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In functional analysis and related areas of mathematics, distinguished spaces are topological vector spaces (TVSs) having the property that weak-* bounded subsets of their biduals (that is, the strong dual space of their strong dual space) are contained in the weak-* closure of some bounded subset of the bidual.

Suppose that ${\displaystyle X}$ is a locally convex space and let ${\displaystyle X^{\prime }}$ and ${\displaystyle X_{b}^{\prime }}$ denote the strong dual of ${\displaystyle X}$ (that is, the continuous dual space of ${\displaystyle X}$ 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 ${\displaystyle W}$ 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 ${\displaystyle W}$ in the TVS ${\displaystyle X_{\sigma }^{\prime \prime }}$ the ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$-closure of ${\displaystyle W}$. If ${\displaystyle B}$ is a subset of ${\displaystyle X}$ then the polar of ${\displaystyle B}$ 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 space ${\displaystyle X}$ is called a distinguished space if it satisfies any of the following equivalent conditions:

1. If ${\displaystyle W\subseteq X^{\prime \prime }}$ is a ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$-bounded subset of ${\displaystyle X^{\prime \prime }}$ then there exists a bounded subset ${\displaystyle B}$ of ${\displaystyle X_{b}^{\prime \prime }}$ whose ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$-closure contains ${\displaystyle W}$. ^[1]
2. If ${\displaystyle W\subseteq X^{\prime \prime }}$ is a ${\displaystyle \sigma \left(X^{\prime \prime },X^{\prime }\right)}$-bounded subset of ${\displaystyle X^{\prime \prime }}$ then there exists a bounded subset ${\displaystyle B}$ of ${\displaystyle X}$ such that ${\displaystyle W}$ 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 ${\displaystyle X}$ is a barrelled space. ^[1]

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

4. ( Grothendieck) The strong dual of ${\displaystyle X}$ is a bornological space. ^[1]

All normed spaces and semi-reflexive spaces are distinguished spaces. ^[2] LF spaces are distinguished spaces.

The strong dual space ${\displaystyle X_{b}^{\prime }}$ of a Fréchet space ${\displaystyle X}$ is distinguished if and only if ${\displaystyle X}$ 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 space of a distinguished Fréchet space is not necessarily metrizable. ^[1] There exists a distinguished semi-reflexive non- reflexive non- quasibarrelled Mackey space ${\displaystyle X}$ whose strong dual is a non-reflexive Banach space. ^[1] There exist H-spaces that are not distinguished spaces. ^[1]

Fréchet Montel spaces are distinguished spaces.

• Montel space – Barrelled space where closed and bounded subsets are compact
• Semi-reflexive space

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