Multiple Focus and Hopf Bifurcations in a Predator-Prey System with Nonmonotonic Functional Response
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In this paper, we develop a criterion to calculate the multiplicity of a multiple focus for general predator prey systems. As applications of this criterion, we calculate the most multiplicity of a multiple focus in a predator prey system with nonmonotonic functional response p(x) = x/(ax^2+bx+1) studied by Zhu, Campbell and Wolkowicz  and prove that the degenerate Hopf bifurcation is of codimension two. Furthermore, we show that there exist parameter values for which this system has a unique positive hyperbolic stable equilibrium and exactly two limit cycles, the inner one is unstable and outer one is stable. Numerical simulations for the existence of the two limit cycles bifurcated from the multiple focus were also given in support of the criterion.