Ei(x)

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

where PV denotes the principal value of the integral.