Hu, Zhigang, and Wenbin Liu. 2013. “Solvability of a Coupled System of Fractional Differential Equations With Periodic Boundary Conditions at Resonance”. Ukrains’kyi Matematychnyi Zhurnal 65 (11), 1463–1475. https://umj.imath.kiev.ua/index.php/umj/article/view/2526.