(1) Section de Mathématiques, Université de Genève, Genève, Suisse;(2) Department of Mathematics, University of Pennsylvania, Philadelphia, PA 19104-6395, USA;(3) Department of Mathematics, Bar Ilan University, Ramat Gan, 52900, Israel
Abstract:
We construct a counterexample to a conjectured inequality L ≤ 2D, relating the diameter D and the least length L of a nontrivial closed geodesic, for a Riemannian metric on the 2-sphere. The construction relies on Guillemin’s theorem concerning the existence of Zoll surfaces integrating an arbitrary infinitesimal odd deformation of the round metric. Thus the round metric is not optimal for the ratio L/D. F.B. supported by the Swiss National Science Foundation. C.C. supported by NSF grants DMS 02-02536 and DMS 07-04145. M.K. supported by the Israel Science Foundation (grants 84/03 and 1294/06)