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en() double en ( unsigned int  n, double  x  ) Exponential integral En(x) Purpose en computes the exponential integral En(x) for the argument x. En(x) = ∫ exp(-xt)/t^n dt [1, ∞] Returns Exponential integral En(x). Parameters [in] n Argument n. (n >= 0) [in] x Argument x. (x >= 0) Error handling If x < 0, domain error (EDOM) occurs. Range error (ERANGE) may occur. Reference boost/math/special_functions