function f=track(t,X)
global p;
f=zeros(4,1);
x1=X(1);x2=X(2);x3=X(3);x4=X(4);
f(1)=-x2-x4;
f(2)=x1+0.25*x2+x3-(1-p)*x1-1.25*x2-x3-x4-x1*x4-3+(1-p)*sin(t)-cos(t);
f(3)=0.05*x3-0.5*x4;
f(4)=x1*x4+3;