Adjusted prose in example facts.
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Bokuan Li
2026-06-03 17:08:55 -04:00
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Equipped with pointwise operations and the uniform norm, $BC(X; \complex)$ forms a commutative Banach algebra.
\end{definition}
\begin{theorem}
\label{theorem:multiplicative-functional-bc}
Let $X$ be a completely regular space and $\beta X$ be its \hyperref[Stone-Čech compactification]{definition:stone-cech}. For each $f \in BC(X; \complex)$, let $\beta f \in BC(\beta X; \complex)$ be its unique extension to $\beta X$, then the mapping
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E: \beta X \to \Omega(BC(X; \complex)) \quad E(x)(f) = (\beta f)(x)
\]
is a homeomorphism.
is a homeomorphism. Under the identification $\beta X = \Omega(BC(X; \complex))$, $\Gamma_{BC(X; \complex)} = \beta$.
\end{theorem}
\begin{proof}
Let $\phi \in BC(X; \complex)^* \setminus \ol{E(X)}$, then there exists $\seqf{f_k} \subset BC(X; \complex)$ and $\eps > 0$ such that for every $x \in X$,

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(F(z))(f) = \sum_{n \in \integer}f(n)z^n
\]
is a homeomorphism.
is a homeomorphism. Under the identification $\partial B_\complex(0, 1) = \Omega(\ell^1(\integer))$, the Gelfand transform has the explicit form
\[
\Gamma(f)(z) = \sum_{n \in \integer} f(n)z^n
\]
\end{theorem}
\begin{proof}{Proof, {{\cite[Theorem 6.3]{Zhu}}}. }
Let $z \in \mathbf{S}^1$ and $f, g \in \ell^1(\integer)$, then by \hyperref[Fubini's Theorem]{theorem:fubini-tonelli},
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Finally, by the \hyperref[Dominated Convergence Theorem]{theorem:dct}, $F: \textbf{S}^1 \to \Omega(\ell^1(\integer))$ is continuous. Since $\textbf{S}^1$ is compact, $\Omega(\ell^1(\integer))$ is Hausdorff, and $F$ is a bijection, $F$ is a homeomorphism by \autoref{proposition:compact-hausdorff-homeomorphism}.
\end{proof}
\begin{proposition}
\label{proposition:convolution-integer-spectrum}
Let $\ell^1(\integer)$ be the convolution algebra on $\integer$ and $f \in \ell^1(\integer)$, then
\[
\sigma(f) = \bracs{\sum_{n \in \integer}f(n)z^n \bigg | z \in \partial B_\complex(0, 1)}
\]
\end{proposition}
\begin{proof}
By \autoref{theorem:convolution-integer-gelfand} and (4) of \autoref{proposition:gelfand-transform-gymnastics}.
\end{proof}

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\input{./hardy.tex}
\input{./disk.tex}
\input{./convolution.tex}
\input{./bc.tex}