function Mar13()
% Differentiate the recursion
%          p_{r}(x) = (a(r)-x)p_{r-1}(x) - b(r-1)^2p_{r-2}(x)  
% to get
%   p_{r}'(x) = (a(r)-x)p_{r-1}'(x) -p_{r-1}(x) - b(r-1)^2 * p_{r-2}'(x)
%
% Then insert this thing in the loop that is used to evaluate the
% p_{0},p_{1},...,p_{n} taking care to get the initialization right.