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arXiv · 2608.09063

The minimal obstruction modulus for quadratic forms

Naoki Ochi
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Abstract

Let $Q = ax^2+bxy+cy^2$ be a primitive positive definite integral binary quadratic form with discriminant $\Delta = b^2-4ac$. It is known that $Q$ admits a local obstruction; that is, there exist $k,l \in \mathbb{Z}$ such that $Q \not\equiv l \pmod k$. We study the minimal obstruction modulus $\kappa_Q := \min \{k \in \mathbb{Z}_{\geq 1} \mid \text{there exists } l \text{ such that } Q \not\equiv l \pmod k \}$, and we determine $\kappa_Q$ completely, treating the cases $\Delta \equiv 0 \pmod 4$ and $\Delta \equiv 1 \pmod 4$ separately. We also determine the analogous invariants for primitive ternary diagonal forms.

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Naoki Ochi. 2026-08-10. The minimal obstruction modulus for quadratic forms. https://arxiv.org/abs/2608.09063

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