ZHANG, Lingling and TIAN, Huimin. 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. Online. 10 April 2022. Vol. 59, no. 2B, pp. 719 - 735. [Accessed 3 July 2024]. DOI 10.12775/TMNA.2021.008.