Some cubic systems with several limit cycles

Two-dimensional systems x=P(x, y), y=Q(x,y) in which P and Q are cubic polynomials, are considered, and a number of classes with several limit cycles are described. Examples of systems with six small-amplitude limit cycles are given. Other classes of systems with several limit cycles are obtained by considering simultaneous bifurcation from a finite critical point and infinity. Simultaneous bifurcation from several critical points is investigated.

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