diff --git a/src/op/system/complete-upgrade.tex b/src/op/system/complete-upgrade.tex index fae2f83..13aa67d 100644 --- a/src/op/system/complete-upgrade.tex +++ b/src/op/system/complete-upgrade.tex @@ -34,60 +34,6 @@ The following is a slight generalisation of one form of the above example, but s Let $A$ be a $C^*$-algebra, $S, T \subset A$ be operator systems, and $\phi: S \to T$. If there exists $x \in A$ such that $\phi(s) = x\phi(s)x^*$ for all $s \in S$, then $\phi$ is completely positive. \end{proposition} - - -\begin{proposition} -\label{proposition:completely-positive-norm} - Let $A, B$ be $C^*$-algebras with $A$ being unital, $S \subset A$ be an operator system, and $\phi: S \to B$ be a completely positive map, then - \[ - \norm{\phi}_{\text{cb}} = \norm{\phi}_{L(S; B)} = \norm{\phi(1_A)}_B - \] -\end{proposition} -\begin{proof} - After taking a \hyperref[unitisation]{proposition:c-star-unitisation}, assume without loss of generality that $B$ is unital. - - It is sufficient to show that $\norm{\phi}_{\text{cb}} \le \norm{\phi(1_A)}_B$. To this end, let $T \in M_n(S)$ with $\norm{T}_{M_n(S)} \le 1$, then - \[ - \begin{bmatrix} - 1_{M_n(A)} &T \\ - T^* &1_{M_n(A)} - \end{bmatrix} \ge 0 - \] - - in $M_{2}(M_n(S)) = M_{2n}(S)$ by \autoref{lemma:norm-matrix-test}. Since $\phi$ is completely positive, - \[ - \phi_{2n}\begin{bmatrix} - 1_{M_n(A)} &T \\ - T^* &1_{M_n(A)} - \end{bmatrix} = \begin{bmatrix} - \phi_n(1_{M_n(A)}) &\phi_n(T) \\ - \phi_n(T)^* & \phi_n(1_{M_n(A)}) - \end{bmatrix} \ge 0 - \] - - First suppose that $\phi_n(1_{M_n(A)})$ is invertible, then - - \begin{align*} - \phi_n(1_{M_n(A)}) &\le \normn{\phi_n(1_{M_n(A)})}_{M_n(B)} \cdot 1_{M_n(B)} \\ - 1_{M_n(B)} &\le \normn{\phi_n(1_{M_n(A)})}_{M_n(B)} \cdot \phi_n(1_{M_n(A)})^{-1} \\ - \normn{\phi_n(1_{M_n(A)})}_{M_n(B)}^{-1} &\le \phi_n(1_{M_n(A)})^{-1} - \end{align*} - - By the \hyperref[Schur complement condition]{proposition:schur-complement-positive}, - \begin{align*} - \phi_n(1_{M_n(A)}) &\ge \phi_n(T)\phi_n(1_{M_n(A)})^{-1}\phi_n(T)^* \\ - &\ge \normn{\phi_n(1_{M_n(A)})}_{M_n(B)}^{-1}\phi_n(T)\phi_n(T)^* \\ - \normn{\phi_n(1_{M_n(A)})}_{M_n(B)}^2 &\ge \normn{\phi_n(T)\phi_n(T)^*}_{M_n(B)} = \normn{\phi_n(T)}_{M_n(B)}^2 - \end{align*} - - and $\normn{\phi_n(T)}_{M_n(B)} \le \normn{\phi_n(1_{M_n(A)})}_{M_n(B)}$. Therefore $\norm{\phi}_{\text{cb}} \le \normn{\phi(1_A)}_B$. - - Now suppose that $\phi$ is an arbitrary completely positive map. Let $\psi \in S(A)$ be a state and $\eps > 0$, then $\phi + \eps 1_B\psi$ is also a completely positive map. In which case, - \[ - \norm{\phi}_{\text{cb}} \le \inf_{\eps > 0}\norm{\phi + \eps1_B\psi}_{\text{cb}} \le \inf_{\eps > 