In functional analysis, a topological vector space (TVS) is said to be countably barrelled if every weakly bounded countable union of equicontinuous subsets of its continuous dual space is again equicontinuous. This property is a generalization of barrelled spaces.

Definition

A TVS X with continuous dual space ${\displaystyle X^{\prime ))$ is said to be countably barrelled if ${\displaystyle B^{\prime }\subseteq X^{\prime ))$ is a weak-* bounded subset of ${\displaystyle X^{\prime ))$ that is equal to a countable union of equicontinuous subsets of ${\displaystyle X^{\prime ))$, then ${\displaystyle B^{\prime ))$ is itself equicontinuous.^[1] A Hausdorff locally convex TVS is countably barrelled if and only if each barrel in X that is equal to the countable intersection of closed convex balanced neighborhoods of 0 is itself a neighborhood of 0.^[1]

σ-barrelled space

A TVS with continuous dual space ${\displaystyle X^{\prime ))$ is said to be σ-barrelled if every weak-* bounded (countable) sequence in ${\displaystyle X^{\prime ))$ is equicontinuous.^[1]

Sequentially barrelled space

A TVS with continuous dual space ${\displaystyle X^{\prime ))$ is said to be sequentially barrelled if every weak-* convergent sequence in ${\displaystyle X^{\prime ))$ is equicontinuous.^[1]

Properties

Every countably barrelled space is a countably quasibarrelled space, a σ-barrelled space, a σ-quasi-barrelled space, and a sequentially barrelled space.^[1] An H-space is a TVS whose strong dual space is countably barrelled.^[1]

Every countably barrelled space is a σ-barrelled space and every σ-barrelled space is sequentially barrelled.^[1] Every σ-barrelled space is a σ-quasi-barrelled space.^[1]

A locally convex quasi-barrelled space that is also a 𝜎-barrelled space is a barrelled space.^[1]

Examples and sufficient conditions

Every barrelled space is countably barrelled.^[1] However, there exist semi-reflexive countably barrelled spaces that are not barrelled.^[1] The strong dual of a distinguished space and of a metrizable locally convex space is countably barrelled.^[1]

Counter-examples

There exist σ-barrelled spaces that are not countably barrelled.^[1] There exist normed DF-spaces that are not countably barrelled.^[1] There exists a quasi-barrelled space that is not a 𝜎-barrelled space.^[1] There exist σ-barrelled spaces that are not Mackey spaces.^[1] There exist σ-barrelled spaces that are not countably quasi-barrelled spaces and thus not countably barrelled.^[1] There exist sequentially barrelled spaces that are not σ-quasi-barrelled.^[1] There exist quasi-complete locally convex TVSs that are not sequentially barrelled.^[1]