Analytic solution to the Eliashberg equation for the superconducting critical temperatures

The analytic solutions to the Eliashberg equation for the superconducting T_c obtained by the Chinese physicists, mainly Wu and his co-workers, during the years 1977–1985 are systematically reviewed. Two independent and mutually complementary formulae which hold in the regions λ > Λ and λ < Λ, respectively are obtained. Here, Λ is a material dependent parameter and its definition will be given in the paper. The analytic derivations and the properties of these two formulae are discussed and analysed in detail. The values of T_c calculated from them are compared with the experimental values and the numerical solutions of the Eliashberg equation. They are compared further with those given by other T_c formulae proposed by previous authors and some comments are given.