Sur la densité d'état de l'opérateur de Schrödinger quasi-périodique unidimensionnel

We prove two results on the density of states of the discrete one dimensional quasi-periodic Schrödinger equation with an analytic potential and Diophantine frequencies in the perturbed regime. On the one hand, we prove that this function has the behavior of a Hölder-$\frac{1}{2}$ function. On the other, we show that the length of the gaps has a sub-exponential estimate which depends on its label given by the gap-labeling theorem. To cite this article: S. Hadj Amor, C. R. Acad. Sci. Paris, Ser. I 343 (2006).     

Un théorème de Vitali-Hahn-Saks pour les semimartingales

Summary Let (, , P) be a complete probability space; let _t0 be an increasing right-continuous family of -complete sub--fields of ; let be a sequence of semimartingales. Assume that for all positive t and for all bounded predictable processes H, the r.v.'s converge in probability to a limit J(t, H) when n tends to infinity. Then there exists a semimartingale X such that, for all t and H, J(t, H)= .     

Un théorème de dualité

Sans résumé Herrn H. Hopf zum sechzigsten Geburtstag gewidmet     

Un théorème de Choquet-Deny pour les groupes moyennables

Résumé SoitG un groupe moyennable connexe, locallement compact, à base dénombrable. Soit une mesure positive sur les boréliens deG. Nous étudions les fonctions boréliennes positivesh vérifiant: g G, . Sous de bonnes hypothèses sur , nous obtenons, pour ces fonctions, une représentation intégrale à l'aide d'exponentielles.

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Summary LetG be a connected locally compact separable amenable group. Let be a positive measure on the Borel -field ofG. We study the positive Borel functionsh onG which satisfy: g G, . Under smooth assumptions on , we establish an integral representation of these functions in term of exponentials.

     

Un théorème de représentation pour les martingales discontinues

We consider a fixed integer-valued random measure (also called a random point process). We represent any local martingale as the sum of a stochastic integral with respect to this random measure, and of a local martingale which does not jump on the support of the random measure.     

Un théorème de décomposition pour les polaires génériques d'une courbe plane

In this Note we state a decomposition theorem into bunches of branches (i.e. analytically irreducible germs) for the generic polar curves of a reduced germ of a plane analytic curve, with equation ƒ(x, y) = 0. They are the curves with equation ∂f(x, y)/∂yτ∂f(x, y)/∂x = 0 with generic τ. All the branches of the same bunch have the same contact with each branch of C. A number of the first terms of the Puiseux expansion of each branch of the polar is therefore independent of τ; this number depends only on the bunch to which the branch belongs. This generalizes results of H.J.S. Smith [8], M. Merle [7] (where C is a branch), E. Casas [1], F. Delgado [2]. We show by an example that it is also optimal. We have shown elsewhere that it implies the results of Lê-Michel-Weber [6].     

Un algorithme dérivé de l'algorithme de Metropolis

Statistical physics and stochastic modelling in economic sciences share the same mathematical bases given by the Gibbs distribution, but system characteristics are different. For instance, an economic system can be described by a Bose–Einstein statistics with few non-degenerate states and an infinitesimal “temperature”; under such conditions, the approximation of the most probable configuration is invalid. Therefore, the calculus of the exact solution needs using a Metropolis algorithm, which estimates a Gibbs distribution. This paper presents a much more efficient algorithm. For small systems, the exact distribution on the canonical set can be computed, and then this distribution is compared to the solutions of the old and new algorithms.     

Paradoxe de Klein pour l'équation de Klein–Gordon chargée : superradiance et opérateur de diffusion

We develop the scattering theory for the charged Klein–Gordon equation on ${\mathbb{R}}_t\times {\mathbb{R}}_x$, when the electrostatic potential $A(x)$ has different asymptotics $a^{\pm }$ as $x\to \pm \infty$. In this case, the conserved energy is not positive definite (Klein Paradox). We construct the spectral representation for the harmonic equation, and we establish the existence of a Scattering Operator the symbol of which has a norm strictly larger than 1, for the frequencies in $(a^{?}\text{,}{a}^{+})$. These results can be applied to the DeSitter–Reissner–Nordstrøm metric, to justify the notion of superradiance of the charged black-holes. To cite this article: A. Bachelot, C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

Comportement semi-classique pour l'opérateur de Schrödinger à potentiel périodique

In this paper, the Schrödinger operator with a periodic potential is considered. Let V a smooth periodic function, we study the semi-classical behavior for a continuum spectrum of −h² Δ + V (h → 0). We are interested by localization and width of bands. We give the interaction matrix up to an exponentially small error, measured by Agmon's distance between the wells. A detailed investigation of the spectrum is made for the case where V has one nondegenerate minima per unit cell. We also investigate the spectral properties of −h² Δ + V + ΔV, where ΔV is a smooth positive perturbation with compact support.     

Un théorème de Torelli infinitésimal à coefficients

We generalize an infinitesimal Torelli theorem for hypersurfaces of high degree of M. Green (1985) [1], for twisted coefficients.     

Un théorème sur les séries de Dirichlet

We considerk Dirichlet series a _j (n)n ^–s (1jk),k2. We suppose that for eachj the series a _j (n)n ^–s converges fors=s _j =j+it _j , and that Max j<1/(k–1). We prove that the (Dirichlet) product of these series converges uniformly on every bounded segment of the line es = (₁+...+ _k )/k+1–1/k and we estimate the rate of convergence. The number 1–1/k cannot be replaced by a smaller one.     

Sur le théorème de Berry-Esseen pour les suites faiblement dépendantes

Résumé Nous étendons la méthode de démonstration du théorème de Berry-Esseen proposée par Bergström aux suites de variables aléatoires faiblement dépendantes. En particulier, nous montrons que, pour les suites stationnaires de variables aléatoires réelles bornées, la vitesse de convergence dans le théorème limite central en distance de Lévy est de l'ordre den ^–1/2 dès que la suite ( _p)p>0 des coefficients de mélange uniforme satisfait la condition _p>0 p _p <

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About the Berry-Esseen Theorem for weakly dependent sequences

We extend the method of Bergström for the rates of convergence in the central limit theorem to weakly dependent sequences. In particular, we prove that, for stationary and uniformly mixing sequences of real-valued and bounded random variables, the rate of convergence in the central limit theorem is of the order ofn ^–1/2 as soon as the sequence ( _p)p>0 of uniform mixing coefficients satisfies _p>0 p _p <.

     

Un théorème de Harris et Segal pour un anneau d'entiers algébriques exceptionnel

Let K be a number field, O_K the ring of the integers of K, ℓ a prime integer and Z_(ℓ) the localisation of Z at ℓ. Harris and Segal [4] proved that there exists infinitely many primes p of O_K such that the natural morphism K_i(O_K) ⊗ Z_(ℓ) → K_i(O_K /p) ⊗ Z_(ℓ) in algebraic K-theory is split surjective for i > 0, except if ℓ = 1 and K is exceptional. In this Note, we prove that the Harris-Segal theorem is still true for ℓ = 2 in the exceptional case, if we replace algebraic K-theory by orthogonal K-theory defined by Karoubi [5]. Thanks to [3], we can then determine a direct summand of the 2-torsion of KO_n(O_K).