MUSSO, Monica, PEJSACHOWICZ, Jacobo and PORTALURI, Alessandro. A Morse index theorem for perturbed geodesics on semi-Riemannian manifolds. Topological Methods in Nonlinear Analysis. Online. 1 March 2005. Vol. 25, no. 1, pp. 69 - 99. [Accessed 22 July 2024].