— Function: double gsl_cheb_eval (const gsl_cheb_series * cs, double x)

This function evaluates the Chebyshev series cs at a given point x.

— Function: int gsl_cheb_eval_err (const gsl_cheb_series * cs, const double x, double * result, double * abserr)

This function computes the Chebyshev series cs at a given point x, estimating both the series result and its absolute error abserr. The error estimate is made from the first neglected term in the series.

— Function: double gsl_cheb_eval_n (const gsl_cheb_series * cs, size_t order, double x)

This function evaluates the Chebyshev series cs at a given point x, to (at most) the given order order.

— Function: int gsl_cheb_eval_n_err (const gsl_cheb_series * cs, const size_t order, const double x, double * result, double * abserr)

This function evaluates a Chebyshev series cs at a given point x, estimating both the series result and its absolute error abserr, to (at most) the given order order. The error estimate is made from the first neglected term in the series.