Partial connection for $p$-torsion line bundles in charateristic $p>0$
An arXiv administrator withdrew this article because it is a duplicate of math/0608203.
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arXiv subjects
Hélène Esnault
Publications and source records attributed to Hélène Esnault.
Partial connection for $p$-torsion line bundles in characteristic $p>0$
The aim of this brief note for the volume in honor of S. S. Chern is to give a construction for $p$-torsion line bundles in characteristic $p>0$ which plays a similar rôle as the standard connection on an $n$-torsion line bundle in characteristic 0.
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On Nori's Fundamental Group Scheme
We determine the quotient category which is the representation category of the kernel of the homomorphism from Nori's fundamental group scheme to its étale and local parts. Pierre Deligne pointed out an error in the first version of this article. We profoundly thank him, in particular for sending us his enlightning example reproduced in Remark 2.4 2).
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Rational Singularities and Rational Points
If $X$ is a projective, geometrically irreducible variety defined over a finite field $\F_q$, such that it is smooth and its Chow group of 0-cycles fulfills base change, i.e. $CH_0(X\times_{\F_q}\bar{\F_q(X)})=\Q$, then the second author's theorem asserts that its number of rational points satisfies $|X(\F_q)| \equiv 1$ modulo $q$. If $X$ is not smooth, this is no longer true. Indeed J. Kollár constructed an example of a rationally connected surface over $\F_q$ without any rational points. Based on the work by Berthelot-Bloch and the second author computing the slope $<1$ piece of rigid cohomology, we define a notion of Witt-rational singularities in characteristic $p>0$. The theorem is then that if $X/\F_q$ is a projective, geometrically irreducible variety, such that it has Witt-rational singularities and its Chow group of 0-cycles fulfills base change, then $|X(\F_q)| \equiv 1$ modulo $q$.
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Characteristic 0 and p analogies, and some motivic cohomology
The purpose of this survey is to explain some recent results about analogies between characteristic 0 and characteristic $p>0$ geometry, and to discuss an infinitesimal variant of motivic cohomology. More specifically, we review results showing that the big de Rham Witt complex of a field is a complex of 0-cycles (section 2), as well as results showing analogies between char. $p>0$ and char. 0 geometry. Some of those results determine local cohomology invariants of singular varieties (section 3) and others are concerned with congruences modulo $q$-powers for the number of rational points of varieties defined over $\F_q$ (section 1).
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Gauß-Manin connection and Tannaka duality
In analogy with the étale fundamental groups, we express the Gauß-Manin connection for $H^1$ in Tannaka terms. One difficulty is that unlike for fundamental groups, the Tannaka group scheme of relative connections, and the groupoid schemes of absolute connections and of connections on the base do not form an exact sequence. We added a theorem due to Pierre Deligne which improves the main result of the article. It allows to compare the Tannaka group scheme of relative connections with the kernel of the map from the groupoid scheme of absolute connections to the one of the base.
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On Witt vector cohomology for singular varieties
Over a perfect field $k$ of characteristic $p > 0$, we construct a ``Witt vector cohomology with compact supports'' for separated $k$-schemes of finite type, extending (after tensorisation with $\mathbb{Q}$) the classical theory for proper $k$-schemes. We define a canonical morphism from rigid cohomology with compact supports to Witt vector cohomology with compact supports, and we prove that it provides an identification between the latter and the slope $< 1$ part of the former. Over a finite field, this allows one to compute congruences for the number of rational points in special examples. In particular, the congruence modulo the cardinality of the finite field of the number of rational points of a theta divisor on an abelian variety does not depend on the choice of the theta divisor. This answers positively a question by J.-P. Serre.
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On Motives Associated to Graph Polynomials
The appearance of multiple zeta values in anomalous dimensions and $β$-functions of renormalizable quantum field theories has given evidence towards a motivic interpretation of these renormalization group functions. In this paper we start to hunt the motive, restricting our attention to a subclass of graphs in four dimensional scalar field theory which give scheme independent contributions to the above functions.
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A remark on higher congruences for the number of rational points of varieties defined over a finite field
We show that there is no simple congruence formula for the number of points of the mod p reduction of a regular model of a smooth proper variety defined over a local field with $\ell$-adic cohomology supported in codimension $\ge κ$.
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Künneth projectors for open varieties
We consider correspondences on smooth quasiprojective varieties $U$. An algebraic cycle inducing the Künneth projector onto $H^1(U)$ is constructed. Assuming normal crossings at infinity, the existence of relative motivic cohomology is shown to imply the independence of $\ell$ for traces of open correspondences.
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Appendix to ``Deligne's integrality theorem in unequal characteristic and rational points over finite fields'' by Hélène Esnault
One extends P. Deligne's notion of integrality over a finite field for a $\ell$-adic sheaf on a scheme of finite type over a local field with finite residue field. One shows that this integrality notion is preserved by $Rf_!$, as it is over a finite field after P. Deligne.
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Variation on Artin's vanishing theorem
We give a proof of Artin's vanishing theorem in characteristic zero, based on Deligne's Riemann-Hilbert correspondence. Just as a curiosity.
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Appendix to "Congruences for rational points on varieties over finite fields" by N. Fakhruddin and C. S. Rajan
We give a positive answer to the questions 4.4 on Hodge type and 4.2 on divisibility of the eigenvalues of Frobenius of the article by N. Fakhruddin and C. S. Rajan (NT/0402230).
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Local Fourier transforms and rigidity for D-modules
Local Fourier trnasforms, analogous to the $\ell$-adic Fourier transforms, are constructed for connections over $k((t))$. Following a program of Katz, a meromorphic connection on a curve is shown to be rigid, i.e. determined by local data at the singularities, if and only if a certain cohomological condition is satisfied. We show that the index of rigidity is invariant under Fourier transform.
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Some elementary theorems about divisibility of 0-cycles on abelian varieties defined over finite fields
If $X$ is an abelian variety over a field and $L$ is an invertible sheaf, we know that the degree of the 0-cycle $L^g$ is divisible by $g!$. As a 0-cycle, it is not, even over a field of cohomological dimension 1. But we show that over a finite field there is perhaps some hope.
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$ε$-factors for Gauss-Manin determinants
This is the last version of AG/0111277. Here the old abstract: We define $ε$-factors in the de Rham setting and calculate the determinant of the Gauß-Manin connection for a family of (affine) curves and a vector bundle equipped with a flat connection.
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Characteristic classes of flat bundles and determinant of the Gauss-Manin connection
The purpose of this note is to give a survey on recent progress on characteristic classes of flat bundles, and how they behave in a family.
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Cohomological divisibility and point count divisibility
We show that Ax-Katz divisibility on the number of rational points of a variety defined over a finite field by equations of low degrees comes from divisibility of the eigenvalues of the Frobenius action of the $\ell$-adic cohomology with compact supports. This completes the motivc picture, as we knew already that in characteristic 0, the Hodge type was the expected one. [version 2 as the title contained a typo..., else no changes]
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