A Multiplicity Theorem for Double Resonant Periodic Problems
Leszek Gasiński, Νικόλαος Παπαγεωργίου · Advanced Nonlinear Studies · 2010
Abstract We consider semilinear periodic ordinary differential equations with the nonlinearity exhibiting double resonance at infinity in the spectral interval [λ k , λ k+1 ], k ≥ 1. Using minimax methods based on the critical point theory together with Morse theory, we show that the problem has at least four nontrivial solutions.