Upgraded Kadison-Schwarz.
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Bokuan Li
2026-10-14 20:04:57 -04:00
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\begin{proposition}[Kadison-Schwarz Inequality]
\label{proposition:cauchy-schwarz-2-positive}
Let $A, B$ be unital $C^*$-algebras, $\phi: A \to B$ be a unital $2$-positive map, then for each $x \in A$,
Let $A, B$ be unital $C^*$-algebras, $\phi: A \to B$ be a $2$-positive map, then for each $x \in A$,
\[
\phi(x)^*\phi(x) \le \phi(x^*x)
\phi(x)^*\phi(x) \le \norm{\phi(1_A)}_B \phi(x^*x)
\]
xxx
\end{proposition}
\begin{proof}
Since
\[
\begin{bmatrix}
1 &x \\
1_A &x \\
x^* &x^*x
\end{bmatrix} = \begin{bmatrix}
1 &x \\
1_A &x \\
0 &0
\end{bmatrix}^*\begin{bmatrix}
1 &x \\
1_A &x \\
0 &0
\end{bmatrix} \ge 0
\]
and $\phi$ is unital and $2$-positive,
and $\phi$ is $2$-positive,
\[
\begin{bmatrix}
1 &\phi(x) \\
1_B\norm{\phi(1_A)}_B &\phi(x) \\
\phi(x)^* &\phi(x^*x)
\end{bmatrix} \ge \begin{bmatrix}
\phi(1_A) &\phi(x) \\
\phi(x)^* &\phi(x^*x)
\end{bmatrix} = \phi_2\begin{bmatrix}
1 &x \\
1_A &x \\
x^* &x^*x
\end{bmatrix} \ge 0
\]
By \autoref{lemma:norm-matrix-test}, $\phi(x)^*\phi(x) \le \phi(x^*x)$.
By \autoref{lemma:norm-matrix-test}, $\phi(x)^*\phi(x) \le \norm{\phi(1_A)}_B\phi(x^*x)$.
\end{proof}