### Eta Function

The eta function is defined by \eta(s) = (1-2^{1-s}) \zeta(s).

Function: double gsl_sf_eta_int (int n)
Function: int gsl_sf_eta_int_e (int n, gsl_sf_result * result)
  These routines compute the eta function \eta(n) for integer n.

Function: double gsl_sf_eta (double s)
Function: int gsl_sf_eta_e (double s, gsl_sf_result * result)
  These routines compute the eta function \eta(s) for arbitrary s.