“…Hence, the isochronous center problem is equivalent to find a transversal commutator in a punctured neighborhood of the origin. There are only a few families of polynomial differential systems in which a complete classification of the isochronous centers is known, and almost all of them have a polynomial commutator, see for instance [5,10,15,16,18,19]. Moreover, several works are devoted to find polynomial commutators for different families of polynomial differential systems, see [10,18].…”