Abstract
Let ind(c) be the Morse index of a closed geodesic c in an (n+1)-dimensional Riemannian manifold \( \mathcal{M} \). We prove that an oriented closed geodesic c is unstable if n + ind(c) is odd and a non-oriented closed geodesic c is unstable if n+ind(c) is even. Our result is a generalization of the famous theorem due to Poincaré which states that the closed minimizing geodesic on a Riemann surface is unstable.
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Hu, X., Sun, S. Morse index and the stability of closed geodesics. Sci. China Math. 53, 1207–1212 (2010). https://doi.org/10.1007/s11425-010-0064-0
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DOI: https://doi.org/10.1007/s11425-010-0064-0