The Borsuk-Ulam theorem for planar polygon spaces

Abstract

The moduli space of planar polygons with generic side lengths is a closed, smooth manifold. Mapping a polygon to its reflected image across the X-axis defines a fixed-point-free involution on these moduli spaces, making them into free Z2-spaces. There are some important numerical parameters associated with free Z2-spaces, like index and coindex. In this paper, we compute these parameters for some moduli spaces of polygons. We also determine for which of these spaces a generalized version of the Borsuk-Ulam theorem holds. Moreover, we obtain a formula for the Stiefel-Whitney height in terms of the genetic code, a combinatorial data associated with side lengths.