AMSTER, Pablo, EPSTEIN, Julián and SANJUÁN CUÉLLAR, Arturo. Periodic solutions for systems of functional-differential semilinear equations at resonance. Topological Methods in Nonlinear Analysis. Online. 5 December 2021. Vol. 58, no. 2, pp. 591 - 607. [Accessed 7 July 2025]. DOI 10.12775/TMNA.2020.078.