function z = vdpjac(x,y,mu)
% computes the Jacobian matrix for the VanDerPol system
% to be used in connection with the vpeig.m demo.
z(1,1)=0; z(1,2)=1;
z(2,1) = -2*mu*y(1)*y(2)-1;
z(2,2) =mu*(1-y(1)^2);