The polygamma functions of order @math{m} defined by @math{\psi^{(m)}(x) = (d/dx)^m \psi(x) = (d/dx)^{m+1} \log(\Gamma(x))}, where @math{\psi(x) = \Gamma'(x)/\Gamma(x)} is known as the digamma function. These functions are declared in the header file `gsl_sf_psi.h'.