Polynomial invariants and moduli of generic two-dimensional commutative algebras

Abstract

Let $V$ be a two-dimensional vector space over a field $\mathbb F$ of characteristic not $2$ or $3$. We show there is a canonical surjection $\nu$ from the set of suitably generic commutative algebra structures on $V$ modulo the action of $GL(V)$ onto the plane $\mathbb F^2$. In these coordinates, which are quotients of invariant quartic polynomials, properties such as associativity and the existence of zero divisors are described by simple algebraic conditions. The map $\nu$ is a bijection over the complement of a degenerate elliptic curve $\Gamma$ and over $\Gamma$ we give an explicit parametrisation of the fibre in terms of Galois extensions of $\mathbb F$. Algebras in $\nu^{-1}(\Gamma)$ are exactly those which admit non-trivial automorphisms. We show how $\nu$ can be lifted to a map from the $SL(V)-$moduli space to an algebraic hypersurface $\Gamma'$ in a four-dimensional vector space whose equation is essentially the classical Eisenstein equation for the covariants of a binary cubic. This map is the restriction of a surjective map from the set of stable commutative algebras on $V$ modulo the action of $SL(V)$ onto $\Gamma'$.