PDpsider computes derivative of psi function (second derivative of rho function) for minimum power divergence estimator

`psider=PDpsider(u,alpha)`

example

Riani, M. Atkinson, A.C., Corbellini A. and Perrotta A. (2020), Robust Regression with Density Power Divergence: Theory, Comparisons and Data Analysis, Entropy, Vol. 22, 399.