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# Documents  Karpenko, Nikita | enregistrements trouvés : 8

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## $H^{3}$ non ramifié et cycles de codimension 2 Colliot-Thélène, Jean-Louis | CIRM H

Post-edited

Research talks;Algebraic and Complex Geometry;Topology

Le troisième groupe de cohomologie non ramifiée d'une variété lisse, à coefficients dans les racines de l'unité tordues deux fois, intervient dans plusieurs articles récents, en particulier en relation avec le groupe de Chow de codimension 2. On fera un tour d'horizon : espaces homogènes de groupes algébriques linéaires; variétés rationnellement connexes sur les complexes; images d'applications cycle sur les complexes, sur un corps fini, sur un corps de nombres. Le troisième groupe de cohomologie non ramifiée d'une variété lisse, à coefficients dans les racines de l'unité tordues deux fois, intervient dans plusieurs articles récents, en particulier en relation avec le groupe de Chow de codimension 2. On fera un tour d'horizon : espaces homogènes de groupes algébriques linéaires; variétés rationnellement connexes sur les complexes; images d'applications cycle sur les complexes, sur un corps fini, sur un ...

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## Interview au CIRM : Jean-Pierre Serre avec Jean-Louis Colliot-Thélène Serre, Jean-Pierre ; Colliot-Thélène, Jean-Louis | CIRM H

Post-edited

Outreach;Mathematics Education and Popularization of Mathematics

Jean-Pierre Serre est un mathématicien français, plus jeune Médaille Fields en 1954, il fut également le premier lauréat du Prix Abel en 2003.

Jean-Louis Colliot-Thélène est un mathématicien français, Directeur de recherches à l'Université Paris-Sud, il étudie principalement la théorie des nombres et la géométrie algébrique.

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## The rationality problem for forms of moduli spaces of stable marked curves Reichstein, Zinovy | CIRM H

Multi angle

Research talks;Algebraic and Complex Geometry;Topology

Let $\overline{M_{g,n}}$ be the moduli space of stable curves of genus $g$ with $n$ marked points. It is a classical problem in algebraic geometry to determine which of these spaces are rational over $\mathbb{C}$. In this talk, based on joint work with Mathieu Florence, I will address the rationality problem for twisted forms of $\overline{M_{g,n}}$ . Twisted forms of $\overline{M_{g,n}}$ are of interest because they shed light on the arithmetic geometry of $\overline{M_{g,n}}$, and because they are coarse moduli spaces for natural moduli problems in their own right. A classical result of Yu. I. Manin and P. Swinnerton-Dyer asserts that every form of $\overline{M_{0,5}}$ is rational. (Recall that the $F$-forms $\overline{M_{0,5}}$ are precisely the del Pezzo surfaces of degree 5 defined over $F$.) Mathieu Florence and I have proved the following generalization of this result.
Let $n\geq 5$ is an integer, and $F$ is an infinite field of characteristic $\neq$ 2.
(a) If $n$ is odd, then every twisted $F$-form of $\overline{M_{0,n}}$ is rational over $F$.
(b) If $n$ is even, there exists a field extension $F/k$ and a twisted $F$-form of $\overline{M_{0,n}}$ which is unirational but not retract rational over $F$.
We also have similar results for forms of $\overline{M_{g,n}}$ , where $g \leq 5$ (for small $n$ ). In the talk, I will survey the geometric results we need about $\overline{M_{g,n}}$ , explain how our problem reduces to the Noether problem for certain twisted goups, and how this Noether problem can (sometimes) be solved.

Keywords: rationality - moduli spaces of marked curves - Galois cohomology - Noether's problem
Let $\overline{M_{g,n}}$ be the moduli space of stable curves of genus $g$ with $n$ marked points. It is a classical problem in algebraic geometry to determine which of these spaces are rational over $\mathbb{C}$. In this talk, based on joint work with Mathieu Florence, I will address the rationality problem for twisted forms of $\overline{M_{g,n}}$ . Twisted forms of $\overline{M_{g,n}}$ are of interest because they shed light on the ...

