— Function: double gsl_sf_bessel_Inu (double nu, double x)
— Function: int gsl_sf_bessel_Inu_e (double nu, double x, gsl_sf_result * result)

These routines compute the regular modified Bessel function of fractional order \nu, I_\nu(x) for x>0, \nu>0.

— Function: double gsl_sf_bessel_Inu_scaled (double nu, double x)
— Function: int gsl_sf_bessel_Inu_scaled_e (double nu, double x, gsl_sf_result * result)

These routines compute the scaled regular modified Bessel function of fractional order \nu, \exp(-|x|)I_\nu(x) for x>0, \nu>0.