Paper detail

Extremal polygons in R^3

The oriented area function $A$ is (generically) a Morse function on the space of planar configurations of a polygonal linkage. We are lucky to have an easy description of its critical points as cyclic polygons and a simple formula for the Morse index of a critical point. However, for planar polygons, the function $A$ in many cases is not a perfect Morse function. In particular, for an equilateral pentagonal linkage it has one extra local maximum (except for the global maximum) and one extra local minimum. In the present paper we consider the space of 3D configurations of a polygonal linkage. For an appropriate generalization $S$ of the area function $A$ the situation becomes nicer: we again have an easy description of critical points and a simple formula for the Morse index. In particular, unlike the planar case, for an equilateral linkage with odd number of edges the function $S$ is always a perfect Morse function and fits the lacunary principle. Therefore cyclic equilateral polygons can be interpreted as independent generators of the homology groups of the (decorated) configuration space.