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## Subtle Stiefel-Whitney classes and the J-invariant of quadrics Vishik, Alexander | CIRM H

Multi angle

Research Talks;Algebraic and Complex Geometry

I will discuss the new ?subtle? version of Stiefel-Whitney classes introduced by Alexander Smirnov and me. In contrast to the classical classes of Delzant and Milnor, our classes see the powers of the fundamental ideal, as well as the Arason invariant and its higher analogues, and permit to describe the motives of the torsor and the highest Grassmannian associated to a quadratic form. I will consider in more details the relation of these classes to the J-invariant of quadrics. This invariant defined in terms of rationality of the Chow group elements of the highest Grassmannian contains the most basic qualitative information on a quadric. I will discuss the new ?subtle? version of Stiefel-Whitney classes introduced by Alexander Smirnov and me. In contrast to the classical classes of Delzant and Milnor, our classes see the powers of the fundamental ideal, as well as the Arason invariant and its higher analogues, and permit to describe the motives of the torsor and the highest Grassmannian associated to a quadratic form. I will consider in more details the relation of these classes ...

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## On examples of varieties that are not stably rational Pirutka, Alena | CIRM H

Multi angle

Research Talks;Algebraic and Complex Geometry;Mathematical Physics;Topology

A variety X is stably rational if a product of X and some projective space is rational. There exists examples of stably rational non rational complex varieties. In this talk we will discuss recent series of examples of varieties, which are not stably rational and not even retract rational. The proofs involve studying the properties of Chow groups of zero-cycles and the diagonal decomposition. As concrete examples, we will discuss some quartic double solids (C. Voisin), quartic threefolds (a joint work with Colliot-Thélène), some hypersurfaces (Totaro) and others. A variety X is stably rational if a product of X and some projective space is rational. There exists examples of stably rational non rational complex varieties. In this talk we will discuss recent series of examples of varieties, which are not stably rational and not even retract rational. The proofs involve studying the properties of Chow groups of zero-cycles and the diagonal decomposition. As concrete examples, we will discuss some quartic ...

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## Decomposition of the diagonal and applications Totaro, Burt | CIRM H

Multi angle

Research talks;Algebraic and Complex Geometry

Decomposition of the diagonal is a basic method in the theory of algebraic cycles. The method relates the birational geometry of a variety to properties of the Chow groups. One recent application is that the Chow ring of a finite group can depend nontrivially on the base field, even for fields containing the algebraic closure of $Q$. Another application is that a very general complex hypersurface in $P^{n+1}$ of degree at least about 2n/3 is not stably rational. Decomposition of the diagonal is a basic method in the theory of algebraic cycles. The method relates the birational geometry of a variety to properties of the Chow groups. One recent application is that the Chow ring of a finite group can depend nontrivially on the base field, even for fields containing the algebraic closure of $Q$. Another application is that a very general complex hypersurface in $P^{n+1}$ of degree at least about 2n/3 is not ...

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## The rational motivic sphere spectrum and motivic Serre finiteness Levine, Marc | CIRM H

Multi angle

Research talks;Algebraic and Complex Geometry;Topology

After inverting 2, the motivic sphere spectrum splits into a plus part and a minus part with respect to a certain natural involution. Cisinsky and Déglise have shown that, with rational coefficients, the plus part is given by rational motivic cohomlogy. With Ananyevskiy and Panin, we have computed the minus part with rational coefficients as being given by rational Witt-theory. In particular, this shows that the rational bi-graded homotopy sheaves of the minus sphere are concentrated in bi-degree (n,n). This may be rephrased as saying that the graded homotopy sheaves of the minus sphere in strictly positive topological degree are torsion. Combined with the result of Cisinski-Déglise mentioned above, this shows that the graded homotopy sheaves of the sphere spectrum in strictly positive topological degree and non-negative Tate degree are torsion, an analog of the classical theorem of Serre, that the stable homotopy groups of spheres in strictly positive degree are finite. After inverting 2, the motivic sphere spectrum splits into a plus part and a minus part with respect to a certain natural involution. Cisinsky and Déglise have shown that, with rational coefficients, the plus part is given by rational motivic cohomlogy. With Ananyevskiy and Panin, we have computed the minus part with rational coefficients as being given by rational Witt-theory. In particular, this shows that the rational bi-graded homotopy ...

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## The algebraic and geometric theory of quadratic forms Elman, Richard S. ; Karpenko, Nikita ; Merkurjev, Alexander | Amercian Mathematical Society 2008

Ouvrage

- viii, 435 p.
ISBN 978-0-8218-4329-1

American mathematical society colloquium publications , 0056

Localisation : Collection 1er étage

forme quadratique # forme bilinéaire # théorie de Chow # motif Chow # opération Steenrod # cycle algébrique # intersection

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