Amster, Pablo, and Julián Haddad. “A Hartman-Nagumo Type Condition for a Class of Contractible Domains”. Topological Methods in Nonlinear Analysis 41, no. 2 (April 22, 2013): 287–304. Accessed January 23, 2022. https://apcz.umk.pl/TMNA/article/view/TMNA.2013.013.