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arXiv · 2610.36291

Pointwise to global good reduction problems

François Gatine
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Abstract

Given a smooth proper family of varieties over some $p$-adic field, if (sufficiently many of) the closed fibers have good reduction, does the family as a whole have good reduction? This question is asked and positively answered in recent work of Cadoret-Tamagawa for curves in general, and abelian varieties if the base field has absolute ramification at most $p-1$. In this paper we define a general framework for studying this question, yielding new positive answers for families of K3 surfaces and cubic fourfoulds, again with ramification at most $p-1$. Our strategy is to study several extension properties of moduli spaces and how they transfer under various morphisms. Of particular importance are integral canonical models of Shimura varieties, and whether or not their extension properties are preserved under ramified scalar extension. Additionally, building on earlier counterexamples by Raynaud-Ogus-Gabber and Vasiu-Zink, we show that the ramification assumption is optimal for the result of Cadoret-Tamagawa on families of abelian varieties. This reflects the fact that the extension property of Siegel Shimura varieties no longer holds over a field of absolute ramification at least $p$.

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François Gatine. 2026-10-26. Pointwise to global good reduction problems. https://arxiv.org/abs/2610.36291

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