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.
A TVS X with continuous dual space is said to be countably barrelled if is a weak-* bounded subset of that is equal to a countable union of equicontinuous subsets of , then 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]
A TVS with continuous dual space is said to be σ-barrelled if every weak-* bounded (countable) sequence in is equicontinuous. [1]
A TVS with continuous dual space is said to be sequentially barrelled if every weak-* convergent sequence in is equicontinuous. [1]
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]
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]
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]