Order convergence

In mathematics, specifically in order theory and functional analysis, a filter $ℱ$ in an order complete vector lattice $X$ is order convergent if it contains an order bounded subset (that is, a subset contained in an interval of the form $[a,b]:=\{x\in X:a\le x\text{ and }x\le b\}$) and if $$\sup \{\inf S:S\in \mathrm{OBound}(X)\cap ℱ\}=\inf \{\sup S:S\in \mathrm{OBound}(X)\cap ℱ\},$$ where $\mathrm{OBound}(X)$ is the set of all order bounded subsets of X, in which case this common value is called the order limit of $ℱ$ in $X.$

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

Definition

A net ${\left(x_{\alpha }\right)}_{\alpha \in A}$ in a vector lattice $X$ is said to decrease to $x_0\in X$ if $\alpha \le \beta$ implies $x_{\beta }\le x_{\alpha }$ and $x_0=inf\{x_{\alpha }:\alpha \in A\}$ in $X.$ A net ${\left(x_{\alpha }\right)}_{\alpha \in A}$ in a vector lattice $X$ is said to order-converge to $x_0\in X$ if there is a net ${\left(y_{\alpha }\right)}_{\alpha \in A}$ in $X$ that decreases to $0$ and satisfies $|x_{\alpha }-x_0|\le y_{\alpha }$ for all $\alpha \in A$.

Order continuity

A linear map $T:X\to Y$ between vector lattices is said to be order continuous if whenever ${\left(x_{\alpha }\right)}_{\alpha \in A}$ is a net in $X$ that order-converges to $x_0$ in $X,$ then the net ${\left(T\left(x_{\alpha }\right)\right)}_{\alpha \in A}$ order-converges to $T\left(x_0\right)$ in $Y.$ $T$ is said to be sequentially order continuous if whenever ${\left(x_n\right)}_{n\in \mathbb{N}}$ is a sequence in $X$ that order-converges to $x_0$ in $X,$then the sequence ${\left(T\left(x_n\right)\right)}_{n\in \mathbb{N}}$ order-converges to $T\left(x_0\right)$ in $Y.$

Related results

In an order complete vector lattice $X$ whose order is regular, $X$ is of minimal type if and only if every order convergent filter in $X$ converges when $X$ is endowed with the order topology.

See also

• Banach lattice – Banach space with a compatible structure of a lattice
• Fréchet lattice – Topological vector lattice
• Vector lattice – Partially ordered vector space, ordered as a latticePages displaying short descriptions of redirect targets