— Function: size_t gsl_cheb_order (const gsl_cheb_series * cs)

This function returns the order of Chebyshev series cs.

— Function: size_t gsl_cheb_size (const gsl_cheb_series * cs)
— Function: double * gsl_cheb_coeffs (const gsl_cheb_series * cs)

These functions return the size of the Chebyshev coefficient array c[] and a pointer to its location in memory for the Chebyshev series cs.