— Function: double gsl_sf_Si (const double x)
— Function: int gsl_sf_Si_e (double x, gsl_sf_result * result)

These routines compute the Sine integral Si(x) = \int_0^x dt \sin(t)/t.

— Function: double gsl_sf_Ci (const double x)
— Function: int gsl_sf_Ci_e (double x, gsl_sf_result * result)

These routines compute the Cosine integral Ci(x) = -\int_x^\infty dt \cos(t)/t for x > 0.