Changements de temps de processus de markov

Sans résumé     

Tours de corps de classes et estimations de discriminants

Sans résuméLaboratoire Associé au C.N.R.S. n^o 226     

Sur certains sous-groupes de torsion de jacobiennes de courbes hyperelliptiques de genreg ≥ 1

We construct a family of hyperelliptic curves of genusg defined over Q whose Jacobians have a rational point of order 2g(2g+1). Forl = 2g ² + 5g + 5, we construct a family of genusg hyperelliptic curves defined over Q, such that their Jacobians have a rational point of orderl orl / 2 orl / 4. We also construct a hyperelliptic curve of genusg defined over Q, which does not belong to the previous family, and whose Jacobian has a rational point of orderl.      

Majorations de constantes de Steklov

Résumé Des majorations pour des constantes de Steklov sont établies. Sont considérés l'opérateur laplacien et l'opérateur type de l'élasticité. La méthode utilise comme intermédiaire des majorations de constantes de Poincaré.

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Summary Bounds for Steklov constants are given. One considers the Laplacian operator and the typical operator in elasticity. Results are obtained through bounds of Poincaré constants.

     

Etude de l'algorithme de Rémès en l'absence de condition de Haar

Summary Using a geometric interpretation, the unicity of a best approximation and the convergence of the Remes-algorithm without the Haar-condition are investigated (in the case of the Tchebycheff-approximation of a continuous function by a linear combination of two continuous functions). Replacing the Haar-condition by a weaker one which is generally satisfied, one obtains a convergence theorem that is weaker than the classical convergence theorem but is sufficient for the applicatons. A generalization of these results is indicated.     

Ensembles de Julia quadratiques de mesure de Lebesgue strictement positive

In this Note, we prove a variant of a conjecture stated in the thesis of Chéritat. The proof is based on results announced by Inou and Shishikura, and on earlier results of McMullen and of Chéritat. According to Chéritat's thesis, this allows us to complete a plan initiated by Douady and to show that there exist quadratic polynomials having a Julia set of positive Lebesgue measure. To cite this article: X. Buff, A. Chéritat, C. R. Acad. Sci. Paris, Ser. I 341 (2005).     

Des resultats de non existence de filtre de dimension finie

We show that a necessary condition for the existence of a universal finite dimensionally computable filter is that the Lie algebra $ naturally associated with the Zakai' equation, be finite dimensional at each point and that there exists a homomorphism from a Lie algebra of vectors fields onto.Conversely, we show that, if the signal is one dimensional and if S is infinite dimensional at each point of R, then only the constant functions are such the filter ntf can be realized as theimage of a diffusion.     

Limite de convexité d'une classe de Nevanlinna de fonctions analytiques

Sans résumé     

Sur l'analyse de l'évolution et de la stabilité de l'onde de choc dans la classe de perturbations exponentielles

Résumé On considère le problème de stabilité de l'onde de choc normale stationnaire dans un fluide idéal. On a prouvé, contrairement à [1], que le problème linéaire étudié par D'yakov [2] était correctement posé. On a également démontré que la stabilité exponentielle de l'onde de choc [2, 4, 5] n'existe pas. On a étudié tous les cas de dégénérescence des solutions du problème. On a établie que la considération détaillée de ces cas entraîne le changement de dépendance fonctionnelle de la solution des coordonnées de l'espace.     

Normalisation formelle de structures de Poisson

For any Poisson structure vanishing at a point, we give a local formal normal form which arises from the Levi decomposition of the dual of its linear approximation. So, we generalize the result of Weinstein which says that singular Poisson structures with linear approximation having a semi-simple dual, are formally linearizable.     

Rang moyen de familles de courbes elliptiques et lois de Sato-Tate

The goal of this note is to give some complements to an article of Fouvery and Pomykala: by anad-hoc method, they bound on average the rank of elliptic curves over in polynomial families:y ²=x ³=a(t)x+b(t) whent varies in under some generic conditions on the polynomials (over a(t),b(t). Here, by a more systematic treatment, we are able to relax most of these conditions, keeping only the natural one (the family is not geometricaly trivial). However, this result, specialized to the case treated by Fouvry and Pomykala, yields a better bound; our method depends on the distribution of the number of points in families of elliptic curves over finite fields (known as the vertical Sato-Tate law), which itself depends on the work of Deligne on the Weil conjectures.

     

Estimation de la densité de la solution de l'équation de Zakaï robuste

We study the behaviour in small time of the density of the robust Zakaï equation under the weak Hörmander's hypothesis. We avoid large deviations phenomena by supposing that the drift at the departure is equal to zero. We get an asymptotic expansion over the diagonal of the density where the observation is involved. In the case where all the terms of that asymptotic expansion are equal to zero, a pathology which is related to the Bismut's condition, we give an estimation of the decay by using a probabilistic analogous of the Gevrey methods.