Zhang, Lingling, and Huimin Tian. “New Fixed Point Theorems for Sum Operators in Set $P_{h,e}$ and Their Applications to Nonlinear Fractional Differential Problems”. Topological Methods in Nonlinear Analysis 59, no. 2B (April 10, 2022): 719–735. Accessed April 20, 2024. https://apcz.umk.pl/TMNA/article/view/37958.