Distinguished space

Suppose that X is a locally convex space and let ${\displaystyle X^{\prime }}$ and ${\displaystyle X_{b}^{\prime }}$ denote the strong dual of X (i.e. the continuous dual space of X endowed with the strong dual topology). Let ${\displaystyle X^{\prime \prime }}$ denote the continuous dual space of ${\displaystyle X_{b}^{\prime }}$ and let ${\displaystyle X_{b}^{\prime \prime }}$ denote the strong dual of ${\displaystyle X_{b}^{\prime }.}$ Let ${\displaystyle X_{\sigma }^{\prime \prime }}$ denote ${\displaystyle X^{\prime \prime }}$ endowed with the weak-* topology induced by ${\displaystyle X^{\prime },}$ where this topology is denoted by ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$ (that is, the topology of pointwise convergence on ${\displaystyle X^{\prime }}$). We say that a subset W of ${\displaystyle X^{\prime \prime }}$ is ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$-bounded if it is a bounded subset of ${\displaystyle X_{\sigma }^{\prime \prime }}$ and we call the closure of W in the TVS ${\displaystyle X_{\sigma }^{\prime \prime }}$ the ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$-closure of W. If B is a subset of X then the polar of B is ${\displaystyle B^{\circ }:=\left\{x^{\prime }\in X^{\prime }:\sup _{b\in B}\left\langle b,x^{\prime }\right\rangle \leq 1\right\}.}$

A Hausdorff locally convex TVS X is called a distinguished space if it satisfies any of the following equivalent conditions:

1. If W ⊆ ${\displaystyle X^{\prime \prime }}$ is a ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$-bounded subset of ${\displaystyle X^{\prime \prime }}$ then there exists a bounded subset B of ${\displaystyle X_{b}^{\prime \prime }}$ whose ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$-closure contains W.[1]
2. If W ⊆ ${\displaystyle X^{\prime \prime }}$ is a ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$-bounded subset of ${\displaystyle X^{\prime \prime }}$ then there exists a bounded subset B of X such that W is contained in ${\displaystyle B^{\circ \circ }:=\left\{x^{\prime \prime }\in X^{\prime \prime }:\sup _{x^{\prime }\in B^{\circ }}\left\langle x^{\prime },x^{\prime \prime }\right\rangle \leq 1\right\},}$ which is the polar (relative to the duality ${\displaystyle \left\langle X^{\prime },X^{\prime \prime }\right\rangle }$) of ${\displaystyle B^{\circ }.}$[1]
3. The strong dual of X is a barrelled space.[1]

If in addition X is a metrizable locally convex topological vector space then this list may be extended to include:

4. (Grothendieck) The strong dual of X is a bornological space.[1]

Normed spaces and semi-reflexive spaces is a distinguished spaces.[2] LF spaces are distinguished spaces.

The strong dual space of a Fréchet space is distinguished if and only if it is quasibarrelled.[3]

Every locally convex distinguished space is an H-space.[2]

There exist distinguished Banach spaces spaces that are not semi-reflexive.[1] The strong dual of a distinguished Banach space is not necessarily separable; ${\displaystyle l^{1}}$ is such a space.[4] The strong dual of a distinguished Fréchet space is not necessarily metrizable.[1] There exists a distinguished semi-reflexive non-reflexive non-quasibarrelled Mackey space X whose strong dual is a non-reflexive Banach space.[1] There exist H-spaces that are not distinguished spaces.[1]

• Montel space – A barrelled topological vector space in which every closed and bounded subset is compact.

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• Husain, Taqdir; Khaleelulla, S. M. (1978). Barrelledness in Topological and Ordered Vector Spaces. Lecture Notes in Mathematics. 692. Berlin, New York, Heidelberg: Springer-Verlag. ISBN 978-3-540-09096-0. OCLC 4493665.
• Jarchow, Hans (1981). Locally convex spaces. Stuttgart: B.G. Teubner. ISBN 978-3-519-02224-4. OCLC 8210342.
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• Schaefer, Helmut H.; Wolff, Manfred P. (1999). Topological Vector Spaces. GTM. 8 (Second ed.). New York, NY: Springer New York Imprint Springer. ISBN 978-1-4612-7155-0. OCLC 840278135.
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