A new technique for establishing the existence of periodic solutions of (E) was developed for the 2-dimensional case in publication 1) listed below. The technique consists in studying the behavior of solutions of (E) near the point at infinity by studying the stability of the origin as a critical point of the system obtained by performing an inversion transformation on (E) Practical sufficient conditions for stability and asymptotic stability (i.e., conditions which can be verified by straightforward computation) were derived in publication 2). In publication 3), the technique developed in 1) for the 2-dimensional case was extended to the n-dimensional case and new existence theorems for periodic solutions were derived.
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