We study the periodic boundary value problem associated with the second order nonlinear equation u{\textquoteright}{\textquoteright}+(λa+(t)-μa-(t))g(u)=0, where g(u) has superlinear growth at zero and sublinear growth at infinity. For λ,μ positive and large, we prove the existence of 3^m-1 positive T-periodic solutions when the weight function a(t) has m positive humps separated by m negative ones (in a T-periodicity interval). As a byproduct of our approach we also provide abundance of positive subharmonic solutions and symbolic dynamics. The proof is based on coincidence degree theory for locally compact operators on open unbounded sets and also applies to Neumann and Dirichlet boundary conditions. Finally, we deal with radially symmetric positive solutions for the Neumann and the Dirichlet problems associated with elliptic PDEs.

}, url = {http://urania.sissa.it/xmlui/handle/1963/35264}, author = {Alberto Boscaggin and Guglielmo Feltrin and Fabio Zanolin} } @article {doi:10.1142/S0219199717500213, title = {Positive subharmonic solutions to nonlinear ODEs with indefinite weight}, journal = {Communications in Contemporary Mathematics}, volume = {20}, number = {01}, year = {2018}, pages = {1750021}, abstract = {We prove that the superlinear indefinite equation u" + a(t)up = 0, where p \> 1 and a(t) is a T-periodic sign-changing function satisfying the (sharp) mean value condition ∫0Ta(t)dt \< 0, has positive subharmonic solutions of order k for any large integer k, thus providing a further contribution to a problem raised by Butler in its pioneering paper [Rapid oscillation, nonextendability, and the existence of periodic solutions to second order nonlinear ordinary differential equations, J. Differential Equations 22 (1976) 467{\textendash}477]. The proof, which applies to a larger class of indefinite equations, combines coincidence degree theory (yielding a positive harmonic solution) with the Poincar{\'e}{\textendash}Birkhoff fixed point theorem (giving subharmonic solutions oscillating around it).

}, doi = {10.1142/S0219199717500213}, url = {https://doi.org/10.1142/S0219199717500213}, author = {Alberto Boscaggin and Guglielmo Feltrin} } @article {feltrin2017, title = {An application of coincidence degree theory to cyclic feedback type systems associated with nonlinear differential operators}, journal = {Topol. Methods Nonlinear Anal.}, volume = {50}, number = {2}, year = {2017}, pages = {683{\textendash}726}, publisher = {Nicolaus Copernicus University, Juliusz P. Schauder Centre for Nonlinear Studies}, doi = {10.12775/TMNA.2017.038}, url = {https://doi.org/10.12775/TMNA.2017.038}, author = {Guglielmo Feltrin and Fabio Zanolin} } @article {1534-0392_2017_3_1083, title = {Multiple positive solutions of a sturm-liouville boundary value problem with conflicting nonlinearities}, journal = {Communications on Pure \& Applied Analysis}, volume = {16}, number = {1534-0392_2017_3_108}, year = {2017}, pages = {1083}, abstract = {We study the second order nonlinear differential equation

\begindocument $ u{\textquoteright}{\textquoteright} + \sum\limits_i = 1^m α_ia_i(x)g_i(u) - \sum\limits_j = 1^m + 1 β_jb_j(x)k_j(u) = 0,\rm $ \enddocument

where $\alpha_i, \beta_j\>0$, $a_i(x), b_j(x)$ are non-negative Lebesgue integrable functions defined in $\mathopen[0, L\mathclose]$, and the nonlinearities $g_i(s), k_j(s)$ are continuous, positive and satisfy suitable growth conditions, as to cover the classical superlinear equation $u"+a(x)u.p = 0$, with $p\>1$.When the positive parameters $\beta_j$ are sufficiently large, we prove the existence of at least $2.m-1$positive solutions for the Sturm-Liouville boundary value problems associated with the equation.The proof is based on the Leray-Schauder topological degree for locally compact operators on open and possibly unbounded sets.Finally, we deal with radially symmetric positive solutions for the Dirichlet problems associated with elliptic PDEs.

}, keywords = {Leray-Schauder topological degree;, positive solutions, Sturm-Liouville boundary conditions, Superlinear indefinite problems}, issn = {1534-0392}, doi = {10.3934/cpaa.2017052}, url = {http://aimsciences.org//article/id/1163b042-0c64-4597-b25c-3494b268e5a1}, author = {Guglielmo Feltrin} } @article {FELTRIN20174255, title = {Multiplicity of positive periodic solutions in the superlinear indefinite case via coincidence degree}, journal = {Journal of Differential Equations}, volume = {262}, number = {8}, year = {2017}, pages = {4255 - 4291}, abstract = {We study the periodic boundary value problem associated with the second order nonlinear differential equationu"+cu'+(a+(t)-μa-(t))g(u)=0, where g(u) has superlinear growth at zero and at infinity, a(t) is a periodic sign-changing weight, c∈R and μ\>0 is a real parameter. Our model includes (for c=0) the so-called nonlinear Hill{\textquoteright}s equation. We prove the existence of 2m-1 positive solutions when a(t) has m positive humps separated by m negative ones (in a periodicity interval) and μ is sufficiently large, thus giving a complete solution to a problem raised by G.J. Butler in 1976. The proof is based on Mawhin{\textquoteright}s coincidence degree defined in open (possibly unbounded) sets and applies also to Neumann boundary conditions. Our method also provides a topological approach to detect subharmonic solutions.

