— Function: double gsl_sf_expint_Ei (double x)
— Function: int gsl_sf_expint_Ei_e (double x, gsl_sf_result * result)

These routines compute the exponential integral Ei(x),

          Ei(x) := - PV(\int_{-x}^\infty dt \exp(-t)/t)

where PV denotes the principal value of the integral.