The dynamics of the Hesse derivative on the $j$-invariant

Abstract

The $j$-invariant of a cubic curve is an isomorphism invariant parameterized by the moduli space of elliptic curves. The Hesse derivative of a curve $V(f)$ given by the homogeneous polynomial $f$ is $V(\mathcal{H}(f))$ where $\mathcal{H}(f)$ is a the determinant of the Hesse matrix of $f$. In this paper, we compute the $j$-invariant of the Hesse derivative of a cubic curve $C$ in terms of the $j$-invariant of $C$, getting a rational function on the Riemann sphere. We then analyze the dynamics of this rational function, and investigate when a cubic curve is isomorphic to its $n$-fold Hesse derivative.