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Algèbre et géométrie

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Aujourd'hui

Lie groups definable in o-minimal theories

Sophie Germain salle 1016.

In this talk we will work out a complete characterization of which Lie groups admit a “definable copy”. This is, characterize for which Lie groups G one can find a group H definable in an o-minimal expansion of the real field, and such that G and H are isomorphic. When the answer is positive, the definable copy H that we find is definable in the language of exponential ordered fields, and it is such that any Lie automorphism of H is definable.

Piecewise Interpretable Hilbert Spaces (II)

Sophie Germain salle 1016.

We continue the discussion of piecewise interpretable Hilbert spaces from the Monday seminar. We will prove the main structure theorem of `Piecewise Interpretable Hilbert Spaces' (C., Hrushovski) which analyses a scattered piecewise interpretable Hilbert space into asymptotically free subspaces. We will clarify the model theoretic content of this theorem, highlighting the roles of one-basedness and strong minimality. We will also study its representation theoretic content, establishing a connection with induced represetnations. We will see that this theorem generalises a theorem of Tsankov about unitary representations of oligomorphic groups. This is […]

Quasi-groupes de Frobenius dimensionnels

Sophie Germain salle 1016.

Dans cet exposé, nous présenterons une généralisation des groupes de Frobenius : les quasi-groupes de Frobenius. On dit qu'une paire de groupes C < G est un quasi-groupe de Frobenius si C est d'indice fini dans son normalisateur (dans G) et s'il satisfait la propriété TI, i.e, deux conjugués distincts de C s'intersectent trivialement. Du point de vue de la théorie des modèles, nous travaillerons dans un contexte où l'existence d'une bonne notion de dimension (finie) sur les ensembles définissables est assurée (ce qui englobe les univers rangés et les […]

Metric valued fields in continuous logic

Sophie Germain salle 1016.

By work of Itaï Ben Yaacov complete valued fields with value groups embedded in the real numbers can be viewed as metric structures in continuous logic. For technical reasons one has to consider the projective line over such a field rather than the field itself. In this talk we introduce the above setting and give a classification of the complete theories of metric valued fields in equicharacteristic 0 in terms of their residue field and value group. This can also be seen as an approximate Ax-Kochen-Ershov principle. If time permits, […]

Olivier de Gaay Fortman, raconte-moi la conjecture de Hodge entière !

En salle W au DMA, ou sur Zoom

La conjecture de Hodge reste une conjecture largement ouverte et mystérieuse. Dans cet exposé je parlerai d’un énoncé encore plus fort : la « Conjecture de Hodge Entière ». Bien que fausse en général, il est important de se demander pour quel type de variétés complexes projectives elle est vraie. Je la prouverai pour les classes de homologie de degré deux sur la jacobienne d’une courbe. Enfin, je parlerai de son analogue pour les variétés algébriques réelles: la « Conjecture de Hodge Entière Réelle ».

Existential theories of henselian fields, parameters welcome

Sophie Germain salle 1016

The first-order theories of local fields of positive characteristic, i.e. fields of Laurent series over finite fields, are far less well understood than their characteristic zero analogues: the fields of real, complex and p-adic numbers. On the other hand, the existential theory of an equicharacteristic henselian valued field in the language of valued fields is controlled by the existential theory of its residue field. One is decidable if and only if the other is decidable. When we add a parameter to the language, things get more complicated. Denef and Schoutens […]

Complexity of l-adic sheaves

To a complex of l-adic sheaves on a quasi-projective variety one associate an integer, its complexity. The main result on the complexity is that it is continuous with tensor product, pullback and pushforward, providing effective version of the constructibility theorems in l-adic cohomology. Another key feature is that the complexity bounds the dimensions of the cohomology groups of the complex. This can be used to prove equidistribution results for exponential sums over finite fields. This is due to Will Sawin, written up in collaboration with Javier Fresán and Emmanuel Kowalski.

Skew-invariant curves and algebraic independence

A σ-variety over a difference field (K,σ) is a pair (X,φ) consisting of an algebraic variety X over K and φ:X → X^σ is a regular map from X to its transform Xσ under σ. A subvariety Y ⊆ X is skew-invariant if φ(Y) ⊆ Y^σ. In earlier work with Alice Medvedev we gave a procedure to describe skew-invariant varieties of σ-varieties of the form (𝔸^n,φ) where φ(x_1,...,x_n) = (P_1(x_1),...,P_n(x_n)). The most important case, from which the others may be deduced, is that of n = 2. In the present […]

Sharp o-minimality: towards an arithmetically tame geometry

Salle W (ENS) et Zoom

Over the last 15 years a remarkable link between o-minimality and algebraic/arithmetic geometry has been unfolding following the discovery of Pila-Wilkie's counting theorem and its applications around unlikely intersections, functional transcendence etc. While the counting theorem is nearly optimal in general, Wilkie has conjectured a much sharper form in the structure R_exp. There is a folklore expectation that such sharper bounds should hold in structures "coming from geometry", but for lack of a general formalism explicit conjectures have been made only for specific structures. I will describe a refinement of […]

Le théorème du corps gauche de Zilber / Zilber’s Skew-Field Theorem (joint with Frank Wagner)

Sophie Germain salle 1016.

Le théorème du corps est l'observation qu'un groupe de rang de Morley fini connexe, résoluble, et non nilpotent, interprète un corps infini. Par d'autres résultats classiques, le corps est commutatif et même algébriquement clos. Le théorème du corps est souvent vu comme corollaire du «théorème d'engendrement par des indécomposables» mais c'est une erreur car il en est indépendant. Il a quelques variantes, des théorèmes de linéarisation d'actions de groupes. Je donnerai un énoncé qui généralise naturellement tous les résultats «à la Zilber». C'est un résultat de linéarisation de bimodules, dans […]

Cercles isométriques mais contractiles dans les cônes asymptotiques des groupes

salle 1016 Sophie Germain

La contractilité de tous les cercles dans les cônes asymptotiques d’un groupe G de type fini implique que G est de présentation finie avec fonction de Dehn au plus polynomiale.  Le distorsion métrique de tous ces cercles est une propriété plus forte qui implique que G est fortement raccourci (“strongly shortcut”).  La propriété fortement raccourci est satisfaite par diverses familles de groupes de courbure négative ou nulle, notamment les groupes hyperboliques, CAT(0), Helly, et systoliques, mais elle est aussi satisfaite par le groupe de Heisenberg discret.     Je discuterai d'un […]

On the Borel complexity of modules

Sophie Germain salle 1016.

We prove that among all countable, commutative rings R (with unit) the theory of R-modules is not Borel complete if and only if there are only countably many non-isomorphic countable R-modules. From the proof, we obtain a succinct proof that the class of torsion free abelian groups is Borel complete. The results above follow from some general machinery that we expect to have applications in other algebraic settings. Here, we also show that for an arbitrary countable ring R, the class of left R-modules equipped with an endomorphism is Borel […]