Barrelled space

Template:Short description In functional analysis and related areas of mathematics, a barrelled space (also written barreled space) is a topological vector space (TVS) for which every barrelled set in the space is a neighbourhood for the zero vector. A barrelled set or a barrel in a topological vector space is a set that is convex, balanced, absorbing, and closed. Barrelled spaces are studied because a form of the Banach–Steinhaus theorem still holds for them. Barrelled spaces were introduced by Template:Harvs.

Barrels

A convex and balanced subset of a real or complex vector space is called a Template:Em and it is said to be Template:Em, Template:Em, or Template:Em.

A Template:Em or a Template:Em in a topological vector space (TVS) is a subset that is a closed absorbing disk; that is, a barrel is a convex, balanced, closed, and absorbing subset.

Every barrel must contain the origin. If $\dim X\ge 2$ and if $S$ is any subset of $X,$ then $S$ is a convex, balanced, and absorbing set of $X$ if and only if this is all true of $S\cap Y$ in $Y$ for every $2$-dimensional vector subspace $Y;$ thus if $\dim X>2$ then the requirement that a barrel be a closed subset of $X$ is the only defining property that does not depend Template:Em on $2$ (or lower)-dimensional vector subspaces of $X.$

If $X$ is any TVS then every closed convex and balanced neighborhood of the origin is necessarily a barrel in $X$ (because every neighborhood of the origin is necessarily an absorbing subset). In fact, every locally convex topological vector space has a neighborhood basis at its origin consisting entirely of barrels. However, in general, there Template:Em exist barrels that are not neighborhoods of the origin; "barrelled spaces" are exactly those TVSs in which every barrel is necessarily a neighborhood of the origin. Every finite dimensional topological vector space is a barrelled space so examples of barrels that are not neighborhoods of the origin can only be found in infinite dimensional spaces.

Examples of barrels and non-barrels

The closure of any convex, balanced, and absorbing subset is a barrel. This is because the closure of any convex (respectively, any balanced, any absorbing) subset has this same property.

A family of examples: Suppose that $X$ is equal to $ℂ$ (if considered as a complex vector space) or equal to ${ℝ}^2$ (if considered as a real vector space). Regardless of whether $X$ is a real or complex vector space, every barrel in $X$ is necessarily a neighborhood of the origin (so $X$ is an example of a barrelled space). Let $R:[0,2\pi )\to (0,\mathrm{\infty }]$ be any function and for every angle $\theta \in [0,2\pi ),$ let $S_{\theta }$ denote the closed line segment from the origin to the point $R(\theta )e^{i\theta }\in ℂ.$ Let $S:=\bigcup _{\theta \in [0,2\pi )}{S}_{\theta }.$ Then $S$ is always an absorbing subset of ${ℝ}^2$ (a real vector space) but it is an absorbing subset of $ℂ$ (a complex vector space) if and only if it is a neighborhood of the origin. Moreover, $S$ is a balanced subset of ${ℝ}^2$ if and only if $R(\theta )=R(\pi +\theta )$ for every $0\le \theta <\pi$ (if this is the case then $R$ and $S$ are completely determined by $R$'s values on $[0,\pi )$) but $S$ is a balanced subset of $ℂ$ if and only it is an open or closed ball centered at the origin (of radius $0<r\le \mathrm{\infty }$). In particular, barrels in $ℂ$ are exactly those closed balls centered at the origin with radius in $(0,\mathrm{\infty }].$ If $R(\theta ):=2\pi -\theta$ then $S$ is a closed subset that is absorbing in ${ℝ}^2$ but not absorbing in $ℂ,$ and that is neither convex, balanced, nor a neighborhood of the origin in $X.$ By an appropriate choice of the function $R,$ it is also possible to have $S$ be a balanced and absorbing subset of ${ℝ}^2$ that is neither closed nor convex. To have $S$ be a balanced, absorbing, and closed subset of ${ℝ}^2$ that is Template:Em convex nor a neighborhood of the origin, define $R$ on $[0,\pi )$ as follows: for $0\le \theta <\pi ,$ let $R(\theta ):=\pi -\theta$ (alternatively, it can be any positive function on $[0,\pi )$ that is continuously differentiable, which guarantees that $\underset{\theta \searrow 0}{\lim }R(\theta )=R(0)>0$ and that $S$ is closed, and that also satisfies $\underset{\theta \nearrow \pi }{\lim }R(\theta )=0,$ which prevents $S$ from being a neighborhood of the origin) and then extend $R$ to $[\pi ,2\pi )$ by defining $R(\theta ):=R(\theta -\pi ),$ which guarantees that $S$ is balanced in ${ℝ}^2.$

