Iterating the hessian: a dynamical system on the moduli space of elliptic curves and dessins d'enfants

TL;DR

This paper studies a rational dynamical system on the moduli space of elliptic curves induced by the Hessian construction, analyzing its critical points, iterates, and associated dessins d'enfants.

Contribution

It introduces and explicitly computes the hessian dynamical system on elliptic curves using the $j$-invariant, revealing its critical values and iterative behavior.

Findings

The map has 3 critical values including infinity and two special elliptic curves.

It maps its critical values into themselves, maintaining the set under iteration.

A new algorithm constructs a sequence of dessins d'enfants related to the system.

Abstract

Each elliptic curve can be embedded uniquely in the projective plane, up to projective equivalence. The hessian curve of the embedding is generically a new elliptic curve, whose isomorphism type depends only on that of the initial elliptic curve. One gets like this a rational map from the moduli space of elliptic curves to itself. We call it the hessian dynamical system. We compute it in terms of the $j$-invariant of elliptic curves. We deduce that, seen as a map from a projective line to itself, it has 3 critical values, which correspond to the point at infinity of the moduli space and to the two elliptic curves with special symmetries. Moreover, it sends the set of critical values into itself, which shows that all its iterates have the same set of critical values. One gets like this a sequence of dessins d'enfants. We describe an algorithm allowing to construct this sequence.