Stable closed geodesics and stable figure-eights in convex hypersurfaces

For each odd $n\ge 3$, we construct a closed convex hypersurface of ${\mathbb{R}}^{n+1}$ that contains a non-degenerate closed geodesic with Morse index zero. A classical theorem of J. L. Synge would forbid such constructions for even $n$, so in a sense we prove that Synge’s theorem is “sharp.” We also construct stable figure-eights: that is, for each $n\ge 3$ we embed the figure-eight graph in a closed convex hypersurface of ${\mathbb{R}}^{n+1}$, such that sufficiently small variations of the embedding either preserve its image or must increase its length. These index-zero geodesics and stable figure-eights are mainly derived by constructing explicit billiard trajectories with “controlled parallel transport” in convex polytopes.