E1 Ei En Li Spence

E1() Function E1 ( X As  Double, Optional Info As  Long  ) Exponential integral E1(x) Purpose This routine computes the exponential integral E1(x) for the argument x. E1(x) = ∫ exp(-xt)/t dt [1, ∞] = ∫ exp(-u)/u du [x, ∞] = -Ei(-x) Returns Double Exponential integral E1(x). Parameters [in] X Argument x. (X <> 0) [out] Info (Optional) = 0: Successful exit. = 1: Floating point range error. Reference boost/math/special_functions