisher = Brooks/Cole Publishing Company
  | year = 1998
  | isbn = 0-534-34330-9}}</ref>{{rp|§ 11.5}}
 
:<math>\frac{dy}{dx} = -\frac{\partial F / \partial x}{\partial F / \partial y} = -\frac {F_x}{F_y},</math>
 
where ''F<sub>x</sub>'' and ''F<sub>y</sub>'' indicate the derivatives of ''F'' with respect to ''x'' and ''y''.
 
The above formula comes from using the [[Chain_rule#Chain_rule_for_several_variables|generalized chain rule]] to obtain the [[total derivative]]—with respect to ''x''—of both sides of ''F''(''x'', ''y'') = 0:
 
:<math>\frac{\partial F}{\partial x} \frac{dx}{dx} + \frac{\partial F}{\partial y} \frac{dy}{dx} = 0,</math>
 
and hence
 
<math>\frac{\partial F}{\partial x} + \frac{\partial F}{\partial y} \frac{dy}{dx} =0.</math>
 
==Implicit function theorem==
{{main|Implicit function theorem}}
It can be shown that if ''R''(''x'', ''y'') is given by a [[smooth manifold|smooth]] [[submanifold]] ''M'' in '''R'''<sup>2</sup>, and (''a'', ''b'') is a point of this submanifold such that the [[tangent space]] there is not vertical 
 
(that is, <math>\frac{\partial R}{\partial y}\ne0</math>), then ''M'' in some small enough [[neighbourhood (mathematics)|neighbourhood]] of (''a'', ''b'') is given by a [[parametrization]] (''x'', ''f''(''x'')) where ''f'' is a [[smooth function]]. In less technical language, implicit functions exist and can be differentiated, unless the tangent to the supposed graph would be vertical. In the standard case where we are given an equation
 
<math>R(x,y) = 0</math>
 
the condition on ''R'' can be checked by means of [[partial derivative]]s.<ref name="Stewart1998"/>{{rp|§ 11.5}}
 
==Applications in economics==
===Marginal rate of substitution===
{{main|Marginal rate of substitution}}
 
In [[economics]], when the level set ''R''(''x'', ''y'') = 0 is an [[indifference curve]] for the quantities ''x'' and ''y'' consumed of two goods, the absolute value of the implicit derivative is interpreted as the [[marginal rate of substitution]] of the two goods: how much more of ''y'' one must receive in order to be indifferent to a loss of 1 unit of&nbsp;''x''.
 
==See also==
*[[Functional equation]]
*[[Level set]]
**[[Isocontour]]
**[[Isosurface]]
*[[Marginal rate of substitution]]
*[[Implicit function theorem]]
*[[Logarithmic differentiation]]
*[[Iteration]] (Iterative solutions for implicit functions)
 
== References ==
<references/>
*{{cite book
| last=Rudin
| first=Walter
| authorlink=Walter Rudin
| title=Principles of Mathematical Analysis
| publisher=[[McGraw-Hill]]
| year=1976
| isbn=0-07-054235-X}}
*{{cite book
| last=Spivak
| first=Michael
| authorlink=Michael Spivak
| title=Calculus on Manifolds
| publisher=[[HarperCollins]]
| year=1965
| isbn=0-8053-9021-9}}
*{{cite book
| last=Warner
| first=Frank
| title=Foundations of Differentiable Manifolds and Lie Groups
| publisher=[[Springer Science+Business Media|Springer]]
| year=1983
| isbn=0-387-90894-3}}
 
[[Category:Differential calculus]]
[[Category:Theorems in analysis]]
[[Category:Multivariable calculus]]
[[Category:Differential topology]]
[[Category:Algebraic geometry]]

Revision as of 14:09, 8 February 2014

Making a computer run fast is actually pretty simple. Most computers run slow because they are jammed up with junk files, that Windows has to search through every time it wants to locate anything. Imagine having to find a book in a library, but all library books are in a big huge pile. That's what it's like for your computer to find something, whenever a system is full of junk files.

Another answer is to provide the computer program with a unique msvcr71 file. Frequently, once the file has been corrupted or damaged, it might no longer be capable to function like it did before thus it's only all-natural to substitute the file. Just download another msvcr71.dll file within the internet. Often, the file usually come inside a zip structure. Extract the files within the zip folder and destination them accordingly inside this location: C:\Windows\System32. Afterward, re-register the file. Click Start and then choose Run. When the Run window appears, type "cmd". Press Enter plus then type "regsvr32 -u msvcr71.dll" followed by "regsvr32 msvcr71.dll". Press Enter again plus the file ought to be registered accordingly.

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