diff --git a/src/op/vn/topologies.tex b/src/op/vn/topologies.tex index 7b578b6..524c7b9 100644 --- a/src/op/vn/topologies.tex +++ b/src/op/vn/topologies.tex @@ -35,17 +35,14 @@ Now, a few facts about the more familiar operator topologies: \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 composition map $(S, T) \mapsto ST$ is left-hypocontinuous with respect to the strong operator topology and strong-operator bounded subsets of $B(H)$. \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 $\angles{S_\alpha}_{\alpha \in A} \subset B$, $\angles{T_\alpha}_{\alpha \in A} \subset B(H)$, and $(S, T) \in B \times B(H)$ 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 $x \in H$, - \[ - \lim_{\alpha \in A} S_\alpha T_\alpha x = \lim_{\alpha \in A}S_\alpha Tx = \lim_{\alpha \in A}STx - \] + (4): By the \hyperref[Banach-Steinhaus Theorem]{theorem:banach-steinhaus}, every strong-operator bounded subset of $B(H)$ is equicontinuous. \end{proof} diff --git a/src/op/vn/vn.tex b/src/op/vn/vn.tex index 2379745..803b862 100644 --- a/src/op/vn/vn.tex +++ b/src/op/vn/vn.tex @@ -6,18 +6,17 @@ Let $H$ be a complex Hilbert space and $A \subset B(H)$ be a strong-operator closed $C^*$-subalgebra, then: \begin{enumerate} \item For any bounded directed family $\cf \subset A_{sa}$, $\sup(\cf) = \sotlim_{T \in \cf}T \in A_{sa}$. - \item For any bounded commuting family $\cf \subset A_{sa}$, $\sup(\cf)$ exists in $A_{sa}$. \item For any family of projections $\mathcal{P} \subset A_{sa}$, $\sup(\mathcal{P}) \in A$ is the projection onto $\ol{\bigcup_{P \in \mathcal{P}}P(H)}$. \end{enumerate} and - \begin{enumerate}[start=3] + \begin{enumerate}[start=2] \item Let $T \in A_{sa}$ with $0 \le T \le I$ and $P \in B(H)$ be the orthogonal projection onto $\ol{T(H)}$, then $P = \sotlim_{n \to \infty}T^{1/n} \in A$. \item For each $T \in A$, the orthogonal projection onto $\ol{T(H)}$ is in $A$. \end{enumerate} Finally, - \begin{enumerate}[start=5] + \begin{enumerate}[start=4] \item There exists a maximum projection $P \in A$ such that $PT = TP = T$ for all $T \in A$, which is the multiplicative unit of $A$. \end{enumerate} @@ -47,9 +46,7 @@ for all $S \in \cf$ with $S \ge T$. As such a $T$ exists for all $\eps > 0$, $R = \sotlim_{T \in \cf}T$. - (2): Assume without loss of generality that $A$ is the smallest strong-operator closed $C^*$-subalgebra of $B(H)$ containing $\cf$. In which case, by separate continuity of composition in the strong operator topology, $A$ is also commutative. Thus $A_{sa}$ is a lattice by the \hyperref[Gelfand-Naimark Theorem]{section:gelfand-naimark}, and $\cf$ may be extended into a directed family. By (1), $\sup(\cf)$ exists in $A$. - - (3): Assume without loss of generality that $\mathcal{P}$ is directed. By (1), $\sup(\mathcal{P})$ exists in $A$. Since the set of projections in $B(H)$ is strong-operator closed, $\sup(\mathcal{P}) = \sotlim_{P \in \mathcal{P}}P$ is also a projection. For each $x \in \bigcup_{P \in \mathcal{P}}P(H)$, there exists $P \in \mathcal{P}$ with $Px = x$. In which case, + (2): Assume without loss of generality that $\mathcal{P}$ is directed. By (1), $\sup(\mathcal{P})$ exists in $A$. Since the set of projections in $B(H)$ is strong-operator closed, $\sup(\mathcal{P}) = \sotlim_{P \in \mathcal{P}}P$ is also a projection. For each $x \in \bigcup_{P \in \mathcal{P}}P(H)$, there