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Genus of abstract modular curves with level-ℓ structures

  • Anna Cadoret EMAIL logo and Akio Tamagawa
Published/Copyright: November 8, 2016

Abstract

We prove – in arbitrary characteristic – that the genus of abstract modular curves associated to bounded families of continuous geometrically perfect 𝔽-linear representations of étale fundamental groups of curves goes to infinity with . This applies to the variation of the Galois image on étale cohomology groups with coefficients in 𝔽 in 1-dimensional families of smooth proper schemes or, under certain assumptions, to specialization of first Galois cohomology groups.

Award Identifier / Grant number: ANR-10-JCJC 0107

Award Identifier / Grant number: DMS-1155

Award Identifier / Grant number: 22340006

Award Identifier / Grant number: 15H03609

Funding statement: This work was partly supported by RIMS, Kyoto University. The first author was also partly supported by the project ANR-10-JCJC 0107 from the Agence Nationale de la Recherche and the NSF grant DMS-1155. Most of this work was elaborated during stays of the first author at RIMS and IAS. She would like to thank both institutes for their hospitality. The second author was partly supported by JSPS KAKENHI Grant Numbers 22340006, 15H03609.

Acknowledgements

The first author would like to thank Michael Larsen for his interest and stimulating discussions on the topics of this paper while she visited him at Indiana University. The authors are grateful to the referee for numerous constructive comments.

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Received: 2014-12-13
Revised: 2016-08-05
Published Online: 2016-11-08
Published in Print: 2019-07-01

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