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arXiv · 2606.06713

Tamagawa number formula for Jacobians

Tim Dokchitser
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Abstract

We give a product formula for the Tamagawa numbers of Jacobians over a discrete valuation field with perfect residue field k. It comes as a product of four terms - unipotent, toric, arithmetic and (somewhat intricate) cohomological. It is proved by (1) extending the classical flow-cut construction from semistable to arbitrary curves, which, by Raynaud's results, gives the formula when k is algebraically closed; (2) observing that this formula respects the natural metric on the edges of the dual graph, which allows to quotient out the Galois action; (3) extending Bosch-Liu's description of the cohomological term when k is finite. In particular, this answers a question of Bosch-Liu, and gives an alternative description of Poonen-Stoll's cohomological obstruction in terms of characters.

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Tim Dokchitser. 2026-06-04. Tamagawa number formula for Jacobians. https://arxiv.org/abs/2606.06713

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