RT Journal Article T1 The Poincaré–Birkhoff Theorem for a Class of Degenerate Planar Hamiltonian Systems A1 López Gómez, Julián A1 Muñoz Hernández, Eduardo A1 Zanolin, Fabio AB In this paper, we investigate the problem of the existence and multiplicity of periodic solutions to the planar Hamiltonian system x' = −λα(t)f (y), y' = λβ(t)g(x), where α, β are non-negative T-periodic coefficients and λ > 0. We focus our study to the so-called “degenerate” situation, namely when the set Z := supp α ∩ supp β has Lebesgue measure zero. It is known that, in this case, for some choices of α and β, no nontrivial T-periodic solution exists. On the opposite, we show that, depending of some geometric configurations of α and β, the existence of a large number of T-periodic solutions (aswell as subharmonic solutions) is guaranteed (for λ > 0 and large). Our proof is based on the Poincaré–Birkhoff twist theorem. Applications are given to Volterra’s predator-prey model with seasonal effects. PB De Gruyter SN 1536-1365 YR 2021 FD 2021-07-17 LK https://hdl.handle.net/20.500.14352/4981 UL https://hdl.handle.net/20.500.14352/4981 LA eng NO Ministerio de Ciencia e Innovación (MICINN) DS Docta Complutense RD 2 may 2025