Soit $F(X,Y)$ une forme binaire à coefficients entiers, de discriminant non nul, de degré $\geq 3$.
A quelle condition, nécessaire et suffisante, existe-t-il une forme $G (X,Y)$, non $GL(2, Z)$-équivalente à $F(X,Y)$, telle qu’on ait l’égalité des images $F(Z^2) = G(Z^2)$ ?
La condition trouvée repose sur l’existence d’un élément d’ordre $3$, d’un certain type, dans le groupe d’automorphismes de $F$.
Travail en commun avec Peter Koymans.

Pour modéliser les premiers, nous considérerons au cours de l’exposé de telles questions de nature statistique concernant la suite des entiers sans facteur carré (SFC), parmi d’autres suites « criblées ». J’espère pouvoir motiver et expliquer notre résultat principal inconditionnel qui énonce que le nombre de SFC dans les intervalles courts uniformément aléatoires suit en effet une loi Gaussienne, ce faisant résolvant plusieurs problèmes de R.R. Hall.

Ceci est un travail en commun avec O. Gorodetsky et B. Rodgers.

We present a new elementary algorithm for computing $M(x) = \sum_{n \leq x} \mu(n),$ where $\mu(n)$ is the Möbius function. Our algorithm takes
\[\begin{aligned}
\mathrm{time} \ \ O_\epsilon\left(x^{\frac{3}{5}} (\log x)^{\frac{3}{5}+\epsilon} \right)
\ \ \mathrm{and}\ \ \mathrm{space} \ \ O\left(x^{\frac{3}{10}} (\log x)^{\frac{13}{10}}
\right)\end{aligned},\] which improves on existing combinatorial algorithms. While there is an analytic algorithm due to Lagarias-Odlyzko with computations based on the integrals of $\zeta(s)$ that only takes time $O(x^{1/2 + \epsilon})$, our algorithm has the advantage of being easier to implement. The new approach roughly amounts to analyzing the difference between a model that we obtain via Diophantine approximation and reality, and showing that it has a simple description in terms of congruence classes and segments. This simple description allows us to compute the difference quickly by means of a table lookup. This talk is based on joint work with Harald Andrés Helfgott.

Les journées SL2R « Théorie des Représentations et Analyse Harmonique » se tiendront à l’Université du Lorraine les jeudi 12 et vendredi 13 Mai 2020. Ce colloque tournant — entre les universités de Strasbourg, Lorraine, Luxembourg et Reims (=SL2R) — regroupe deux à trois fois par an les mathématiciens de ces quatre universités travaillant en théorie des représentations et en analyse harmonique.

Cette édition sera en l’honneur du Professeur Jacques Faraut, l’un des fondateurs du précurseur des journées SL2R : le séminaire Nancy-Strasbourg (organisé par P. Eymard – R. Takahashi de Nancy et J. Faraut – G. Schiffmann de Strasbourg).

Pour participer, merci de remplir le formulaire form et de l’envoyer à khalid.koufany@univ-lorraine.fr et wolfgang.bertram@univ-lorraine.fr

Le programme, les participants et toutes les informations pratiques sont à retrouver sur le site web de l’événement : http://khalid-koufany.perso.math.cnrs.fr/SL2R2022/index.html

Given an arithmetic function $\theta$, we consider the set
$$ \mathcal{B}_\theta = \Bigl\{n\ge 1: p|n \Rightarrow p\le \theta\Bigl(\prod_{q<p \atop q^\alpha || n} q^\alpha \Bigr) \Bigr\},$$
where $p$ and $q$ denote primes. Depending on the choice of $\theta$, the possible sets $\mathcal{B}_\theta$ include the set of prime powers, almost primes, friable numbers, dense numbers, and practical numbers.
We will discuss (1) asymptotic results for the counting function of $\mathcal{B}_\theta$, (2) a generalization of the Siegel-Walfisz theorem, and (3) the normal order of the number of prime factors of integers in $\mathcal{B}_\theta$.

Let G be a non-compact connected semisimple real Lie group with finite center. Suppose L is a non-compact connected closed subgroup of G acting transitively on a symmetric space G/H such that L\cap H is compact. We will describe the action on L/L\cap H of a Dirac operator D_{G/H}(E) acting on sections of an E-twist of the spin bundle over G/H. We will illustrate the results in an SL(2) example.  This is joint work with Salah Mehdi.

The quantum gauge theory is the current basis for explaining the interactions of elementary particles. Among the four fundamental interactions: electromagnetic, weak, strong, and gravitational, for the last two, a proper mathematical construction does not yet exist. Quantum gauge theory is based on three different branches of mathematics: 1) geometry, 2) group theory, and 3) functional analysis.

The strong interactions are studied in the SU(3) quantum Yang-Mills theory but an axiomatic QFT with the mass gap and color confinement does not yet exist. In the classical field theory model, the gluon^1 is a massless particle but one can not see the color-charge^2 of a particle in nature, and then the gluons must be in the bound states, forming massive particles. This is the mass gap problem. The mass gap is the difference in energy between the vacuum and the next lowest energy state. The energy of the vacuum is zero by definition, and the mass gap is the mass of the lightest particle. However, the color confinement postulated permits only bound states of gluons, it is an obvious contradiction.

In this presentation, I would like to discuss the fact that if we replace: 1)  Minkowskian geometry with the de Sitterian geometry, 2) Poincaré group with de Sitter group and 3) Hilbert space quantization with the Krein space quantization, an axiomatic quantum Yang-Mills theory with a mass gap and the color confinement can be constructed. It is important to note that the ambient space formalism allows us to make this construction possible.

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1 . A gluon is an elementary particle that acts as the exchange particle (or gauge boson) for the strong force between quarks.

2 . Color charge is a property of quarks and gluons that is related to the particles’ strong interactions in the theory of quantum chromodynamics

Considering partial character sums as defining a family of random processes (by choosing the characters randomly from some set), it becomes natural to ask questions about the limiting distribution. I’ll describe this in some contexts and give examples of what we can find in the support. This is largely work of Ayesha Hussain, but also some joint work.