On the number of limit cycles in quadratic perturbations of quadratic codimension four centers
This paper is concerned with the bifurcation of limit cycles in general quadratic perturbations of quadratic codimension-four centers $Q_4$. Gavrilov and Iliev set an upper bound of {\it eight} for the number of limit cycles produced from the period annulus around the center. Based on Gavrilov-Iliev's proof, we prove in this paper that the perturbed system has at most five limit cycles which emerge from the period annulus around the center. We also show that there exists a perturbed system with three limit cycles produced by the period annulus of $Q_4$.