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research article

Pseudo-effectivity of the relative canonical divisor and uniruledness in positive characteristic

Patakfalvi, Zsolt  
2025
Epijournal de Geometrie Algebrique

We show that if f : X → T is a surjective morphism between smooth projective varieties over an algebraically closed field k of characteristic p > 0 with geometrically integral and non-uniruled generic fiber, then KX/T is pseudo-effective. The proof is based on covering X with rational curves, which gives a contradiction as soon as both the base and the generic fiber are not uniruled. However, we assume only that the generic fiber is not uniruled. Hence, the hardest part of the proof is to show that there is a finite smooth non-uniruled cover of the base for which we show the following: If T is a smooth projective variety over k and A is an ample enough line bundle, then a cyclic cover of degree p l d given by a general element of | Ad | is not uniruled. For this we show the following cohomological uniruledness condition, which might be of independent interest: A smooth projective variety T of dimenion n is not uniruled whenever the dimension of the semi-stable part of Hn(T ,OT ) is greater than that of Hn-1(T ,OT ). Additionally, we also show singular versions of all the above statements.

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Type
research article
DOI
10.46298/epiga.2025.11595
Scopus ID

2-s2.0-105002766625

Author(s)
Patakfalvi, Zsolt  

École Polytechnique Fédérale de Lausanne

Date Issued

2025

Published in
Epijournal de Geometrie Algebrique
Volume

9

Article Number

15512

Subjects

relative canonical divisor

•

Semi-positivity

•

subadditivity of Kodaira dimension

•

uniruledness

Editorial or Peer reviewed

REVIEWED

Written at

EPFL

EPFL units
CAG  
Available on Infoscience
April 29, 2025
Use this identifier to reference this record
https://infoscience.epfl.ch/handle/20.500.14299/249509
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