“…In order to calculate the average of the Green function G(z) we need to find its evolution equation, this can be done by using a projector operator technique, see for example [Hernandez et al, 1990a;Hernandez et al, 1990b;Cáceres et al, 1997;Cáceres, 2003Cáceres, , 2004. The average of G(z), i.e.…”