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| {{redirect|Vector lattice|the concept of <u>lattice vector</u>|Bravais lattice}}
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| In [[mathematics]], a '''Riesz space''', '''lattice-ordered vector space''' or '''vector lattice''' is a [[ordered vector space|partially ordered vector space]] where the [[order structure]] is a [[lattice (order)|lattice]].
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| Riesz spaces are named after [[Frigyes Riesz]] who first defined them in his 1928 paper ''Sur la décomposition des opérations fonctionelles linéaires''.
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| Riesz spaces have wide ranging applications. They are important in [[measure theory]], in that important results are special cases of results for Riesz Spaces. E.g. the [[Radon–Nikodym theorem]] follows as a special case of the [[Freudenthal spectral theorem]]. Riesz spaces have also seen application in [[Mathematical economics]] through the work of Greek-American economist and mathematician [[Charalambos D. Aliprantis]].
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| == Definition ==
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| A Riesz space ''E'' is defined to be a vector space endowed with a [[partial order]] "<math>\leq</math>" that, for any <math>x,y,z\in E</math>, satisfies:
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| # <math>x\leq y</math> implies <math>x + z\leq y + z</math> ([[translation invariance]]).
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| # For any scalar <math>\alpha \geq 0</math>, <math>x\leq y</math> implies <math>\alpha x \leq \alpha y </math> ([[Homogeneous_function#Positive_homogeneity|positive homogeneity]]).
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| # For any pair of vectors <math>x,y\in E</math> there exists a [[supremum]] (denoted <math>x\vee y</math>) in ''E'' with respect to the partial order "<math>\leq</math>" ([[Lattice (order)|lattice structure]])
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| == Basic properties ==
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| Every Riesz space is a [[ordered vector space|partially ordered vector space]], but not every partially ordered vector space is a Riesz space.
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| Every element ''f'' in a Riesz space, ''E'', has unique positive and negative parts, written <math>f^+:=f\vee 0</math> and <math>f^-:=(-f)\vee 0</math>. Then it can be shown that, <math>f=f^+-f^-</math> and an absolute value can be defined by <math>|f|:=f^++f^-</math>. Every Riesz space is a [[distributive lattice]] and has the [[Approximately finite dimensional C*-algebra#The Effros-Handelman-Shen theorem|Riesz decomposition property]].
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| == Order convergence ==
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| There are a number of meaningful non-equivalent ways to define convergence of sequences or nets with respect to the order structure of a Riesz space. A sequence <math>\{x_n\}</math> in a Riesz space ''E'' is said to '''converge monotonely''' if it is a [[monotone sequence|monotone]] decreasing (increasing) sequence and its [[infimum]] (supremum) ''x'' exists in ''E'' and denoted <math>x_n\downarrow x</math> (<math>x_n\uparrow x</math>).
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| A sequence <math>\{x_n\}</math> in a Riesz space ''E'' is said to '''converge in order''' to ''x'' if there exists a monotone converging sequence <math>\{p_n\}</math> in ''E'' such that <math>|x_n-x|\leq p_n\downarrow 0</math>.
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| If ''u'' is a positive element a Riesz space ''E'' then a sequence <math>\{x_n\}</math> in ''E'' is said to '''converge u-uniformly''' to ''x'' for any <math>\varepsilon >0</math> there exists an ''N'' such that <math>|x_n-x|<\varepsilon u</math> for all ''n>N''.
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| == Subspaces ==
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| Being vector spaces, it is also interesting to consider subspaces of Riesz spaces. The extra structure provided by these spaces provide for distinct kinds of Riesz subspaces. The collection of each kind structure in a Riesz space (e.g. the collection of all Ideals) forms a [[distributive lattice]].
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| === Ideals ===
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| A vector subspace ''I'' of a Riesz space ''E'' is called an ''ideal'' if it is ''solid'', meaning if for any element ''f'' in ''I'' and any ''g'' in ''E'', ''|g| ≤ |f|'' implies that ''g'' is actually in ''I''. The intersection of an arbitrary collection of ideals is again an ideal, which allows for the definition of a smallest ideal containing some non-empty subset ''A'' of ''E'', and is called the ideal ''generated'' by ''A''. An Ideal generated by a singleton is called a '''principal ideal'''.
