We prove existence results for systems of boundary value problems involving elliptic second-order differential operators. The assumptions involve lower and upper solutions, which may be either well-ordered, or not at all. The results are stated in an abstract framework, and can be translated also for systems of parabolic type.We prove existence results for systems of boundary value problems involving elliptic second-order differential operators. The assumptions involve lower and upper solutions, which may be either well-ordered, or not at all. The results are stated in an abstract framework, and can be translated also for systems of parabolic type.

SN - 0219-1997 UR - https://doi.org/10.1142/S0219199721500802 JO - Commun. Contemp. Math. ER - TY - JOUR T1 - Periodic Solutions of Second-Order Differential Equations in Hilbert Spaces Y1 - 2021 A1 - Alessandro Fonda A1 - Giuliano Klun A1 - Andrea Sfecci AB -We prove the existence of periodic solutions of some infinite-dimensional systems by the use of the lower/upper solutions method. Both the well-ordered and non-well-ordered cases are treated, thus generalizing to systems some well-established results for scalar equations.

VL - 18 SN - 1660-5454 UR - https://doi.org/10.1007/s00009-021-01857-8 IS - 5 JO - Mediterranean Journal of Mathematics ER - TY - JOUR T1 - Well-Ordered and Non-Well-Ordered Lower and Upper Solutions for Periodic Planar Systems JF - Advanced Nonlinear Studies Y1 - 2021 A1 - Alessandro Fonda A1 - Giuliano Klun A1 - Andrea Sfecci VL - 21 UR - https://doi.org/10.1515/ans-2021-2117 IS - 2 ER - TY - JOUR T1 - Periodic solutions of nearly integrable Hamiltonian systems bifurcating from infinite-dimensional tori JF - NONLINEAR ANALYSIS Y1 - 2020 A1 - Alessandro Fonda A1 - Giuliano Klun A1 - Andrea Sfecci AB -We prove the existence of periodic solutions of some infinite-dimensional nearly integrable Hamiltonian systems, bifurcating from infinite-dimensional tori, by the use of a generalization of the Poincaré–Birkhoff Theorem.

UR - https://doi.org/10.1016/j.na.2019.111720 ER - TY - JOUR T1 - On the topological degree of planar maps avoiding normal cones JF - TOPOLOGICAL METHODS IN NONLINEAR ANALYSIS Y1 - 2019 A1 - Alessandro Fonda A1 - Giuliano Klun AB -The classical Poincaré-Bohl theorem provides the existence of a zero for a function avoiding external rays. When the domain is convex, the same holds true when avoiding normal cones.

We consider here the possibility of dealing with nonconvex sets having inward corners or cusps, in which cases the normal cone vanishes. This allows us to deal with situations where the topological degree may be strictly greater than $1$.

We provide a geometric assumption which unifies and generalizes the conditions proposed in [11], [12], so to obtain a higher dimensional version of the Poincaré–Birkhoff fixed point Theorem for Poincaré maps of Hamiltonian systems.

VL - 262 UR - http://www.sciencedirect.com/science/article/pii/S0022039616303278 ER - TY - JOUR T1 - Generalizing the Poincaré–Miranda theorem: the avoiding cones condition JF - Annali di Matematica Pura ed Applicata (1923 -) Y1 - 2016 A1 - Alessandro Fonda A1 - Paolo Gidoni AB -After proposing a variant of the Poincaré–Bohl theorem, we extend the Poincaré–Miranda theorem in several directions, by introducing an avoiding cones condition. We are thus able to deal with functions defined on various types of convex domains, and situations where the topological degree may be different from \$\$\backslashpm \$\$±1. An illustrative application is provided for the study of functionals having degenerate multi-saddle points.

VL - 195 UR - https://doi.org/10.1007/s10231-015-0519-6 ER - TY - JOUR T1 - Periodic perturbations of Hamiltonian systems JF - Advances in Nonlinear Analysis Y1 - 2016 A1 - Alessandro Fonda A1 - Maurizio Garrione A1 - Paolo Gidoni AB -We prove existence and multiplicity results for periodic solutions of Hamiltonian systems, by the use of a higher dimensional version of the Poincaré–Birkhoff fixed point theorem. The first part of the paper deals with periodic perturbations of a completely integrable system, while in the second part we focus on some suitable global conditions, so to deal with weakly coupled systems.

