Critical temperature of anisotropic superconductors

An approximate analytic formula for the increase in the superconducting critical temperature caused by anisotropy is derived and compared with both the results of exact numerical solutions of Eliashberg equations and with a previous approximate formula due to Allen and Butler. In all the cases considered our new formula is found to be more accurate than the previous one and to agree reasonably well with the exact results for the intermediate and weak coupling superconductors considered