Giannoni, F., and A. Masiello. “Morse Relations for Geodesics on Stationary Lorentzian Manifolds With Boundary”. Topological Methods in Nonlinear Analysis, vol. 6, no. 1, Sept. 1995, pp. 1-30, https://apcz.umk.pl/TMNA/article/view/TMNA.1995.029.