exists $P \in \mathcal{P}$ with $Px = x$. In which case, \[ \dpn{\sup(\mathcal{P})x, x}{H} \ge \dpn{Px, x}{H} = \dpn{x, x}{H} = \norm{x}_H^2 \] @@ -61,7 +58,7 @@ \ol{\bigcup_{P \in \mathcal{P}}P(H)} = Q(H) \supset \sup(\mathcal{P})(H) \] - (4): As $0 \le T \le I$, $\sigma_{B(H)}(T) \subset [0, 1]$ by the \hyperref[Gelfand-Naimark Theorem]{theorem:gelfand-naimark}. For each $n \in \natp$ and $t \in [0, 1]$, let $f_n(t) = t^{1/n}$, then $f_n$ is an increasing sequence of continuous functions on $[0, 1]$. By the \hyperref[continuous functional calculus]{definition:continuous-functional-calculus}, $\bracsn{f_n(T)}_1^\infty = \bracsn{T^{1/n}}_1^\infty$ is an increasing sequence that lies between $0$ and $I$. + (3): As $0 \le T \le I$, $\sigma_{B(H)}(T) \subset [0, 1]$ by the \hyperref[Gelfand-Naimark Theorem]{theorem:gelfand-naimark}. For each $n \in \natp$ and $t \in [0, 1]$, let $f_n(t) = t^{1/n}$, then $f_n$ is an increasing sequence of continuous functions on $[0, 1]$. By the \hyperref[continuous functional calculus]{definition:continuous-functional-calculus}, $\bracsn{f_n(T)}_1^\infty = \bracsn{T^{1/n}}_1^\infty$ is an increasing sequence that lies between $0$ and $I$. Let $n \in \natp$. By the \hyperref[Stone-Weierstrass Theorem]{theorem:stone-weierstrass}, there exist polynomials $\seq{p_{n, k}} \subset \real[x]$ such that: \begin{enumerate}[label=(\roman*)] @@ -78,9 +75,9 @@ and $Q$ is self-adjoint, $Q$ is a projection. - For each $x \in T(H) \setminus \bracs{0}$, $\dpn{Qx, x}{H} \ge \dpn{Tx, x}{H} > 0$ because $T$ is positive. As such, $\ker(Q) \subset \ker(T)$. For any $x \in \ker(T)$, $\dpn{Qx, x}{H} = \limv{n}\dpn{T^{1/n}x, x}{H} = 0$, so $\ker(Q) \supset \ker(T)$. Since both operators are self-adjoint, $\ol{Q(H)} = \ol{T(H)} = P(H)$, and $P = Q$. + For each $x \in H \setminus \ker(T)$, $\dpn{Qx, x}{H} \ge \dpn{Tx, x}{H} > 0$ because $T$ is positive. As such, $\ker(Q) \subset \ker(T)$. For any $x \in \ker(T)$, $\dpn{Qx, x}{H} = \limv{n}\dpn{T^{1/n}x, x}{H} = 0$, so $\ker(Q) \supset \ker(T)$. Since both operators are self-adjoint, $\ol{Q(H)} = \ol{T(H)} = P(H)$, and $P = Q$. - (5): Assume without loss of generality that $T \ne 0$. Let $x \in H$, then + (4): Assume without loss of generality that $T \ne 0$. Let $x \in H$, then \[ \norm{T^*x}_H^2 = \dpn{T^*x, T^*x}{H} = \dpn{TT^*x, x}{H} \] @@ -90,12 +87,78 @@ \ol{T(H)} = \ker(T^*)^\perp = \ker(TT^*)^\perp = \ol{TT^*(H)} \] - By (4) applied to $TT^*/\norm{TT^*}_{B(H)}$, the orthogonal projection onto $\ol{T(H)}$ is in $A$. + By (3) applied to $TT^*/\norm{TT^*}_{B(H)}$, the orthogonal projection onto $\ol{T(H)}$ is in $A$. - (6): Let $\mathcal{P}$ be the set of all projections in $A$, then $\mathcal{P} \subset A_{sa}$ is bounded and directed. By (3), $P = \sup_{Q \in \mathcal{P}}Q \in A$, which is the maximum projection in $A$. + (5): Let $\mathcal{P}$ be the set of all projections in $A$, then $\mathcal{P} \subset A_{sa}$ is bounded and directed. By (2), $P = \sup_{Q \in \mathcal{P}}Q \in A$, which is the maximum projection in $A$. - Let $T \in A$, then by (5), $P$ is greater than the projection onto $\ol{T(H)}$, so $PT = T$. On the other hand, since $PT^* = T^*$, $TP = T$ as well. Therefore $P$ is the multiplicative identity in $A$. + Let $T \in A$, then by (4), $P$ is greater than the projection onto $\ol{T(H)}$, so $PT = T$. On the