Review of Precompact Sets and Compact Sets

# Review of Precompact Sets and Compact Sets

Let $E$ be a locally convex topological vector space and let $A \subseteq E$.

• Recall from the Precompact Sets in a LCTVS page that if $U$ is a neighbourhood of the origin then $A$ is said to be Small of Order $U$ if for all $x, y \in A$ we have that $x - y \in U$.
• We said that $A$ is Precompact if for every absolutely convex neighbourhood $U$ of the origin there exists a finite collection $A_1$, $A_2$, …, $A_n$ of sets that are all small of order $U$ and such that:
(1)
\begin{align} \quad A \subseteq \bigcup_{i=1}^{n} A_i \end{align}
• We summarize some equivalent criterion for precompactness of a set $A$ in a locally convex topological vector space.
 Equivalent Criteria for Precompactness (1) $A$ is precompact. (2) For every absolutely convex neighbourhood $U$ of the origin there exists $a_1, a_2, ..., a_n \in E$ such that $\displaystyle{A \subseteq \bigcup_{i=1}^{n} (a_i + U)}$.
 Properties of Precompact Sets (1) If $A$ is precompact then $\overline{A}$ is precompact. (2) If $A$ is precompact then for all $\lambda \in \mathbf{F}$, $\lambda A$ is precompact. (3) If $A$ is precompact and $B \subseteq A$ then $B$ is precompact. (4) Union of finite collections of precompact sets are precompact. (5) Intersections of arbitrary collections of precompact sets are precompact. (6) Sums of finite collections of precompact sets are precompact.

If $E$ is a locally convex topological vector space then every precompact set is a bounded set.

If $E$ is a Hausdorff locally convex topological vector space and if $E$ has a precompact neighbourhood of the origin then $E$ is finite-dimensional.

 Results on Precompactness (1) If $(E, F)$ is a dual pair and $A$ is $\sigma(E, F)$-weakly bounded then $A$ is weakly precompact. (2) If $A$ is precompact then $\mathrm{conv}(A)$ is precompact. (3) If $A$ is precompact then $\mathrm{abs \: conv}(A)$ is precompact. (4) If $A$ is precompact then $\overline{\mathrm{abs \: conv}(A)}$ is precompact.
• On the Compact Sets in a Topological Space page we defined $A$ to be Compact if for every open cover of $A$ there is a finite subcollection of the cover that still covers $A$.
 Properties of Compactness (1) Unions of finite collections of compact sets are compact. (2) Closed subsets of compact sets are compact. (3) If $E$ is Hausdorff and $K$ is compact then $K$ is closed. (4) Continuous images of compact sets are compact. (5) If $K$ is compact then $\lambda K$ is compact for each $\lambda \in \mathbf{F}$. (6) Finite sums of compact sets are compact. (7) If $K$ is compact and $C$ is closed then $K + C$ is closed.