The estimation of the relativistic Coulomb wave function is a time-consuming procedure, when the experimental data on the double beta decays are analyzed or the Monte Carlo calculation is performed. In order to avoid this complication, some simple approximated formulas are derived for three modes: the double beta decay with two neutrino emission, the neutrinoless double beta decay, and the neutrinoless double beta decay with Majoron emission. The errors of these approximations are of order of 1% or less in the region where decay events are measured practically.

• Received 9 October 1987

DOI:https://doi.org/10.1103/PhysRevC.37.2104