“…For example, reference papers [Li et al, 2009;Llibre & Pessoa, 2009;Aziz et al, 2014;Xiong & Han, 2015] investigated their centers, [García et al, 2013;Algaba et al, 2011;Giné et al, 2013;Llibre & Zhang, 2002;Cairó & Llibre, 2007;Edneral & Romanovski, 2011] studied their integrability, [Li et al, 2009;Gavrilov et al, 2009] discussed the limit cycle problem, and [Algaba et al, 2010] concerned normal forms, to name but a few. More precisely, in the paper [Li et al, 2009], the authors presented a necessary condition for the existence of a center of (3) at the origin, and when (3) has a center at the origin, they investigated the problem of the maximal number of limit cycles bifurcating from the period annulus surrounding the origin.…”