From 56f3ae37f7844096fdf9bdf0e2f8a21199801440 Mon Sep 17 00:00:00 2001 From: Bokuan Li Date: Wed, 3 Jun 2026 14:37:13 -0400 Subject: [PATCH] Slight adjustments in known results. --- src/op/banach/fc.tex | 13 +++++++++---- src/op/example/hardy.tex | 3 +-- 2 files changed, 10 insertions(+), 6 deletions(-) diff --git a/src/op/banach/fc.tex b/src/op/banach/fc.tex index dd438d7..88eb925 100644 --- a/src/op/banach/fc.tex +++ b/src/op/banach/fc.tex @@ -23,12 +23,18 @@ \] for all $f \in H(U; \complex)$. + \item For each $U \subset \complex$ open and $f \in H(U; \complex)$, the mapping + \[ + \bracs{x \in A| \sigma_A(x) \subset U} \to A \quad x \mapsto f(x) + \] + + is continuous. \end{enumerate} The mapping $f \mapsto f(x)$ is the \textbf{holomorphic functional calculus} of $x$. \end{definition} \begin{proof}[Proof, {{\cite[Proposition I.2.7]{Takesaki1}}}. ] - (Definition): Let $U, V \in \cn_\complex(\sigma_A(x))$ such that $\ol V$ is a compact subset of $U$, then by \autoref{proposition:existence-curves}, there exists closed rectifiable curves $\seqf{\gamma_j}$ on $V \setminus \sigma_A(x)$ such that + (Definition), (3): Let $U, V \in \cn_\complex(\sigma_A(x))$ such that $\ol V$ is a compact subset of $U$, then by \autoref{proposition:existence-curves}, there exists closed rectifiable curves $\seqf{\gamma_j}$ on $V \setminus \sigma_A(x)$ such that \begin{enumerate}[label=(\alph*)] \item For all $f \in H(V; \complex)$ and $z_0 \in \sigma_A(x)$, \[ @@ -89,7 +95,6 @@ \] (Uniqueness): By (2), the homomorphism extends uniquely to $\complex(z) \cap H(\sigma_A(x); \complex)$. By \hyperref[Runge's Theorem]{corollary:runge-rational-approximation}, it extends uniquely to $H(\sigma_A(x); \complex)$ by continuity. - \end{proof} \begin{theorem}[Spectral Mapping Theorem (Holomorphic)] @@ -103,7 +108,7 @@ \begin{proof} (1): Let $\lambda \in \complex \setminus f(\sigma_A(x))$, then $1/(\lambda - f) \in H(\sigma_A(x); \complex)$. Since the holomorphic functional calculus is a homomorphism, $[1/(\lambda - f)](x) = (\lambda - f(x))^{-1}$. Thus $\sigma_A(f(x)) \subset f(\sigma_A(x))$. - On the other hand, let $\lambda \in \sigma_A(x)$, then the function $h(\mu) = [f(\mu) - f(\lambda)]/(\mu - \lambda)$ extends to an element of $H(\sigma_A(x); \complex)$ by \autoref{proposition:zero-finite-multiplicity}. Therefore by the homomorphism property, + On the other hand, let $\lambda \in \sigma_A(x)$, then the function $h(\mu) = [f(\mu) - f(\lambda)]/(\mu - \lambda)$ extends to an element of $H(\sigma_A(x); \complex)$ by \autoref{proposition:zero-finite-multiplicity}. By the homomorphism property, \[ f(x) - f(\lambda) = (x - \lambda)h(x) = h(x)(x - \lambda) \] @@ -120,7 +125,7 @@ By \hyperref[Runge's Theorem]{corollary:runge-rational-approximation}, there exists $h \in U \cap \complex(z)$. Thus $\norm{g(f(x)) - (g \circ f)(x)}_A < \eps$ as well. As this holds for all $\eps > 0$, $g(f(x)) = (g \circ f)(x)$ for any $f \in H(\sigma_A(x); \complex)$ and $g \in \complex(x) \cap H(f(\sigma_A(x)); \complex)$. - Through a second application of \hyperref[Runge's Theorem]{corollary:runge-rational-approximation} again and continuity of the holomorphic functional calculus, the above also holds for all $f \in H(\sigma_A(x); \complex)$ and $g \in H(f(\sigma_A(x)); \complex)$. + Through a second application of \hyperref[Runge's Theorem]{corollary:runge-rational-approximation} and continuity of the holomorphic functional calculus, the above also holds for all $f \in H(\sigma_A(x); \complex)$ and $g \in H(f(\sigma_A(x)); \complex)$. \end{proof} \begin{proposition} diff --git a/src/op/example/hardy.tex b/src/op/example/hardy.tex index e39c1b7..f4ba611 100644 --- a/src/op/example/hardy.tex +++ b/src/op/example/hardy.tex @@ -23,12 +23,11 @@ \item If $\fU \to x_0 \in \ol{D}$, then $f \in A(D)$, $\phi_{\fU}(f) = f(z_0)$. \item $\phi_{\fU}$ is a multiplicative linear functional on $H^\infty(D)$. \end{enumerate} - \end{proposition} \begin{proof} Let $f \in H^\infty(D)$, then by \autoref{proposition:imagefilterbase}, $f(\fU)$ is an ultrafilter base. Since $f$ is bounded, $f(\fU)$ converges to exactly one element of $\complex$. Hence the limit is well-defined. - (2): By \autoref{proposition:operator-space-completeness}. + (2): By \autoref{definition:multiplicative-linear-functional-space}. \end{proof}