The half-life with respect to two-neutrino double-beta (2 nu beta beta )dec
ay to the final ground state is calculated under the assumption of single-s
tate dominance. To the best of our knowledge, the energy denominators of pe
rturbation theory are considered for the first time without invoking any ap
proximation. The results obtained for the experimentally interesting A = 10
0, 110, 114, 116, and 128 nuclear systems are presented. (C) 2000 MAIK "Nau
ka/Interperiodica".