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ei() double ei ( double  x ) Exponential integral Ei(x) Purpose ei computes the exponential integral Ei(x) for the argument x. Ei(x) = -∫ exp(-t)/t dt [-x, ∞] = -E1(-x) Returns Exponential integral Ei(x). Parameters [in] x Argument x. (x != 0) Error handling If x = 0, -inf is returned and range error (ERANGE) occurs. Other range error (ERANGE) may occur. Reference boost/math/special_functions