function dx=hopf2(t,x)
% exemplo de ciclo limite
% [t1,y1]=ode45('hopf2',100,[0.35 0.35]);
% [t2,y2]=ode45('hopf2',6.4,[0.51 0]);
% [t3,y3]=ode45('hopf',20,[0.5 0]);
% plot(y1(:,1),y1(:,2),'b',y2(:,1),y2(:,2),'r',y3(:,1),y3(:,2),'k');
u=0.5^2;
dx=[x(2)-x(1)*(u-x(1)*x(1)-x(2)*x(2));
    -x(1)-x(2)*(u-x(1)*x(1)-x(2)*x(2))];

%NOTA:
% x_ss = [0 0] --> estavel, com D=[-u+i; -u-i]
% x_ss(1)^2 + x_ss(2)^2 = u  --> C.L. instavel
% neste caso o sistema e´ equivalente a dx=[x(2); -x(1)] com D=[0+i; 0-i]
