Fast generation of the gamma and digamma functions of complex argument
Michael A. K. Hamid, S. J TOWAI · International Journal of Electronics · 1973
Using standard recurrence relations for the gamma and digamma functions, the real part is stepped up by integers until it is sufficiently large to permit the use of standard asymptotic expansions. It is shown that the technique is extremely simple and accurate and requires few milliseconds on a modern digital computer.