Collapsing in polygonal dynamics

Abstract: Polygonal dynamics is a family of dynamical systems containing many studied systems, like the famous pentagram map. Similar collapsing phenomena seem to occur in most of these systems. We give a unifying, general definition of polygonal dynamics, and conjecture that a generic orbit collapses towards a predictable point. For the special case of closed polygons'', we show that the collapse point depends algebraicly on the vertices of the starting polygon, using tools called scaling symmetry and infinitesimal monodromy. This holds regardless of the validity of the conjecture. As a corollary, this generalises previous results about the pentagram map. Then, we investigate the case of polygonal dynamics in $\mathbb{P}^1$ for which we give an explicit polynomial equation satisfied by the collapse point. Based on previous works, we define a new dynamical system, thestaircase'' cross-ratio dynamics, for which we study particular configurations.