Mai, Jiehua, and Taixiang Sun. “Periodic Orbits for Multivalued Maps With Continuous Margins of Intervals”. Topological Methods in Nonlinear Analysis 48, no. 2 (August 17, 2016): 453–464. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2016.052.