In this paper we address the persistence of a class of seasonally forced epidemiological models. We use an abstract theorem about persistence by Fonda. Five different examples of application are given.
We study the relationship between the twist condition in the Poincar! e e-Birkhoff fixed point theorem and the assumptions on the Maslov index for asymptotically linear planar Hamiltonian systems. For this aim, we develop a variant of the Poincar! e e-Birkhoff theorem which, together with its classical version, allows to obtain a lower bound for the number of nontrivial periodic solutions in terms of the gap between the Maslov indexes associated to the linearizations of the planar system at zero and at infinity. # 2002 Elsevier Science (USA)
We present a simple topological approach for the search of fixed points and the detection of chaotic dynamics for two-dimensional\ud
maps satisfying a twist condition on linked annuli. Applications to planar Hamiltonian systems are given
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