4.20 ( ) Show that the Hessian matrix for the multiclass logistic regression problem, defined by (4.110), is positive semidefinite. Note that the full Hessian matrix for this problem is of size MK ×MK, where M is the number of parameters and K is the number of classes. To prove the positive semidefinite property, consider the product uTHu where u is an arbitrary vector of lengthMK, and then apply Jensen’s inequality.