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@@ -111,38 +111,43 @@
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\begin{proposition}[Kadison-Schwarz Inequality]
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\label{proposition:cauchy-schwarz-2-positive}
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Let $A, B$ be unital $C^*$-algebras, $\phi: A \to B$ be a unital $2$-positive map, then for each $x \in A$,
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Let $A, B$ be unital $C^*$-algebras, $\phi: A \to B$ be a $2$-positive map, then for each $x \in A$,
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\[
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\phi(x)^*\phi(x) \le \phi(x^*x)
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\phi(x)^*\phi(x) \le \norm{\phi(1_A)}_B \phi(x^*x)
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\]
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xxx
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\end{proposition}
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\begin{proof}
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Since
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\[
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\begin{bmatrix}
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1 &x \\
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1_A &x \\
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x^* &x^*x
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\end{bmatrix} = \begin{bmatrix}
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1 &x \\
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1_A &x \\
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0 &0
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\end{bmatrix}^*\begin{bmatrix}
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1 &x \\
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1_A &x \\
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0 &0
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\end{bmatrix} \ge 0
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\]
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and $\phi$ is unital and $2$-positive,
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and $\phi$ is $2$-positive,
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\[
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\begin{bmatrix}
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1 &\phi(x) \\
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1_B\norm{\phi(1_A)}_B &\phi(x) \\
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\phi(x)^* &\phi(x^*x)
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\end{bmatrix} \ge \begin{bmatrix}
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\phi(1_A) &\phi(x) \\
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\phi(x)^* &\phi(x^*x)
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\end{bmatrix} = \phi_2\begin{bmatrix}
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1 &x \\
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1_A &x \\
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x^* &x^*x
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\end{bmatrix} \ge 0
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\]
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By \autoref{lemma:norm-matrix-test}, $\phi(x)^*\phi(x) \le \phi(x^*x)$.
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By \autoref{lemma:norm-matrix-test}, $\phi(x)^*\phi(x) \le \norm{\phi(1_A)}_B\phi(x^*x)$.
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\end{proof}
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