other hand, since $PT^* = T^*$, $TP = T$ as well. Therefore $P$ is the multiplicative identity in $A$. \end{proof} +\begin{lemma} +\label{lemma:invariant-projection-test} + Let $H$ be a complex Hilbert space, $M \subset H$ be a closed subspace, $P \in B(H)$ be the orthogonal projection onto $M$, and $T \in B(H)$, then the following are equivalent: + \begin{enumerate} + \item $T(M) \subset M$. + \item $PTP = TP$. + \end{enumerate} +\end{lemma} +% Proof omitted due to obviousness +\begin{definition}[Reducing Subspace] +\label{definition:reducing-subspace} + Let $H$ be a complex Hilbert space, $M \subset H$ be a closed subspace, $P \in B(H)$ be the orthogonal projection onto $P$, and $T \in B(H)$, then the following are equivalent: + \begin{enumerate} + \item $T(M) \subset M$ and $T^*(M) \subset M$. + \item $TP = PT$. + \end{enumerate} + + If the above holds, then $M$ is a \textbf{reducing subspace} of $T$. +\end{definition} +\begin{proof}[Proof, {{\cite[Corollary 18.3]{Zhu}}}. ] + (1) $\Rightarrow$ (2): By \autoref{lemma:invariant-projection-test}, $PTP = TP$ and $PT^*P = T^*P$. Thus $TP = PTP = PT$. + + (2) $\Rightarrow$ (1): Since $TP = PT$, $PTP = PT$, and $T(M) \subset M$ by \autoref{lemma:invariant-projection-test}. Similarly, $T^*P = PT^*$ implies that $PT^*P = PT^*$, and $T^*(M) \subset M$ by \autoref{lemma:invariant-projection-test}. +\end{proof} + +\begin{definition}[Commutant] +\label{definition:commutant} + Let $H$ be a complex Hilbert space and $A \subset B(H)$, then + \[ + A' = \bracs{T \in B(H)| TS = ST \forall S \in A} + \] + + is the \textbf{commutant} of $A$. +\end{definition} + +\begin{lemma} +\label{lemma:bicommutant-pointwise} + Let $H$ be a complex Hilbert space and $A \subset B(H)$ be a unital, self-adjoint subalgebra, $T \in A''$, and $x \in H$, then $Tx \in \ol{\bracsn{Sx|S \in A}}$. +\end{lemma} +\begin{proof}[Proof, {{\cite[Lemma 18.4]{Zhu}}}. ] + Let $M = \ol{\bracsn{Sx|S \in A}}$, then since $A$ is self-adjoint, $M$ is a reducing subspace for each element of $A$. Let $P \in B(H)$ be the orthogonal projection onto $M$, then $PS = SP$ for all $S \in A$. As such, $P \in A'$. + + Now, since $T \in A''$, $TP = PT$ as well, so $M$ is a reducing subspace for $T$. As $A$ is unital, $x \in M$, so $Tx \in M = \ol{\bracsn{Sx|S \in A}}$. +\end{proof} + +\begin{theorem}[Von Neumann's Bicommutant Theorem] +\label{theorem:bicommutant} + Let $H$ be a complex Hilbert space and $A \subset B(H)$ be a unital, self-adjoint subalgebra, then $A''$ is the strong-operator closure of $A$. +\end{theorem} +\begin{proof}[Proof, {{\cite[Section 18.3]{Zhu}}}. ] + ($\overline{A}^{\text{\small SOT}} \subset A''$): By separate continuity of composition, $A''$ is a strong-operator closed subset that contains $A$. Hence $A''$ contains the strong-operator closure of $A$. + + ($\overline{A}^{\text{\small SOT}} \supset A''$): Let $T \in A''$ and $x = \seqf{x_j} \in H^n$. For each $S \in B(H)$, denote $S^{(n)} = (S, \cdots, S)$ ($n$-copies), then + \begin{enumerate}[label=(\roman*)] + \item $A^{(n)} = \bracsn{S^{(n)}|S \in A}$ is a unital, self-adjoint subalgebra of $B(H^n)$. + \item $T^{(n)} \in (A^{(n)})''$. + \end{enumerate} + + By \autoref{lemma:bicommutant-pointwise}, + \[ + (Tx_1, \cdots, Tx_n) \in \ol{\bracsn{(Sx_1, \cdots, Sx_n)|S \in A}} + \] + + so $T \in \ol{A}^{\text{\small SOT}}$. +\end{proof} +