% differential equation of Nm

function    Z=cal_z(Z0,Z1,W,kn,ssp,H,N);
Z(1)=Z0;
Z(2)=Z1;
h=1/N;
for ii=3:N+1
    Z(ii)=(2-H*H*h*h*(W*W/ssp(N+2-ii)/ssp(N+2-ii)-kn*kn))*Z(ii-1)-Z(ii-2);
end