Updated Alaoglu's theorem.
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Bokuan Li
2026-05-27 23:53:22 -04:00
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2 changed files with 15 additions and 10 deletions

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@@ -96,13 +96,3 @@
\end{proof}
% TODO: Replace this with a more general version involving polars in the future.
\begin{theorem}[Alaoglu's Theorem]
\label{theorem:alaoglu}
Let $E$ be a normed vector space over $K \in \RC$, then $B^* = \bracsn{\phi \in E^*| \norm{x}_{E^*} \le 1}$ is compact in the weak*-topology.
\end{theorem}
\begin{proof}
For each $x \in E$, $I_x = \bracsn{\dpn{x, \phi}{E}|\phi \in B^*}$ is compact. By \autoref{proposition:operator-space-completeness}, the closure of $B^*$ in $\prod_{x \in E}I_x$ is a subset of $\hom(E; K)$. Since $B^*$ is bounded, $I_x \subset \overline{B_K(0, 1)}$ for all $x \in E$, so the closure of $B^*$ is contained in $B^*$. By \autoref{theorem:tychonoff}, $\prod_{x \in E}I_x$ is compact. Therefore $B^*$ is compact with respect to the weak*-topology.
\end{proof}