E1 Ei En Li Spence

En() Function En ( X As  Double, N As  Long, Optional Info As  Long  ) Exponential integral En(x) Purpose This routine computes the exponential integral En(x) for the argument x. En(x) = ∫ exp(-xt)/t^n dt [1, ∞] Returns Double Exponential integral En(x). Parameters [in] X Argument x. (X >= 0) [in] N Argument n. (N >= 0) [out] Info = 0: Successful exit. = -1: The argument X had an illegal value. (X < 0) = -2: The argument N had an illegal value. (N < 0) = 1: Floating point range error. Reference boost/math/special_functions