Musso, Monica, Jacobo Pejsachowicz, and Alessandro Portaluri. “A Morse Index Theorem for Perturbed Geodesics on Semi-Riemannian Manifolds”. Topological Methods in Nonlinear Analysis 25, no. 1 (March 1, 2005): 69–99. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2005.004.