Slight adjustments in known results.

This commit is contained in:
Bokuan Li
2026-06-03 14:37:13 -04:00
parent 2c1169e55a
commit 56f3ae37f7
2 changed files with 10 additions and 6 deletions

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@@ -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}

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@@ -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}