Essential perturbations of polynomial vector fields with a period annulus

In this paper we first give the explicit definition of essential perturbation. Secondly, given a perturbation of a particular family of centers of polynomial differential systems of arbitrary degree for which we explicitly know its Poincar\'e--Liapunov constants, we give the structure of its $k$-th Melnikov function. This result generalizes the result obtained by Chicone and Jacobs for perturbations of degree at most two of any center of a quadratic polynomial system. Moreover we study the essential perturbations for all the centers of the differential systems $$\dot{x} \, = \, -y + P_{\rm d}(x,y), \quad \dot{y} \, = \, x + Q_{\rm d}(x,y),$$ where $P_{\rm d}$ and $Q_{\rm d}$ are homogeneous polynomials of degree ${\rm d}$, for ${\rm d}=2$ and ${\rm d}=3$.