The strong-coupling theory of the superconducting critical temperature

A Tsub(c) formula is derived analytically from the Eliashberg equation by an iterative method. The Karakozov et al Tsub(c) formula is only a weak-coupling limit of it. The values of Tsub(c) calculated from our formula are in agreement with the experimental values for both crystalline and amorphous superconducting materials. Certain reasonable approximations are made to render our Tsub(c) formula simplified and several useful formulae for Tsub(c) containing only a few parameters are obtained, one of which is just the Rowell empirical Tsub(c) formula. The Tsub(c) formulae obtained in the present paper are compared further with some empirical ones proposed by previous authors and some comments are given. (author)