}, keywords = {Coincidence degree, Multiplicity results, Neumann boundary value problems, Positive periodic solutions, subharmonic solutions, Superlinear indefinite problems}, issn = {0022-0396}, doi = {https://doi.org/10.1016/j.jde.2017.01.009}, url = {http://www.sciencedirect.com/science/article/pii/S0022039617300219}, author = {Guglielmo Feltrin and Fabio Zanolin} } @article {12163, title = {A note on a fixed point theorem on topological cylinders}, journal = {Ann. Mat. Pura Appl.}, number = {Annali di Matematica Pura ed Applicata;}, year = {2017}, note = {AMS Subject Classification: 47H10, 37C25, 47H11, 54H25.}, publisher = {Springer Verlag}, abstract = {We present a fixed point theorem on topological cylinders in normed linear spaces for maps satisfying a property of stretching a space along paths. This result is a generalization of a similar theorem obtained by D. Papini and F. Zanolin. In view of the main result, we discuss the existence of fixed points for maps defined on different types of domains and we propose alternative proofs for classical fixed point theorems, as Brouwer, Schauder and Krasnosel{\textquoteright}skii ones.

}, doi = {10.1007/s10231-016-0623-2}, url = {http://urania.sissa.it/xmlui/handle/1963/35263}, author = {Guglielmo Feltrin} } @article {2016, title = {Pairs of positive periodic solutions of nonlinear ODEs with indefinite weight: a topological degree approach for the super-sublinear case}, journal = {Proc. Roy. Soc. Edinburgh Sect. A 146 (2016), 449{\textendash}474.}, number = {Proceedings of the Royal Society of Edinburgh. Section A. Mathematics;volume 146; issue 3; pages 449-474;}, year = {2016}, note = {AMS Subject Classification: Primary 34B18; 34C25; Secondary 34B15; 47H11;}, publisher = {Cambridge University Press}, abstract = {We study the periodic and Neumann boundary value problems associated with the second order nonlinear differential equation u{\textquoteright}{\textquoteright}+cu{\textquoteright}+λa(t)g(u)=0, where g:[0,+$\infty$[{\textrightarrow}[0,+$\infty$[ is a sublinear function at infinity having superlinear growth at zero. We prove the existence of two positive solutions when ∫a(t)dt 0 is sufficiently large. Our approach is based on Mawhin{\textquoteright}s coincidence degree theory and index computations.

