Amster, P., and J. Haddad. “A Hartman-Nagumo Type Condition for a Class of Contractible Domains”. Topological Methods in Nonlinear Analysis, vol. 41, no. 2, Apr. 2013, pp. 287-04, https://apcz.umk.pl/TMNA/article/view/TMNA.2013.013.