0}\norm{\phi(1_A) + \eps 1_B}_B = \norm{\phi(1_A)}_B - \] -\end{proof} - \begin{proposition} \label{proposition:commutative-mn-facts} Let $X$ be a compact Hausdorff space, then: diff --git a/src/op/system/completely.tex b/src/op/system/completely.tex index 427621c..28fc0c1 100644 --- a/src/op/system/completely.tex +++ b/src/op/system/completely.tex @@ -180,7 +180,58 @@ \end{proof} -% Omitted +\begin{proposition} +\label{proposition:completely-positive-norm} + Let $A, B$ be $C^*$-algebras with $A$ being unital, $S \subset A$ be an operator system, and $\phi: S \to B$ be a completely positive map, then + \[ + \norm{\phi}_{\text{cb}} = \norm{\phi}_{L(S; B)} = \norm{\phi(1_A)}_B + \] +\end{proposition} +\begin{proof} + After taking a \hyperref[unitisation]{proposition:c-star-unitisation}, assume without loss of generality that $B$ is unital. + + It is sufficient to show that $\norm{\phi}_{\text{cb}} \le \norm{\phi(1_A)}_B$. To this end, let $T \in M_n(S)$ with $\norm{T}_{M_n(S)} \le 1$, then + \[ + \begin{bmatrix} + 1_{M_n(A)} &T \\ + T^* &1_{M_n(A)} + \end{bmatrix} \ge 0 + \] + + in $M_{2}(M_n(S)) = M_{2n}(S)$ by \autoref{lemma:norm-matrix-test}. Since $\phi$ is completely positive, + \[ + \phi_{2n}\begin{bmatrix} + 1_{M_n(A)} &T \\ + T^* &1_{M_n(A)} + \end{bmatrix} = \begin{bmatrix} + \phi_n(1_{M_n(A)}) &\phi_n(T) \\ + \phi_n(T)^* & \phi_n(1_{M_n(A)}) + \end{bmatrix} \ge 0 + \] + + First suppose that $\phi_n(1_{M_n(A)})$ is invertible, then + + \begin{align*} + \phi_n(1_{M_n(A)}) &\le \normn{\phi_n(1_{M_n(A)})}_{M_n(B)} \cdot 1_{M_n(B)} \\ + 1_{M_n(B)} &\le \normn{\phi_n(1_{M_n(A)})}_{M_n(B)} \cdot \phi_n(1_{M_n(A)})^{-1} \\ + \normn{\phi_n(1_{M_n(A)})}_{M_n(B)}^{-1} &\le \phi_n(1_{M_n(A)})^{-1} + \end{align*} + + By the \hyperref[Schur complement condition]{proposition:schur-complement-positive}, + \begin{align*} + \phi_n(1_{M_n(A)}) &\ge \phi_n(T)\phi_n(1_{M_n(A)})^{-1}\phi_n(T)^* \\ + &\ge \normn{\phi_n(1_{M_n(A)})}_{M_n(B)}^{-1}\phi_n(T)\phi_n(T)^* \\ + \normn{\phi_n(1_{M_n(A)})}_{M_n(B)}^2 &\ge \normn{\phi_n(T)\phi_n(T)^*}_{M_n(B)} = \normn{\phi_n(T)}_{M_n(B)}^2 + \end{align*} + + and $\normn{\phi_n(T)}_{M_n(B)} \le \normn{\phi_n(1_{M_n(A)})}_{M_n(B)}$. Therefore $\norm{\phi}_{\text{cb}} \le \normn{\phi(1_A)}_B$. + + Now suppose that $\phi$ is an arbitrary completely positive map. Let $\psi \in S(A)$ be a state and $\eps > 0$, then $\phi + \eps 1_B\psi$ is also a completely positive map. In which case, + \[ + \norm{\phi}_{\text{cb}} \le \inf_{\eps > 0}\norm{\phi + \eps1_B\psi}_{\text{cb}} \le \inf_{\eps > 0}\norm{\phi(1_A) + \eps 1_B}_B = \norm{\phi(1_A)}_B + \] +\end{proof} + \begin{proposition} \label{proposition:unital-2-positive-contractive}