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| === Bands and <math>\sigma</math>-Ideals ===
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| A ''band'' ''B'' in a Riesz space ''E'' is defined to be an ideal with the extra property, that for any element ''f'' in ''E'' for which its absolute value ''|f|'' is the supremum of an arbitrary subset of positive elements in ''B'', that ''f'' is actually in ''B''. ''<math>\sigma</math>-Ideals'' are defined similarly, with the words 'arbitrary subset' replaced with 'countable subset'. Clearly every band is a <math>\sigma</math>-ideal, but the converse is not true in general.
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| As with ideals, for every non-empty subset ''A'' of ''E'', there exists a smallest band containing that subset, called ''the band generated by A''. A band generated by a singleton is called a '''principal band'''.
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| === Disjoint complements ===
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| Two elements ''f,g'' in a Riesz space ''E'', are said to be '''disjoint''', written <math>f\bot g</math>, when <math>|f|\wedge |g|=0</math>. For any subset ''A'' of ''E'', its disjoint complement <math>A^d</math> is defined as the set of all elements in ''E'', that are disjoint to all elements in ''A''. Disjoint complements are always bands, but the converse is not true in general.
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| === Projection bands ===
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| A band ''B'' in a Riesz space, is called a ''projection band'', if <math>E=B\oplus B^d</math>, meaning every element ''f'' in ''E'', can be written uniquely as a sum of two elements, <math>f=u+v</math>, with <math>u\in B</math> and <math>v\in B^d</math>. There then also exists a positive linear idempotent, or ''projection'', <math>P_B:E\to E</math>, such that <math>P_B(f)=u</math>.
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| The collection of all projection bands in a Riesz space forms a [[Boolean algebra]]. Some spaces do not have non-trivial projection bands (e.g. <math>C([0,1])</math>), so this Boolean algebra may be trivial.
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| == Projection properties ==
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| There are numerous projection properties that Riesz spaces may have. A Riesz space is said to have the (principal) projection property if every (principal) band is a projection band.
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| The so-called '''main inclusion theorem''' relates these properties. Super [[Dedekind complete]]ness implies Dedekind completeness; Dedekind completeness implies both Dedekind <math>\sigma</math>-completeness and the projection property; Both Dedekind <math>\sigma</math>-completeness and the projection property separately imply the principal projection property; and the principal projection property implies the [[Archimedean property]].
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| None of the reverse implications hold, but Dedekind <math>\sigma</math>-completeness and the projection property together imply Dedekind completeness.
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| == Examples ==
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| * The space of continuous real valued functions with [[compact support]] on a topological space ''X'' with the [[pointwise]] [[partial order]] defined by ''f'' ≤ ''g'' when ''f(x)'' ≤ ''g(x)'' for all ''x'' in ''X'', is a Riesz space. It is Archimedean, but usually does not have the principal projection property unless ''X'' satisfies further conditions (e.g. being [[extremally disconnected]]).
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| * Any [[Lp space]] with the ([[almost everywhere]]) pointwise partial order is a Dedekind complete Riesz space.
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| * The space <math>\mathbb{R}^2</math> with the [[lexicographical order]] is a non-Archimedean Riesz space.
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| == Properties ==
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| * Riesz spaces are [[lattice ordered group]]s
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| * Every Riesz space is a [[distributive lattice]]
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| == See also ==
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| * [[Partially ordered space]]
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| * [[Ordered vector space]]
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| * [[Convex cone]]
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| == References ==
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| * [[Bourbaki, Nicolas]]; <cite>Elements of Mathematics: Integration. Chapters 1–6</cite>; ISBN 3-540-41129-1
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| * Riesz, Frigyes; <cite>Sur la décomposition des opérations fonctionelles linéaires</cite>, Atti congress. internaz. mathematici (Bologna, 1928), 3, Zanichelli (1930) pp. 143–148
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| * {{citation
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| | last=Sobolev
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| | first=V. I.
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| | author-link=
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| | contribution=Riesz space
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| | title=[[Encyclopaedia of Mathematics|Encyclopædia of Mathematics]]
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| | publisher=[[Springer Verlag|Springer]]
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| | year=2001
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| | isbn=978-1-4020-0609-8
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| | url=http://eom.springer.de/R/r082290.htm
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| }}
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| * {{Citation
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| | last = Zaanen | first = Adriaan C.
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| | year = 1996
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| | title = Introduction to Operator Theory in Riesz spaces
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| | publisher = [[Springer Verlag|Springer]]
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| | isbn = 3-540-61989-5
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| }}
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| {{Functional Analysis}}
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| [[Category:Functional analysis]]
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| [[Category:Ordered groups]]
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