}, doi = {10.1017/S0308210515000621}, url = {http://urania.sissa.it/xmlui/handle/1963/35262}, author = {Alberto Boscaggin and Guglielmo Feltrin and Fabio Zanolin} } @mastersthesis {2016, title = {Positive solutions to indefinite problems: a topological approach}, year = {2016}, note = {The research work described in this Ph.D. thesis has produced 10 papers.}, school = {SISSA}, abstract = {The present Ph.D. thesis is devoted to the study of positive solutions to indefinite problems. In particular, we deal with the second order nonlinear differential equation u{\textquoteright}{\textquoteright} + a(t) g(u) = 0, where g : [0,+$\infty$[{\textrightarrow}[0,+$\infty$[ is a continuous nonlinearity and a : [0,T]{\textrightarrow}R is a Lebesgue integrable sign-changing weight. We analyze the Dirichlet, Neumann and periodic boundary value problems on [0,T] associated with the equation and we provide existence, nonexistence and multiplicity results for positive solutions. In the first part of the manuscript, we investigate nonlinearities g(u) with a superlinear growth at zero and at infinity (including the classical superlinear case g(u)=u^p, with p>1). In particular, we prove that there exist 2^m-1 positive solutions when a(t) has m positive humps separated by negative ones and the negative part of a(t) is sufficiently large. Then, for the Dirichlet problem, we solve a conjecture by G{\'o}mez-Re{\~n}asco and L{\'o}pez-G{\'o}mez (JDE, 2000) and, for the periodic problem, we give a complete answer to a question raised by Butler (JDE, 1976). In the second part, we study the super-sublinear case (i.e. g(u) is superlinear at zero and sublinear at infinity). If a(t) has m positive humps separated by negative ones, we obtain the existence of 3^m-1 positive solutions of the boundary value problems associated with the parameter-dependent equation u{\textquoteright}{\textquoteright} + λ a(t) g(u) = 0, when both λ>0 and the negative part of a(t) are sufficiently large. We propose a new approach based on topological degree theory for locally compact operators on open possibly unbounded sets, which applies for Dirichlet, Neumann and periodic boundary conditions. As a byproduct of our method, we obtain infinitely many subharmonic solutions and globally defined positive solutions with complex behavior, and we deal with chaotic dynamics. Moreover, we study positive radially symmetric solutions to the Dirichlet and Neumann problems associated with elliptic PDEs on annular domains. Furthermore, this innovative technique has the potential and the generality needed to deal with indefinite problems with more general differential operators. Indeed, our approach apply also for the non-Hamiltonian equation u{\textquoteright}{\textquoteright} + cu{\textquoteright} + a(t) g(u) = 0. Meanwhile, more general operators in the one-dimensional case and problems involving PDEs will be subjects of future investigations.}, keywords = {positive solutions}, author = {Guglielmo Feltrin} } @article {2015, title = {Existence of positive solutions in the superlinear case via coincidence degree: the Neumann and the periodic boundary value problems}, journal = {Adv. Differential Equations 20 (2015), 937{\textendash}982.}, number = {Advances in Differential Equations;volume 20; issue 9/10; pages 937-982;}, year = {2015}, note = {AMS Subject Classification: 34B18, 34B15, 34C25, 47H11.}, publisher = {Khayyam Publishing}, abstract = {We prove the existence of positive periodic solutions for the second order nonlinear equation u{\textquoteright}{\textquoteright} + a(x) g(u) = 0, where g(u) has superlinear growth at zero and at infinity. The weight function a(x) is allowed to change its sign. Necessary and sufficient conditions for the existence of nontrivial solutions are obtained. The proof is based on Mawhin{\textquoteright}s coincidence degree and applies also to Neumann boundary conditions. Applications are given to the search of positive solutions for a nonlinear PDE in annular domains and for a periodic problem associated to a non-Hamiltonian equation.

}, url = {http://projecteuclid.org/euclid.ade/1435064518}, author = {Guglielmo Feltrin and Fabio Zanolin} } @article {0133-0189_2015_special_436, title = {Existence of positive solutions of a superlinear boundary value problem with indefinite weight}, journal = {Conference Publications}, volume = {2015}, number = {0133-0189_2015_special_43}, year = {2015}, pages = {436}, abstract = {We deal with the existence of positive solutions for a two-point boundary value problem associated with the nonlinear second order equation $u{\textquoteright}{\textquoteright}+a(x)g(u)=0$. The weight $a(x)$ is allowed to change sign. We assume that the function $g\colon\mathopen[0,+$\infty$\mathclose[\to\mathbb{R}$ is continuous, $g(0)=0$ and satisfies suitable growth conditions, including the superlinear case $g(s)=s^p$, with $p\>1$. In particular we suppose that $g(s)/s$ is large near infinity, but we do not require that $g(s)$ is non-negative in a neighborhood of zero. Using a topological approach based on the Leray-Schauder degree we obtain a result of existence of at least a positive solution that improves previous existence theorems.

}, keywords = {boundary value problem, indefinite weight, Positive solution; existence result., superlinear equation}, issn = {0133-0189}, doi = {10.3934/proc.2015.0436}, url = {http://aimsciences.org//article/id/b3c1c765-e8f5-416e-8130-05cc48478026}, author = {Guglielmo Feltrin} } @article {2015, title = {Multiple positive solutions for a superlinear problem: a topological approach}, journal = {J. Differential Equations 259 (2015), 925{\textendash}963.}, number = {Journal of Differential Equations;volume 259; issue 3; pages 925-963;}, year = {2015}, note = {Work presented at the "Special Session 21" of the "10th AIMS Conference on Dynamical Systems, Differential Equations and Applications" (Madrid, July 7-11, 2014).}, publisher = {Elsevier}, abstract = {We study the multiplicity of positive solutions for a two-point boundary value problem associated to the nonlinear second order equation u{\textquoteright}{\textquoteright}+f(x,u)=0. We allow x ↦ f(x,s) to change its sign in order to cover the case of scalar equations with indefinite weight. Roughly speaking, our main assumptions require that f(x,s)/s is below λ_1 as s{\textrightarrow}0^+ and above λ_1 as s{\textrightarrow}+$\infty$. In particular, we can deal with the situation in which f(x,s) has a superlinear growth at zero and at infinity. We propose a new approach based on the topological degree which provides the multiplicity of solutions. Applications are given for u{\textquoteright}{\textquoteright} + a(x) g(u) = 0, where we prove the existence of 2^n-1 positive solutions when a(x) has n positive humps and a^-(x) is sufficiently large.

}, doi = {10.1016/j.jde.2015.02.032}, url = {http://urania.sissa.it/xmlui/handle/1963/35147}, author = {Guglielmo Feltrin and Fabio Zanolin} }