Quasibarrelled space

In functional analysis and related areas of mathematics, quasibarrelled spaces are topological vector spaces (TVS) for which every bornivorous barrelled set in the space is a neighbourhood of the origin. Quasibarrelled spaces are studied because they are a weakening of the defining condition of barrelled spaces, for which a form of the Banach–Steinhaus theorem holds. A subset of a topological vector space (TVS) is called bornivorous if it absorbs all bounded subsets of ; that is, if for each bounded subset of there exists some scalar such that A barrelled set or a barrel in a TVS is a set which is convex, balanced, absorbing and closed. A quasibarrelled space is a TVS for which every bornivorous barrelled set in the space is a neighbourhood of the origin. A locally convex Hausdorff quasibarrelled space that is sequentially complete is barrelled. A locally convex Hausdorff quasibarrelled space is a Mackey space, quasi-M-barrelled, and countably quasibarrelled. A locally convex quasibarrelled space that is also a σ-barrelled space is necessarily a barrelled space. A locally convex space is reflexive if and only if it is semireflexive and quasibarrelled. A Hausdorff topological vector space is quasibarrelled if and only if every bounded closed linear operator from into a complete metrizable TVS is continuous. By definition, a linear operator is called closed if its graph is a closed subset of For a locally convex space with continuous dual the following are equivalent: is quasibarrelled. Every bounded lower semi-continuous semi-norm on is continuous. Every -bounded subset of the continuous dual space is equicontinuous. If is a metrizable locally convex TVS then the following are equivalent: The strong dual of is quasibarrelled. The strong dual of is barrelled. The strong dual of is bornological. Every Hausdorff barrelled space and every Hausdorff bornological space is quasibarrelled. Thus, every metrizable TVS is quasibarrelled. Note that there exist quasibarrelled spaces that are neither barrelled nor bornological.