Hu, Zhigang, and Wenbin Liu. “Solvability of a Coupled System of Fractional Differential Equations With Periodic Boundary Conditions at Resonance”. Ukrains’kyi Matematychnyi Zhurnal 65, no. 11 (November 25, 2013): 1463–1475. Accessed November 23, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2526.