Quadratic perturbations of quadratic codimension-four centers

Lubomir Gavrilov, Iliya D. Iliev

We study the stratum in the set of all quadratic differential systems $\dot{x}=P_2(x,y),\dot{y}=Q_2(x,y)$ with a center, known as the codimension-four case $Q_4$. It has a center…