PB - De Gruyter VL - 5 ER - TY - JOUR T1 - A permanence theorem for local dynamical systems JF - Nonlinear Analysis: Theory, Methods & Applications Y1 - 2015 A1 - Alessandro Fonda A1 - Paolo Gidoni KW - Lotka–Volterra KW - permanence KW - Predator–prey KW - Uniform persistence AB -We provide a necessary and sufficient condition for permanence related to a local dynamical system on a suitable topological space. We then present an illustrative application to a Lotka–Volterra predator–prey model with intraspecific competition.

VL - 121 UR - http://www.sciencedirect.com/science/article/pii/S0362546X14003332 N1 - Nonlinear Partial Differential Equations, in honor of Enzo Mitidieri for his 60th birthday ER - TY - JOUR T1 - Generalized Sturm-Liouville boundary conditions for first order differential systems in the plane JF - Topol. Methods Nonlinear Anal. Y1 - 2013 A1 - Alessandro Fonda A1 - Maurizio Garrione AB -We study asymptotically positively homogeneous first order systems in the plane, with boundary conditions which are positively homogeneous, as well. Defining a generalized concept of Fučík spectrum which extends the usual one for the scalar second order equation, we prove existence and multiplicity of solutions. In this way, on one hand we extend to the plane some known results for scalar second order equations (with Dirichlet, Neumann or Sturm-Liouville boundary conditions), while, on the other hand, we investigate some other kinds of boundary value problems, where the boundary points are chosen on a polygonal line, or in a cone. Our proofs rely on the shooting method.

PB - Nicolaus Copernicus University, Juliusz P. Schauder Centre for Nonlinear Studies VL - 42 UR - https://projecteuclid.org:443/euclid.tmna/1461248981 ER - TY - JOUR T1 - Periodic bouncing solutions for nonlinear impact oscillators JF - Advanced Nonlinear Studies Y1 - 2013 A1 - Alessandro Fonda A1 - Andrea Sfecci PB - Advanced Nonlinear Studies, Inc. VL - 13 ER - TY - JOUR T1 - A general method for the existence of periodic solutions of differential systems in the plane JF - Journal of Differential Equations Y1 - 2012 A1 - Alessandro Fonda A1 - Andrea Sfecci KW - Nonlinear dynamics KW - Periodic solutions AB -We propose a general method to prove the existence of periodic solutions for planar systems of ordinary differential equations, which can be used in many different circumstances. Applications are given to some nonresonant cases, even for systems with superlinear growth in some direction, or with a singularity. Systems “at resonance” are also considered, provided a Landesman–Lazer type of condition is assumed.

VL - 252 UR - http://www.sciencedirect.com/science/article/pii/S0022039611003196 ER - TY - JOUR T1 - Periodic solutions of a system of coupled oscillators with one-sided superlinear retraction forces JF - Differential Integral Equations Y1 - 2012 A1 - Alessandro Fonda A1 - Andrea Sfecci PB - Khayyam Publishing, Inc. VL - 25 UR - https://projecteuclid.org:443/euclid.die/1356012248 ER - TY - JOUR T1 - Double resonance with Landesman–Lazer conditions for planar systems of ordinary differential equations JF - Journal of Differential Equations Y1 - 2011 A1 - Alessandro Fonda A1 - Maurizio Garrione KW - Double resonance KW - Landesman–Lazer conditions KW - Nonlinear planar systems AB -We prove the existence of periodic solutions for first order planar systems at resonance. The nonlinearity is indeed allowed to interact with two positively homogeneous Hamiltonians, both at resonance, and some kind of Landesman–Lazer conditions are assumed at both sides. We are thus able to obtain, as particular cases, the existence results proposed in the pioneering papers by Lazer and Leach (1969) [27], and by Frederickson and Lazer (1969) [18]. Our theorem also applies in the case of asymptotically piecewise linear systems, and in particular generalizes Fabry's results in Fabry (1995) [10], for scalar equations with double resonance with respect to the Dancer–Fučik spectrum.

VL - 250 UR - http://www.sciencedirect.com/science/article/pii/S0022039610002901 ER - TY - JOUR T1 - Nonlinear resonance: a comparison between Landesman-Lazer and Ahmad-Lazer-Paul conditions JF - Advanced Nonlinear Studies Y1 - 2011 A1 - Alessandro Fonda A1 - Maurizio Garrione AB -We show that the Ahmad-Lazer-Paul condition for resonant problems is more general than the Landesman-Lazer one, discussing some relations with other existence conditions, as well. As a consequence, such a relation holds, for example, when considering resonant boundary value problems associated with linear elliptic operators, the p-Laplacian and, in the scalar case, with an asymmetric oscillator.

PB - Advanced Nonlinear Studies, Inc. VL - 11 ER -