function f=felet(t,z)
global c q m;


r2=(z(1)^2+z(2)^2);
r=sqrt(r2);
aux=(q/m)*(c/r^3);
f(1)=z(3);
f(2)=z(4);
f(3)= aux*z(1);
f(4)= aux*z(2);
end