Kapteyn series and photon emission

Sums of Kapteyn series of several types are obtained and relations between them are found. Special attention is paid to series whose sums are expressed in terms of transcendent functions. Their asymptotics are determined. In some cases, integral representations were obtained and reduced to the form convenient for numerical calculations. The electron lifetime in a constant electromagnetic field before photon emission is determined. It was indicated that this time increases with the ultrarelativistic electron energy in a strong constant field.