— Function: double gsl_sf_gamma_inc (double a, double x)
— Function: int gsl_sf_gamma_inc_e (double a, double x, gsl_sf_result * result)

These functions compute the unnormalized incomplete Gamma Function \Gamma(a,x) = \int_x^\infty dt t^{a-1} \exp(-t) for a real and x >= 0.

— Function: double gsl_sf_gamma_inc_Q (double a, double x)
— Function: int gsl_sf_gamma_inc_Q_e (double a, double x, gsl_sf_result * result)

These routines compute the normalized incomplete Gamma Function Q(a,x) = 1/\Gamma(a) \int_x^\infty dt t^{a-1} \exp(-t) for a > 0, x >= 0.

— Function: double gsl_sf_gamma_inc_P (double a, double x)
— Function: int gsl_sf_gamma_inc_P_e (double a, double x, gsl_sf_result * result)

These routines compute the complementary normalized incomplete Gamma Function P(a,x) = 1 - Q(a,x) = 1/\Gamma(a) \int_0^x dt t^{a-1} \exp(-t) for a > 0, x >= 0.

Note that Abramowitz & Stegun call P(a,x) the incomplete gamma function (section 6.5).