$\omega^\omega$-Base and infinite-dimensional compact sets in locally convex spaces

Abstract

A locally convex space (lcs) $E$ is said to have an $\omega^{\omega}$-base if $E$ has a neighborhood base $\{U_{\alpha}:\alpha\in\omega^\omega\}$ at zero such that $U_{\beta}\subseteq U_{\alpha}$ for all $\alpha\leq\beta$. The class of lcs with an $\omega^{\omega}$-base is large, among others contains all $(LM)$-spaces (hence $(LF)$-spaces), strong duals of distinguished Fr\'echet lcs (hence spaces of distributions $D'(\Omega)$). A remarkable result of Cascales-Orihuela states that every compact set in a lcs with an $\omega^{\omega}$-base is metrizable. Our main result shows that every uncountable-dimensional lcs with an $\omega^{\omega}$-base contains an infinite-dimensional metrizable compact subset. On the other hand, the countable-dimensional space $\varphi$ endowed with the finest locally convex topology has an $\omega^\omega$-base but contains no infinite-dimensional compact subsets. It turns out that $\varphi$ is a unique infinite-dimensional locally convex space which is a $k_{\mathbb{R}}$-space containing no infinite-dimensional compact subsets. Applications to spaces $C_{p}(X)$ are provided.