Removed parts from Zhu citations.
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@@ -74,7 +74,7 @@
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then $f = 1$.
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\end{proposition}
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\begin{proof}[Proof, {{\cite[Lemma I.4.4]{Zhu}}}. ]
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\begin{proof}[Proof, {{\cite[Lemma 4.4]{Zhu}}}. ]
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By (c) and \autoref{proposition:entire-logarithm}, there exists $g \in H(\complex; \complex)$ such that $f = e^g$. Since $f(0) = 1$, $g(0) = 0$. As $f'(0) = g'(0)f(0) = 0$, $g'(0) = 0$ as well.
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From (c), $|g(z)| \le e^{|z|}$ for each $z \in \complex$. Thus for every $r > 0$ and $z \in B_\complex(0, r)$, $|g(z)| \le |g(z) - 2r|$, and
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@@ -65,7 +65,7 @@
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\label{proposition:swap-invertible}
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Let $A$ be a unital Banach algebra and $x, y \in A$, then $1 - xy \in G(A)$ if and only if $1 - yx \in G(A)$.
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\end{proposition}
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\begin{proof}[Proof, {{\cite[Proposition I.3.4]{Zhu}}}. ]
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\begin{proof}[Proof, {{\cite[Proposition 3.4]{Zhu}}}. ]
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If $1 - xy \in G(A)$, then
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\begin{align*}
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(1 - yx)[y(1 - xy)^{-1}x + 1] &= y(1 - xy)^{-1}x + 1 \\
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@@ -69,7 +69,7 @@
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\item $\phi(1) = 1$ and $\phi(G(A)) \subset \complex \setminus \bracs{0}$.
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\end{enumerate}
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\end{theorem}
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\begin{proof}[Proof, {{\cite[Theorem I.4.5]{Zhu}}}. ]
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\begin{proof}[Proof, {{\cite[Theorem 4.5]{Zhu}}}. ]
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(1) $\Rightarrow$ (2): \autoref{proposition:multiplicative-unit}.
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(2) $\Rightarrow$ (1): Let $x \in A$ and $\lambda \in \complex$ with $|\lambda| > [x]_{sp}$, then $\lambda - x \in G(A)$ and $\phi(\lambda - x) \ne 0$. Therefore $\phi(x) \subset \ol{B(0, [x]_{sp})}$, and $\norm{\phi}_{A^*} = 1$.
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@@ -136,7 +136,7 @@
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\item $\norm{\Phi(x)}_B = \norm{x}_A$.
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\end{enumerate}
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\end{corollary}
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\begin{proof}[Proof, {{\cite[II.10.7]{Zhu}}}. ]
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\begin{proof}[Proof, {{\cite[10.7]{Zhu}}}. ]
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(1): Since $\Phi(G(A)) \subset G(B)$, $\sigma_B(\Phi(x)) \subset \sigma_A(x)$. If $\sigma_B(\Phi(x)) \subsetneq \sigma_A(x)$, then \hyperref[Urysohn's Lemma]{lemma:urysohn} implies that there exists $C(\sigma_A(x); \complex)$ such that $f|_{\sigma_B(\Phi(x))} = 0$ but $f \ne 0$. In which case, by (7) of the \hyperref[continuous functional calculus]{definition:continuous-functional-calculus}, $\Phi(f(x)) = f(\Phi(x)) = 0$, which contradicts the fact that $\Phi$ is injective.
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(2): By \autoref{corollary:c-star-unique-norm}, $\Phi$ is isometric.
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@@ -10,7 +10,7 @@
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is a unital $C^*$-isomorphism.
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\end{theorem}
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\begin{proof}[Proof, {{\cite[Theorem II.9.4]{Zhu}}}. ]
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\begin{proof}[Proof, {{\cite[Theorem 9.4]{Zhu}}}. ]
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By construction $\Gamma_A$ is a unital algebra homomorphism.
