N  = 100 ;
[x,y] = instabile (@fun_exp,1,N) ;
hold off ;
plot(x,y,'.-b');
hold on ;
x = [0:0.01:1];
y = exp(-10.*x) ;
plot(x,y,'-r','LineWidth',2);
axis([0 1 -1 2]);
legend('Instabile','Esatta');