Polygamma should return nans where it is undefined
Polygamma currently returns infinity at its poles where it is undefined. Instead it should return nans (see eigen3/unsupported/Eigen/src/SpecialFunctions/SpecialFunctionsImpl.h).
Polygamma currently returns infinity at its poles where it is undefined. Instead it should return nans (see eigen3/unsupported/Eigen/src/SpecialFunctions/SpecialFunctionsImpl.h).