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To see that $\Gamma_A$ preserves involutions, let $y \in A$ be self-adjoint. By \autoref{proposition:gelfand-transform-gymnastics} and \autoref{proposition:self-adjoint-spectrum}, $\Gamma_A(y)(\Omega(A)) = \sigma_A(y) \subset \real$, so $\Gamma_A(y) \in C(\Omega(A); \real)$. For any $x \in A$, write $x = \text{Re}(x) + i\text{Im}(x)$, where $\text{Re}(x)$ and $\text{Im}(x)$ are both self-adjoint, then
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@@ -36,4 +36,11 @@
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\end{proof}
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\begin{theorem}
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\label{theorem:continuity-of-homomorphism-c-star}
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Let $A, B$ be unital $C^*$-algebras and $\Phi: A \to B$ be a unital *-homomorphism, then $\Phi(A)$ is closed.
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\end{theorem}
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\begin{proof}[Proof, {{\cite[Theorem 11.1]{Zhu}}}. ]
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\end{proof}
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@@ -23,7 +23,7 @@
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\item There exists $\lambda \ge \norm{x}_A$ such that $\norm{\lambda - x}_A \le \lambda$.
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\end{enumerate}
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\end{proposition}
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\begin{proof}[Proof, {{\cite[Lemma II.11.3]{Zhu}}}. ]
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\begin{proof}[Proof, {{\cite[Lemma 11.3]{Zhu}}}. ]
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(1) $\Leftrightarrow$ (2): \autoref{proposition:positive-spectrum}.
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(2) $\Leftrightarrow$ (3): By assumption, $\sigma_A(x) \subset \real$, so \autoref{theorem:c-star-normal-spectral-radius} implies that
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@@ -35,7 +35,7 @@
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\label{theorem:c-star-normal-spectral-radius}
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Let $A$ be a unital $C^*$-algebra and $x \in A$ be normal, then $\norm{x}_A = [x]_{sp}$.
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\end{theorem}
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\begin{proof}[Proof, {{\cite[Theorem II.8.1]{Zhu}}}. ]
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\begin{proof}[Proof, {{\cite[Theorem 8.1]{Zhu}}}. ]
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First suppose that $x$ is self-adjoint. In this case,
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\begin{align*}
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\normn{x^2}_A &= \normn{xx^*}_A = \norm{x}_A^2 \\
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@@ -42,7 +42,7 @@
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\label{proposition:unitary-spectrum}
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Let $A$ be a unital $C^*$-algebra and $x \in A$ be unitary, then $\sigma_A(x) \subset \partial B_\complex(0, 1)$.
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\end{proposition}
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\begin{proof}[Proof, {{\cite[Proposition II.8.2]{Zhu}}}. ]
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\begin{proof}[Proof, {{\cite[Proposition 8.2]{Zhu}}}. ]
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By \autoref{lemma:unitary-unit}, $\norm{x}_A = 1$, so $\sigma_A(x) \subset \ol{B_\complex(0, 1)}$. Thus
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\[
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\bracsn{\ol{\lambda}|\lambda \in \sigma_A(x)} = \sigma_A(x^*) = \sigma_A(x^{-1}) \subset \ol{\complex \setminus B_\complex(0, 1)}
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@@ -17,7 +17,7 @@
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is a homeomorphism. Under the identification $\beta X = \Omega(BC(X; \complex))$, $\Gamma_{BC(X; \complex)} = \beta$.
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\end{theorem}
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\begin{proof}[Proof, {{\cite[Theorem I.6.4]{Zhu}}}. ]
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\begin{proof}[Proof, {{\cite[Theorem 6.4]{Zhu}}}. ]
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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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\[
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f(x) = \sum_{k = 1}^n |f_k(x) - \dpn{f_k, \phi}{BC(X; \complex)}|^2 \ge \eps^2
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@@ -40,7 +40,7 @@
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so $g \le F$. As this holds for all upper bounds of $\cf$, $g$ is the supremum of $\cf$ in $C(X; \real)$.
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($\Leftarrow$, \cite[Theorem II.9.6]{Zhu}): Suppose that $X$ is completely regular and $C(X; \real)$ is order complete. Let $U \subset X$ be open and
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($\Leftarrow$, \cite[Theorem 9.6]{Zhu}): Suppose that $X$ is completely regular and $C(X; \real)$ is order complete. Let $U \subset X$ be open and
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\[
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\cf = \bracs{f \in C(X; [0, 1])| 0 \le f \le \one_U}
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\]
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