The center problem for a 2: -3 resonant cubic Lotka-Volterra system

In this paper we obtain conditions on the coefficients of a cubic Lotka–Volterra system of the form _ x 1⁄4 xð2 a20x a11xy a02yÞ; _ y 1⁄4 yð 3þ b20x þ b11xyþ b02yÞ; ð1Þ which fulfillment yields the existence in a neighborhood of the origin of a first integral of the form /ðx; yÞ 1⁄4 x3y2 þ h:o:t:, in which case the origin is termed a 2 : 3 resonant center. This system was studied in [13], where, due to computational constrains, the consideration was limited to the cases where either one or both coefficients a11;b11 in system (1) were equal to zero, or both coefficients were equal to 1. Here we are studying the case where the coefficient a11 is equal to 1 and b11 is arbitrary. The obtained results represent the study of the center problem for general system (1), since by a linear substitution any system of the form (1) can be transformed either to system (1) with a11 1⁄4 1 or to one of systems studied in [13]. Computation of the resonant saddle quantities (focus quantities) and the decomposition of the variety of the ideal generated by an initial string of them were used to obtain necessary conditions of integrability and the theory of Darboux first integrals and some other methods to show the sufficiency. Since the decompositions of the variety mentioned above was performed using modular computations the obtained 19 conditions of integrability represent the complete list of the integrability conditions only with very high probability and there remains an open problem to verify this statement. 2013 Elsevier Inc. All rights reserved.