Properties of barrels

• In any topological vector space (TVS) $X,$ every barrel in $X$ absorbs every compact convex subset of $X.$Template:Sfn
• In any locally convex Hausdorff TVS $X,$ every barrel in $X$ absorbs every convex bounded complete subset of $X.$Template:Sfn
• If $X$ is locally convex then a subset $H$ of $X^{\prime }$ is $\sigma (X^{\prime },X)$-bounded if and only if there exists a barrel $B$ in $X$ such that $H\subseteq B^{\circ }.$Template:Sfn
• Let $(X,Y,b)$ be a pairing and let $\nu$ be a locally convex topology on $X$ consistent with duality. Then a subset $B$ of $X$ is a barrel in $(X,\nu )$ if and only if $B$ is the polar of some $\sigma (Y,X,b)$-bounded subset of $Y.$Template:Sfn
• Suppose $M$ is a vector subspace of finite codimension in a locally convex space $X$ and $B\subseteq M.$ If $B$ is a barrel (resp. bornivorous barrel, bornivorous disk) in $M$ then there exists a barrel (resp. bornivorous barrel, bornivorous disk) $C$ in $X$ such that $B=C\cap M.$Template:Sfn

Characterizations of barreled spaces

Denote by $L(X;Y)$ the space of continuous linear maps from $X$ into $Y.$

If $(X,\tau )$ is a Hausdorff topological vector space (TVS) with continuous dual space $X^{\prime }$ then the following are equivalent:

1. $X$ is barrelled.
2. Template:Em: Every barrel in $X$ is a neighborhood of the origin.
   • This definition is similar to a characterization of Baire TVSs proved by Saxon [1974], who proved that a TVS $Y$ with a topology that is not the indiscrete topology is a Baire space if and only if every absorbing balanced subset is a neighborhood of Template:Em point of $Y$ (not necessarily the origin).Template:Sfn
3. For any Hausdorff TVS $Y$ every pointwise bounded subset of $L(X;Y)$ is equicontinuous.Template:Sfn
4. For any F-space $Y$ every pointwise bounded subset of $L(X;Y)$ is equicontinuous.Template:Sfn
   • An F-space is a complete metrizable TVS.
5. Every closed linear operator from $X$ into a complete metrizable TVS is continuous.Template:Sfn
   • A linear map $F:X\to Y$ is called closed if its graph is a closed subset of $X\times Y.$
6. Every Hausdorff TVS topology $\nu$ on $X$ that has a neighborhood basis of the origin consisting of $\tau$-closed set is course than $\tau .$Template:Sfn

If $(X,\tau )$ is locally convex space then this list may be extended by appending:

7. There exists a TVS $Y$ not carrying the indiscrete topology (so in particular, $Y\ne \{0\}$) such that every pointwise bounded subset of $L(X;Y)$ is equicontinuous.Template:Sfn
8. For any locally convex TVS $Y,$ every pointwise bounded subset of $L(X;Y)$ is equicontinuous.Template:Sfn
   • It follows from the above two characterizations that in the class of locally convex TVS, barrelled spaces are exactly those for which the uniform boundedness principle holds.
9. Every $\sigma (X^{\prime },X)$-bounded subset of the continuous dual space $X$ is equicontinuous (this provides a partial converse to the Banach-Steinhaus theorem).Template:Sfn
10. $X$ carries the strong dual topology $\beta (X,X^{\prime }).$Template:Sfn
11. Every lower semicontinuous seminorm on $X$ is continuous.Template:Sfn
12. Every linear map $F:X\to Y$ into a locally convex space $Y$ is almost continuous.Template:Sfn
    • A linear map $F:X\to Y$ is called Template:Em if for every neighborhood $V$ of the origin in $Y,$ the closure of $F^{-1}(V)$ is a neighborhood of the origin in $X.$
13. Every surjective linear map $F:Y\to X$ from a locally convex space $Y$ is almost open.Template:Sfn
    • This means that for every neighborhood $V$ of 0 in $Y,$ the closure of $F(V)$ is a neighborhood of 0 in $X.$
14. If $\omega$ is a locally convex topology on $X$ such that $(X,\omega )$ has a neighborhood basis at the origin consisting of $\tau$-closed sets, then $\omega$ is weaker than $\tau .$Template:Sfn

If $X$ is a Hausdorff locally convex space then this list may be extended by appending:

15. Closed graph theorem: Every closed linear operator $F:X\to Y$ into a Banach space $Y$ is continuous.Template:Sfn
    • The linear operator is called Template:Em if its graph is a closed subset of $X\times Y.$
16. For every subset $A$ of the continuous dual space of $X,$ the following properties are equivalent: $A$ is
    1. equicontinuous;
    2. relatively weakly compact;
    3. strongly bounded;
    4. weakly bounded.
17. The 0-neighborhood bases in $X$ and the fundamental families of bounded sets in $X_{\beta }^{\prime }$ correspond to each other by polarity.

