Algorithms 683: a portable FORTRAN subroutine for exponential integrals of a complex argument
Donald E. Amos · ACM Transactions on Mathematical Software · 1990
The algorithm computes exponential integrals E n ( z ) for integer orders n ≥ 1 and complex z in -π < arg z ≤ π. Both single and double precision subroutines are provided. Exponential scaling and stable sequence generation are auxiliary options.