Existence of absolutely continuous spectrum for Galton-Watson random trees

International audience ; We establish a quantitative criterion for an operator defined on a Galton-Watson random tree for having an absolutely continuous spectrum. For the adjacency operator, this criterion requires that the offspring distribution has a relative variance below a threshold. As a byproduct, we prove that the adjacency operator of a supercritical Poisson Galton-Watson tree has a non-trivial absolutely continuous part if the average degree is large enough. We also prove that its Karp and Sipser core has purely absolutely spectrum on an interval if the average degree is large enough. We finally illustrate our criterion on the Anderson model on a d-regular infinite tree with d ≥ 3 and give a quantitative version of Klein's Theorem on the existence of absolutely continuous spectrum at disorder smaller that C √ d for some absolute constant C.