Dual-cyclic polytopes of convex planar polygons with fixed vertex angles

Abstract

If we fix the angles at the vertices of a convex planar $n$-gon, the lengths of its edges must satisfy two linear constraints in order for it to close up. If we also require unit perimeter, our vectors of $n$ edge lengths form a convex polytope of dimension $n-3$, each facet of which consists of those $n$-gons in which the length of a particular edge has fallen to zero. Bavard and Ghys require unit area instead, which gives them a hyperbolic polytope. Those two polytopes are combinatorially equivalent, so either is fine for our purposes. Such a fixed-angles polytope is combinatorially richer when the angles are well balanced. We say that fixed external angles are "majority dominant" when every consecutive string of more than half of them sums to more than $\pi$. When $n$ is odd, we show that the fixed-angles polytope for any majority-dominant angles is dual to the cyclic polytope $C_{n-3}(n)$. To extend that result to even $n$, we require that the angles also have "dipole tie-breaking": None of the $n$ strings of length $n/2$ sums to precisely $\pi$, and the $n/2$ that sum to more than $\pi$ overlap as much as possible, all containing a particular angle. Fixing the vertex angles is uncommon, however; people more often fix the edge lengths. That is harder, in part because fixed-lengths $n$-gons may not be convex, but mostly because fixing the lengths constrains the angles nonlinearly -- so the resulting moduli spaces, called "polygon spaces", are curved. Using Schwarz-Christoffel maps, Kapovich and Millson show that the subset of that polygon space in which the $n$-gons are convex and traversed counterclockwise is homeomorphic to the fixed-angles polytope above, for those same fixed values. Each such subset is thus a topological polytope; and it is dual cyclic whenever the fixed lengths are majority dominant and, for even $n$, have dipole tie-breaking.