“…The values α j give the so-called Lyapunov quantities and can be computed in many other ways, see for instance [2,6,14,18,19,21,22,26]. When all α j = 0, the weak focus is a center; otherwise, if α k = 0, is the first nonzero α j , then it is said that the origin is a weak focus of order k. It is well known that k is the maximum number of limit cycles (isolated periodic orbits) that bifurcate from this type of points and that this amount of limit cycles is attained for some analytic perturbations.…”