We study factorization and dilation properties of Markov maps between von
Neumann algebras equipped with normal faithful states, i.e., completely
positive unital maps which preserve the given states and also intertwine their
automorphism groups. The starting point for our investigation has been the
question of existence of non-factorizable Markov maps, as formulated by C.
Anantharaman-Delaroche.
Let X be a homogeneous tree of degree q+1 (for q between 2 and infinity) and
let f be a complex function on X times X for which f(x,y) only depend on the
distance between x and y in X. Our main result gives a necessary and sufficient
condition for such a function to be a Schur multiplier on X times X. Moreover,
we find a closed expression for the Schur norm of f.