If $X$ is metrizable topological vector space then this list may be extended by appending:

18. For any complete metrizable TVS $Y$ every pointwise bounded Template:Em in $L(X;Y)$ is equicontinuous.Template:Sfn

If $X$ is a locally convex metrizable topological vector space then this list may be extended by appending:

19. (Template:Visible anchor): The weak* topology on $X^{\prime }$ is sequentially complete.Template:Sfn
20. (Template:Visible anchor): Every weak* bounded subset of $X^{\prime }$ is $\sigma (X^{\prime },X)$-relatively countably compact.Template:Sfn
21. (Template:Visible anchor): Every countable weak* bounded subset of $X^{\prime }$ is equicontinuous.Template:Sfn
22. (Template:Visible anchor): $X$ is not the union of an increase sequence of nowhere dense disks.Template:Sfn

Examples and sufficient conditions

Each of the following topological vector spaces is barreled:

1. TVSs that are Baire space.
   • Consequently, every topological vector space that is of the second category in itself is barrelled.
2. F-spaces, Fréchet spaces, Banach spaces, and Hilbert spaces.
   • However, there exist normed vector spaces that are Template:Em barrelled. For example, if the $L^p$-space $L^2([0,1])$ is topologized as a subspace of $L^1([0,1]),$ then it is not barrelled.
3. Complete pseudometrizable TVSs.Template:Sfn
   • Consequently, every finite-dimensional TVS is barrelled.
4. Montel spaces.
5. Strong dual spaces of Montel spaces (since they are necessarily Montel spaces).
6. A locally convex quasi-barrelled space that is also a σ-barrelled space.Template:Sfn
7. A sequentially complete quasibarrelled space.
8. A quasi-complete Hausdorff locally convex infrabarrelled space.Template:Sfn
   • A TVS is called quasi-complete if every closed and bounded subset is complete.
9. A TVS with a dense barrelled vector subspace.Template:Sfn
   • Thus the completion of a barreled space is barrelled.
10. A Hausdorff locally convex TVS with a dense infrabarrelled vector subspace.Template:Sfn
    • Thus the completion of an infrabarrelled Hausdorff locally convex space is barrelled.Template:Sfn
11. A vector subspace of a barrelled space that has countable codimensional.Template:Sfn
    • In particular, a finite codimensional vector subspace of a barrelled space is barreled.
12. A locally convex ultrabarelled TVS.Template:Sfn
13. A Hausdorff locally convex TVS $X$ such that every weakly bounded subset of its continuous dual space is equicontinuous.Template:Sfn
14. A locally convex TVS $X$ such that for every Banach space $B,$ a closed linear map of $X$ into $B$ is necessarily continuous.Template:Sfn
15. A product of a family of barreled spaces.Template:Sfn
16. A locally convex direct sum and the inductive limit of a family of barrelled spaces.Template:Sfn
17. A quotient of a barrelled space.Template:SfnTemplate:Sfn
18. A Hausdorff sequentially complete quasibarrelled boundedly summing TVS.Template:Sfn
19. A locally convex Hausdorff reflexive space is barrelled.

Counter examples

• A barrelled space need not be Montel, complete, metrizable, unordered Baire-like, nor the inductive limit of Banach spaces.
• Not all normed spaces are barrelled. However, they are all infrabarrelled.Template:Sfn
• A closed subspace of a barreled space is not necessarily countably quasi-barreled (and thus not necessarily barrelled).Template:Sfn
• There exists a dense vector subspace of the Fréchet barrelled space ${ℝ}^{\mathbb{N}}$ that is not barrelled.Template:Sfn
• There exist complete locally convex TVSs that are not barrelled.Template:Sfn
• The finest locally convex topology on an infinite-dimensional vector space is a Hausdorff barrelled space that is a meagre subset of itself (and thus not a Baire space).Template:Sfn

Properties of barreled spaces

Banach–Steinhaus generalization

The importance of barrelled spaces is due mainly to the following results.

The Banach-Steinhaus theorem is a corollary of the above result.Template:Sfn When the vector space $Y$ consists of the complex numbers then the following generalization also holds.

Recall that a linear map $F:X\to Y$ is called closed if its graph is a closed subset of $X\times Y.$

Other properties

• Every Hausdorff barrelled space is quasi-barrelled.Template:Sfn
• A linear map from a barrelled space into a locally convex space is almost continuous.
• A linear map from a locally convex space Template:Emto a barrelled space is almost open.
• A separately continuous bilinear map from a product of barrelled spaces into a locally convex space is hypocontinuous.Template:Sfn
• A linear map with a closed graph from a barreled TVS into a $B_r$-complete TVS is necessarily continuous.Template:Sfn

See also

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