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Computation of étale cohomology on curves with constructible coefficients

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Jinbi Jin
University of Leiden/MPI
Wed, 2017-04-19 11:15 - 12:15
MPIM Lecture Hall
Parent event: 
Number theory lunch seminar

The classical point-counting algorithms by Schoof and Pila for curves compute the number
#X(F_q) of F_q-points on a fixed smooth projective curve X in time polynomial in log q.
A motivating question for this talk is whether such efficient algorithms exist for more general
varieties X. Via the Lefschetz trace formula, it suffices to be able to efficiently compute the
étale cohomology groups on X for suitable coefficients. However, the currently known
algorithms (Poonen-Testa-van Luijk, Madore-Orgogozo) have no known upper bounds on
their running time.

In this talk, as a first step towards a more efficient algorithm, we describe how to (algorithmically)
compute the étale cohomology on a smooth affine curve X with coefficients in a constructible
abelian étale sheaf A on X of torsion coprime to the characteristic of the base field, using the
interpretation of the elements of the first étale cohomology as A-torsors on X. Most of this talk
is based on part of my dissertation, which was supervised by Bas Edixhoven and Lenny Taelman.


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