Lengths of geodesics between two points on a Riemannian manifold

Abstract

Let x and y be two (not necessarily distinct) points on a closed Riemannian manifold M of dimension n. According to a celebrated theorem by J.P. Serre there exist infinitely many geodesics between x and y. The length of the shortest of these geodesics is obviously less than the diameter of M. But what can be said about the length of the other geodesics? We conjecture that for every k there are k geodesics between x and y of length not exceeding kd, where d denotes the diameter of M.This conjecture is obviously true for round spheres and it is not difficult to prove it for all closed Riemannian manifolds with non-trivial torsion-free fundamental groups. In this paper we announce two further results in the direction of this conjecture. Our first result is that the length of the second shortest geodesic between x and y does not exceed 2nd. Our second result is that if n=2 and M is diffeomorphic to the two-dimensional sphere, then for every k every two points on M can be connected by k geodesics of length not exceeding $(k^2/2 + 3k/2 +2)d$.