Is the free locally convex space $L(X)$ nuclear?

TL;DR

This paper investigates the nuclearity and related properties of free locally convex spaces $L(X)$, establishing conditions under which they are nuclear, Schwartz, or multi-reflexive, depending on the topological properties of space $X$.

Contribution

It characterizes when $L(X)$ is nuclear, Schwartz, or multi-reflexive, linking these properties to topological features of $X$, and introduces new equivalences and conditions for these properties.

Findings

$L(X)$ is not nuclear if $X$ contains an infinite compact subset.

$L(X)$ is strongly nuclear iff $X$ is countable and discrete for $k$-spaces.

$L(X)$ is multi-reflexive iff $X$ is a $k_ ext{omega}$-space.

Abstract

Given a class $\mathcal P$ of Banach spaces, a locally convex space (LCS) $E$ is called {\em multi-$\mathcal P$} if $E$ can be isomorphically embedded into a product of spaces that belong to $\mathcal P$. We investigate the question whether the free locally convex space $L(X)$ is strongly nuclear, nuclear, Schwartz, multi-Hilbert or multi-reflexive. If $X$ is a Tychonoff space containing an infinite compact subset then, as it follows from the results of \cite{Aus}, $L(X)$ is not nuclear. We prove that for such $X$ the free LCS $L(X)$ has the stronger property of not being multi-Hilbert. We deduce that if $X$ is a $k$-space, then the following properties are equivalent: (1) $L(X)$ is strongly nuclear; (2) $L(X)$ is nuclear; (3) $L(X)$ is multi-Hilbert; (4) $X$ is countable and discrete. On the other hand, we show that $L(X)$ is strongly nuclear for every projectively countable $P$-space (in particular, for every Lindel\"of $P$-space) $X$. We observe that every Schwartz LCS is multi-reflexive. It is known that if $X$ is a $k_\omega$-space, then $L(X)$ is a Schwartz LCS \cite{Chasco}, hence $L(X)$ is multi-reflexive. We show that for any first-countable paracompact (in particular, metrizable) space $X$ the converse is true, so $L(X)$ is multi-reflexive if and only if $X$ is a $k_\omega$-space, equivalently, if $X$ is a locally compact and $\sigma$-compact space. Similarly, we show that for any first-countable paracompact space $X$ the free abelian topological group $A(X)$ is a Schwartz group if and only if $X$ is a locally compact space such that the set $X^{(1)}$ of all non-isolated points of $X$ is $\sigma$-compact.