Spaces of locally convex curves in S^n and combinatorics of the group B_{n+1}

Abstract

In the 1920's Marston Morse developed what is now known as Morse theory trying to study the topology of the space of closed curves on S^2. We propose to attack a very similar problem, which 80 years later remains open, about the topology of the space of closed curves on S^2 which are locally convex (i.e., without inflection points). One of the main difficulties is the absence of the covering homotopy principle for the map sending a non-closed locally convex curve to the Frenet frame at its endpoint. In the present paper we study the spaces of locally convex curves in S^n with a given initial and final Frenet frames. Using combinatorics of B^{+}_{n+1} = B_{n+1} \cap SO_{n+1}, where B_{n+1} \subset O_{n+1} is the usual Coxeter-Weyl group, we show that for any n \ge 2 these spaces fall in at most $\lceil \frac{n}{2}\rceil +1$ equivalence classes up to homeomorphism. We also study this classification in the double cover Spin(n+1). For $n=2$ our results complete the classification of the corresponding spaces into two topologically distinct classes, or three classes in the spin case.

arXiv comment22 pages, 4 figures; full rewrite of previous version