Renormalized boson expansion for the 2 nu beta beta decay

Two-neutrino double-beta (2 nu beta beta) Gamow-Teller transitions, 0(i)(+) --> 0(f)(+) and 0(i)(+) --> 2(f)(+) are treated with a many-body Hamiltoni an involving a Bonn-type realistic two-body interaction. The slates involve d in these transitions are described by a pnRQRPA approach. Transition oper ators are expanded in first order in terms of renormalized bosons. Tn this way the space of intermediate states is enlarged due to the states with two and three bosons. Important effects on both transition amplitudes and Iked a sum rule are pointed out. For illustration the formalism is applied to th e transition Se-82 --> Kr-82.