FENG, Hanying and ZHANG, Xiaofeng. Existence of solutions for a coupled system of nonlinear fractional differential equations at resonance. Topological Methods in Nonlinear Analysis. Online. 2 December 2021. Vol. 58, no. 2, pp. 389 - 401. [Accessed 22 July 2024]. DOI 10.12775/TMNA.2020.061.