function yp=yprim2(x,y)

yp=[0;0;0;0];
k=1;
m=1;
yp(1)=y(3)/m;
yp(2)=y(4)/m;
yp(3)=k*(-2*y(1)+y(2))/m;
yp(4)=k*(y(1)-2*y(2))/m;