A Chebyshev series is stored using the following structure,
typedef struct
{
double * c; /* coefficients c[0] .. c[order] */
int order; /* order of expansion */
double a; /* lower interval point */
double b; /* upper interval point */
...
} gsl_cheb_series
The approximation is made over the range [a,b] using order+1 terms, including the coefficient c[0]. The series is computed using the following convention,
f(x) = (c_0 / 2) + \sum_{n=1} c_n T_n(x)
which is needed when accessing the coefficients directly.