diff --git a/src/op/vn/index.tex b/src/op/vn/index.tex new file mode 100644 index 0000000..d36330f --- /dev/null +++ b/src/op/vn/index.tex @@ -0,0 +1,4 @@ +\chapter{Von Neumann Algebras} +\label{chap:von-neumann-algebras} + +\input{./topologies.tex} diff --git a/src/op/vn/topologies.tex b/src/op/vn/topologies.tex new file mode 100644 index 0000000..d1aa930 --- /dev/null +++ b/src/op/vn/topologies.tex @@ -0,0 +1,52 @@ +\section{Topologies on $B(H)$} +\label{section:topologies-on-bh} + +Let $H$ be a complex Hilbert space. Thanks to its self-duality, there is a natural dual pairing +\[ +B(H) \times (H \otimes H) \quad \dpn{T, \phi \otimes x}{B(H)} = \dpn{Tx, \phi}{H} +\] + +Depending on the topology placed on $H \otimes H$, and the corresponding completion, a handful of different topologies arise on $B(H)$. In fact, the above duality produces a predual for $B(H)$, being the trace class operators: + +\begin{definition}[Ultraweak Topology] +\label{definition:bh-ultraweak-topology} + Let $H$ be a complex Hilbert space, then the dual of $H \wh \otimes_\pi H$ is $B(H)$, and the $\sigma(B(H), H \wh \otimes_\pi H)$-topology is the \textbf{ultraweak}/\textbf{$\sigma$-weak} topology on $B(H)$. +\end{definition} +\begin{proof} + By \autoref{proposition:projective-tensor-product-dual}. +\end{proof} + +Seeing that $B(H)$ is a dual Banach space, the following fact is immediate: + +\begin{proposition} +\label{proposition:bh-ultraweak-bounded} + Let $H$ be a complex Hilbert space, then every bounded subset of $B(H)$ is relatively compact with respect to the ultraweak topology. +\end{proposition} +\begin{proof} + By the \hyperref[Banach-Alaoglu Theorem]{theorem:alaoglu}. +\end{proof} + +Now, a few facts about the more familiar operator topologies: + +\begin{proposition} +\label{proposition:bh-operator-topologies-facts} + Let $H$ be a complex Hilbert space, then: + \begin{enumerate} + \item The dual of $B(H)$ with respect to its strong and weak operator topologies is $H \otimes H$. + \item Every bounded subset of $B(H)$ is relatively compact in the weak operator topology. + \item The composition map $(S, T) \mapsto ST$ is separately continuous in the strong and weak operator topologies. + \item For any bounded subset $B \subset B(H)$, the composition map $(S, T) \mapsto ST$ restricted to $B \times B(H)$ is continuous in the strong operator topology. + \item The adjoint map $T \mapsto T^*$ is continuous in the weak operator topology and the ultraweak topology. + \end{enumerate} +\end{proposition} +\begin{proof} + (2): By \autoref{proposition:bh-ultraweak-bounded}. + + (4): Let $\angles{S_\alpha}_{\alpha \in A} \subset B$, $\angles{T_\alpha}_{\alpha \in A} \subset H$, and $S, T \in H$ such that $S_\alpha \to S$ and $T_\alpha \to T$ in the strong operator topology. Since $\{S_\alpha| \alpha \in A\}$ is equicontinuous, for any $x \in H$, + \[ + \lim_{\alpha \in A} S_\alpha T_\alpha x = \lim_{\alpha \in A}S_\alpha Tx = \lim_{\alpha \in A}STx + \] +\end{proof} + + +