29 Numerics library [numerics]

29.9 Mathematical functions for floating-point types [c.math]

double       expint(double x);
float        expintf(float x);
long double  expintl(long double x);

Effects: These functions compute the exponential integral of their respective arguments x.

Returns:

$E i (x) = - \int_{- x}^{\infty} \frac{e^{- t}}{t} d t$

where x is x.