The Number of Curves of Genus 2 with a Given Refined Humbert Invariant
Let $C/K$ be a curve of genus 2 over an algebraically closed field $K$. Every such curve comes equipped with a canonical quadratic form $q_C$ called its refined Humbert invariant. In the case that the Jacobian $J_C$ of $C$ is isogenous to the self-product $E \times E$ for an elliptic curve $E/K$ with complex multiplication (CM), we provide an explicit formula for the finite number $N_C$ of isomorphism classes of genus $2$ curves $C'/K$ whose refined Humbert invariant is equivalent to $q_C$. This formula implies that $N_C$ is unbounded for such curves, and that there are only finitely many isomorphism classes of such curves $C/K$ with a given value of $N_C$. A key step in our approach is a characterization of when $J_C$ is isogenous to a self product of a CM elliptic curve, formulated purely in terms of properties of the refined Humbert invariant $q_C$. We establish that an analogous characterization also holds for superspecial curves of genus 2. The paper concludes with explicit examples illustrating cases where a genus 2 curve is uniquely determined by the invariant $q_C$.