hold off ;
x = [-2:0.01:2] ;
plot(x,p0(x),'.-b','LineWidth',1.5) ; hold on ;
plot(x,p(x,1),'+-r') ; hold on ;
plot(x,p(x,2),'o-y') ; hold on ;
plot(x,p(x,3),'.-m') ; hold on ;
plot(x,p(x,4),'.-g') ; hold on ;

legend('p0','t=1','t=2','t=3','t=4');
axis([-2,2,-0.5,0.5]);