The Perron-Frobenius Theorem for Markov Semigroups
  Let $P^V_t$, $t\ge0$, be the Schrodinger semigroup associated to a potential $V$ and Markov semigroup $P_t$, $t\ge0$, on $C(X)$. Existence is established of a left eigenvector and right eigenvector corresponding to the spectral radius $e^{\lambda_0t}$ of $P^V_t$, simultaneously for all $t\ge0$. This is derived with no compactness assumption on the semigroup operators. 
Omar Hijab
math.FA