The mean number of $2$-torsion elements in the class groups of cubic orders

Abstract

We determine the mean number of 2-torsion elements in class groups of cubic orders, when such orders are enumerated by discriminant. Specifically, we prove that when isomorphism classes of totally real (resp., complex) cubic orders are enumerated by discriminant, the average $2$-torsion in the class group is $1+\frac{1}{4}\times \frac{\zeta (2)}{\zeta (4)}$ (resp., $1+\frac{1}{2}\times \frac{\zeta (2)}{\zeta (4)}$). In particular, we find that the average $2$-torsion in the class group increases when one ranges over all orders in cubic fields instead of restricting to the subfamily of rings of integers of cubic fields, where the average $2$-torsion in the class group was first determined in work of Bhargava to be $\frac{5}{4}$ (resp., $\frac{3}{2}$).
By work of Bhargava–Varma, proving this result amounts to obtaining an asymptotic count of the number of “reducible” ${\mathrm{SL}}_3(\mathbb{Z})$-orbits on the space ${\mathbb{Z}}^2{\otimes }_{\mathbb{Z}}{\mathrm{Sym}}^2{\mathbb{Z}}^3$ of $3\times 3$ symmetric integer matrices having bounded invariants and satisfying local conditions. In this paper, we resolve the generalization of this orbit-counting problem where the dimension $3$ is replaced by any fixed odd integer $N\ge 3$. More precisely, we determine asymptotic formulas for the number of reducible ${\mathrm{SL}}_N(\mathbb{Z})$-orbits on ${\mathbb{Z}}^2{\otimes }_{\mathbb{Z}}{\mathrm{Sym}}^2{\mathbb{Z}}^N$ satisfying general infinite sets of congruence conditions.