Iteration of closed geodesics in stationary Lorentzian manifolds

Following the lines of a celebrated result by R. Bott (Comm. Pure Appl. Math. 9, 1956) we study the Morse index of the iterated of a closed geodesic in stationary Lorentzian manifolds, or, more generally, of a closed Lorentzian geodesic that admits a timelike periodic Jacobi field. Given one such closed geodesic $\gamma$, we prove the existence of a locally constant integer valued map ${\mathrm{\Lambda }}_{\gamma }$ on the unit circle with the property that the Morse index of the iterated ${\gamma }^N$ is equal, up to a correction term ${ϵ}_{\gamma }\in \{0,1\}$, to the sum of the values of ${\mathrm{\Lambda }}_{\gamma }$ at the $N$-th roots of unity. The discontinuities of ${\mathrm{\Lambda }}_{\gamma }$ occur at a finite number of points of the unit circle, that are special eigenvalues of the linearized Poincar'e map of $\gamma$. We discuss some applications of the theory.