Absolutely continuous spectrum for multi-type Galton Watson trees
  We consider multi-type Galton Watson trees that are close to a tree of finite cone type in distribution. Moreover, we impose that each vertex has at least one forward neighbor. Then, we show that the spectrum of the Laplace operator exhibits almost surely a purely absolutely continuous component which is included in the absolutely continuous spectrum of the tree of finite cone type. 
Matthias Keller
math-ph math.MP math.PR math.SP