function J=Jf(x)

% better to preallocate memory for Jacobian, 
% especially if it is large!
m = length(x);
J = zeros(m);

J(1,1) = 1;
J(1,2) = 2;
J(2,1) = 2*x(1);
J(2,2) = 8*x(2);
