Multiplicative properties of positive maps
  Let $\phi$ be a positive unital normal map of a von Neumann algebra $M$ into itself, and assume there is a family of normal $\phi$-invariant states which is faithful on the von Neumann algebra generated by the image of $\phi$. It is shown that there exists a largest Jordan subalgebra $C_\phi$ of $M$ such that the restriction of $\phi$ to $C_\phi$ is a Jordan automorphhism, and each weak limit point of $(\phi^n (a))$ for $a\in M$ belongs to $C_\phi$. 
Erling Stormer