function  Apr1()
% Let e(j) be the jth column of the identity.
% Assume n = 9 for clarity and that h is small.
%        Divided difference                            Provides apprx to
%    ( F(x+he(1)+h(e(4))+he(7)) - F(x) ) / h         J(:,1)  J(:,4)  J(:,7)
%    ( F(x+he(2)+h(e(5))+he(8)) - F(x) ) / h         J(:,2)  J(:,5)  J(:,8)
%    ( F(x+he(3)+h(e(6))+he(9)) - F(x) ) / h         J(:,3)  J(:,6)  J(:,9)