x0 = 0  ;
y0 = 1  ;
X  = 10 ;
N  = 30 ;
[t,e] = Eulero_esplicito (@cerchio_yp,0,[x0,y0],X,N*2) ;
[t,c] = Collatz          (@cerchio_yp,0,[x0,y0],X*2,N) ;
[t,h] = Heun             (@cerchio_yp,0,[x0,y0],X*2,N) ;
hold off ;
plot(e(:,1),e(:,2),'.-b');
hold on ;
plot(c(:,1),c(:,2),'.-k');
plot(h(:,1),h(:,2),'-gs');
t = [0:0.01:2*pi];
x = cos(t) ;
y = sin(t) ;
plot(x,y,'-r','LineWidth',2);
legend('Eulero',  ...
       'Collatz', ...
       'Heun',    ...
       'Esatta');
