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Connections and genuinely ramified maps of curves

  • Indranil Biswas EMAIL logo , Francois-Xavier Machu and A. J. Parameswaran
Published/Copyright: February 23, 2023

Abstract

Given a singular connection D on a vector bundle E over an irreducible smooth projective curve X, defined over an algebraically closed field, we show that there is a unique maximal subsheaf of E on which D induces a nonsingular connection. Given a generically smooth map ϕ : Y X between irreducible smooth projective curves, and a singular connection ( V , D ) on Y, the direct image ϕ * V has a singular connection. Let 𝐑 ( ϕ * 𝒪 Y ) be the unique maximal subsheaf on which the singular connection on ϕ * 𝒪 Y – corresponding to the trivial connection on 𝒪 Y – induces a nonsingular connection. We prove that the homomorphism of étale fundamental groups ϕ * : π 1 et ( Y , y 0 ) π 1 et ( X , ϕ ( y 0 ) ) induced by ϕ is surjective if and only if 𝒪 X 𝐑 ( ϕ * 𝒪 Y ) is the unique maximal semistable subsheaf. When the characteristic of the base field is zero, this homomorphism ϕ * is surjective if and only if 𝒪 X = 𝐑 ( ϕ * 𝒪 Y ) . For any nonsingular connection D on a vector bundle V over X, there is a natural map V 𝐑 ( ϕ * ϕ * V ) . When the characteristic of the base field is zero, we prove that the map ϕ is genuinely ramified if and only if V = 𝐑 ( ϕ * ϕ * V ) .

MSC 2010: 14H30; 14H60; 53B15

Communicated by Shigeharu Takayama


Acknowledgements

We are very grateful to the referee for Proposition 4.2 and also for helpful comments to improve the manuscript.

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Received: 2022-11-18
Revised: 2023-01-07
Published Online: 2023-02-23
Published in Print: 2024-01-01

© 2023 Walter de Gruyter GmbH, Berlin/Boston

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