_e1 _ei _en _li _spence

_li() double _li ( double  x ) Logarithmic integral li(x) Purpose li computes the logarithmic integral li(x). li(x) = ∫ 1/ln(t) dt [0, x] = Ei(ln(x)) Returns Logarithmic integral li(x). Parameters [in] x Argument x. (x >= 0, x != 1) Error handling If x < 0, domain error (EDOM) occurs. If x = 1, -inf is returned and range error (ERANGE) occurs. Other range error (ERANGE) may occur. Reference boost/math/special_functions