Benevieri, P., and M. Furi. “On the Concept of Orientability for Fredholm Maps Between Real Banach Manifolds”. Topological Methods in Nonlinear Analysis, vol. 16, no. 2, Dec. 2000, pp. 279-06, https://apcz.umk.pl/TMNA/article/view/TMNA.2000.042.