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Order convergence

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In mathematics, specifically inorder theory andfunctional analysis, afilterF{\displaystyle {\mathcal {F}}} in anorder completevector latticeX{\displaystyle X} isorder convergent if it contains anorder bounded subset (that is, a subset contained in an interval of the form[a,b]:={xX:ax and xb}{\displaystyle [a,b]:=\{x\in X:a\leq x{\text{ and }}x\leq b\}}) and ifsup{infS:SOBound(X)F}=inf{supS:SOBound(X)F},{\displaystyle \sup \left\{\inf S:S\in \operatorname {OBound} (X)\cap {\mathcal {F}}\right\}=\inf \left\{\sup S:S\in \operatorname {OBound} (X)\cap {\mathcal {F}}\right\},} whereOBound(X){\displaystyle \operatorname {OBound} (X)} is the set of all order bounded subsets ofX, in which case this common value is called theorder limit ofF{\displaystyle {\mathcal {F}}} inX.{\displaystyle X.}[1]

Order convergence plays an important role in the theory ofvector lattices because the definition of order convergence does not depend on any topology.

Definition

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A net(xα)αA{\displaystyle \left(x_{\alpha }\right)_{\alpha \in A}} in avector latticeX{\displaystyle X} is said todecrease tox0X{\displaystyle x_{0}\in X} ifαβ{\displaystyle \alpha \leq \beta } impliesxβxα{\displaystyle x_{\beta }\leq x_{\alpha }} andx0=inf{xα:αA}{\displaystyle x_{0}=inf\left\{x_{\alpha }:\alpha \in A\right\}} inX.{\displaystyle X.} A net(xα)αA{\displaystyle \left(x_{\alpha }\right)_{\alpha \in A}} in a vector latticeX{\displaystyle X} is said toorder-converge tox0X{\displaystyle x_{0}\in X} if there is a net(yα)αA{\displaystyle \left(y_{\alpha }\right)_{\alpha \in A}} inX{\displaystyle X} that decreases to0{\displaystyle 0} and satisfies|xαx0|yα{\displaystyle \left|x_{\alpha }-x_{0}\right|\leq y_{\alpha }} for allαA{\displaystyle \alpha \in A}.[2]

Order continuity

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A linear mapT:XY{\displaystyle T:X\to Y} between vector lattices is said to beorder continuous if whenever(xα)αA{\displaystyle \left(x_{\alpha }\right)_{\alpha \in A}} is a net inX{\displaystyle X} that order-converges tox0{\displaystyle x_{0}} inX,{\displaystyle X,} then the net(T(xα))αA{\displaystyle \left(T\left(x_{\alpha }\right)\right)_{\alpha \in A}} order-converges toT(x0){\displaystyle T\left(x_{0}\right)} inY.{\displaystyle Y.}T{\displaystyle T} is said to be sequentially order continuous if whenever(xn)nN{\displaystyle \left(x_{n}\right)_{n\in \mathbb {N} }} is a sequence inX{\displaystyle X} that order-converges tox0{\displaystyle x_{0}} inX,{\displaystyle X,}then the sequence(T(xn))nN{\displaystyle \left(T\left(x_{n}\right)\right)_{n\in \mathbb {N} }} order-converges toT(x0){\displaystyle T\left(x_{0}\right)} inY.{\displaystyle Y.}[2]

Related results

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In anorder completevector latticeX{\displaystyle X} whose order isregular,X{\displaystyle X} is ofminimal type if and only if every order convergent filter inX{\displaystyle X} converges whenX{\displaystyle X} is endowed with theorder topology.[1]

See also

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References

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  1. ^abSchaefer & Wolff 1999, pp. 234–242.
  2. ^abKhaleelulla 1982, p. 8.
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