4-variétés parallélisables sans structure complexe dont l'espace twistoriel est complexe

The aim of this Note is to give some applications of twistor theory about existence or non-existence of complex structures. We slightly improve Yau's result [Topology 15 (1976) 51–53] by giving the full list of compact parallelizable real 4-manifolds with a complex structure. On the other hand, we give a family of parallelizable 4-manifolds without complex structure but whose product with the sphere ${\mathbb{S}}^2$ is complex. To cite this article: G. Deschamps, C. R. Acad. Sci. Paris, Ser. I 341 (2005).     

Closed transversals and the genus of closed leaves in foliated 3-manifolds

The classical local theory of integrable 2-plane fields in 3-space leads to interesting qualitative questions about the global properties of solutions surface (i.e., leaves of a foliation) on 3-manifolds. It is now known that foliations admitting a closed leaf of suitably high genus abound on all closed or orientable 3-manifolds that are not rational homology spheres (S. Goodman, Proc. Nat. Acad. Sci. U.S.A.71 (1974), 4414–4415), and this leads to natural questions about the “positions” of such leaves relative to the rest of the foliation. One such question, suggested by Goodman's theorem on closed transversals (S. Goodman, ibid.), is considered here.     

A subspace theorem approach to integral points on curvesPoints entiers sur les courbes et théorème des sous-espaces

We present a proof of Siegel's theorem on integral points on affine curves, through the Schmidt subspace theorem, rather than Roth's theorem. This approach allows one to work only on curves, avoiding the embedding into Jacobians and the subsequent use of tools from the arithmetic of Abelian varieties. To cite this article: P. Corvaja, U. Zannier, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 267–271.     

Opérateurs linéaires continus d'extension dans des intersections de classes ultradifférentiables

Mityagin proved that the Tchebyshev polynomials form a Schauder basis of the space of C^∞ functions on the interval [?1,1]. Thus, he deduced an explicit continuous linear extension operator. These results were extended, by Goncharov, to compact sets which do not satisfy the Markov's inequalities. On the other hand, Tidten gave examples of compact sets for which there is no continuous linear extension operator. In this Note, we generalize these works to ultradifferentiable classes of functions built on the model of the intersection of non quasi-analytic Gevrey classes. We get, among other things, a Whitney linear extension theorem for ultradifferentiable jets of Beurling type. To cite this article: P. Beaugendre, C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

An ergodic theorem for semigroups of nonexpansive mappings in a Hilbert space

J. B. Baillon [C. R. Acad. Sci. Paris Ser. A.280 (1975), 1511–1514] proved an ergodic theorem for a single nonexpansive mapping in a Hilbert space, which is a nonlinear version of von Neumann's mean ergodic theorem. In this paper, we study the ergodic behavior of a semigroup of nonexpansive mappings. We try to find a sequence of means on the semigroup, generalizing the Cesàro means on $\text{N}$, such that the corresponding sequence of nonexpansive mappings converges to a projection onto the set of common fixed-points. Our method of proof is an appropriate modification of A. Pazy's proof [Israel J. Math.26 (1977), 197–204] of Baillon's theorem.     

An extension of Brouwer's fixed point theorem allowing discontinuities

In this article, we extend Brouwer's fixed point theorem – which states that every continuous mapping $\text{f}\text{:B→B}$ (a closed ball of $\text{R}{}^{\mathrm{n}}$) must have a fixed point – by allowing discontinuities of f, and we apply this extension to equilibrium theory in Economics. To cite this article: P. Bich, C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

Étude d'une forme volume naturelle sur l'espace de représentations du groupe d'un nœud dans SU(2)

For a knot K in S³, we construct in the line of Casson – or more precisely taking into account Lin's (J. Differential Geom. 35 (1992) 337–357) and Heusener's (Topology Appl. 127 (2003) 175–197) further works – a 1-volume form on the SU(2)-representation space of the group of K and we show how to interpret this volume form as a Reidemeister torsion. In the last part of this Note, we give an explicit computation of this volume form for torus knots. To cite this article: J. Dubois, C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

Travelling waves and dispersion relation in the spatial unfolding of a periodic orbitOndes progressives et relation de dispersion dans le déploiement spatial d'une orbite périodique

For a partial differential equation in spatial dimension one, admitting a spatially homogeneous time periodic solution, we show the generic existence, close to this solution, of a one-parameter family of travelling waves parametrized by their wave number k (k=0 corresponding to the spatially homogeneous initial solution). The argument is elementary and relies on a direct application of singular perturbation theory (Fenichel's global center manifold theorem). To cite this article: E. Risler, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 833–838.     

Non-Liouville numbers and a Theorem of Hörmander

It is shown in this paper that Theorem 1 of [G. H. Meisters, “Translation-invariant linear forms and a formula for the Dirac measure,” J. Functional Analysis 8 (1971), 173–188] can be deduced from a very general result of Lars Hörmander, namely, Theorem 1 of “Generators for some rings of analytic functions” [Bull. Amer. Math. Soc.73 (1967), 943–949]. However, Hörmander's theorem is evidently not applicable in several other cases where Meisters'-type results have been obtained (e.g., Theorem 1 of G.H. Meisters and Wolfgang M. Schmidt, “Translation-invariant linear forms on L²(G) for compact abelian groups G,” J. Functional Analysis11 (1972), 407–424).     

Counting closed geodesics in globally hyperbolic maximal compact AdS 3-manifolds

We propose a definition for the length of closed geodesics in a globally hyperbolic maximal compact (GHMC) Anti-De Sitter manifold. We then prove that the number of closed geodesics of length less than R grows exponentially fast with R and the exponential growth rate is related to the critical exponent associated to the two hyperbolic surfaces coming from Mess parametrization. We get an equivalent of three results for quasi-Fuchsian manifolds in the GHMC setting: Bowen’s rigidity theorem of critical exponent, Sanders’ isolation theorem and McMullen’s examples lightening the behaviour of this exponent when the surfaces range over Teichmüller space.     

Extending Greene's theorem to directed graphs

The purpose of this note is to prove a theorem on acyclic digraphs, conjectured by Linial (J. Combin. Theory Ser. A, 30 (1981), 331–334), which includes Greene's theorem (J. Combin. Theory Ser. A, 20 (1976), 69–79) on decompositions of a partially ordered set into antichains.     

Separation properties in algebraic categories of topological spaces

Full subcategories C ? Top of the category of topological spaces, which are algebraic over Set in the sense of Herrlich [2], have pleasant separation properties, mostly subject to additional closedness assumptions. For instance, every C-object is a T₁-space, if the two-element discrete space belongs to C. Moreover, if C is closed under the formation of finite powers in Top and even varietal [2], then every C-object is Hausdorff. Hence, the T₂-axiom turns out to be (nearly) superfluous in Herrlich's and Strecker's characterization of the category of compact Hausdorff spaces [1], although it is essential for the proof.If we think of C-objects X as universal algebras (with possibly infinite operations), then the subalgebras of X form the closed sets of a compact topology on X, provided that the ordinal spaces [0, β] belong to C. This generalizes a result in [3]. The subalgebra topology is used to prove criterions for the Hausdorffness of every space in C, if C is only algebraic.     

Families of simply connected 4-manifolds with the same Seiberg-Witten invariants

This article presents several new constructions of infinite families of smooth 4-manifolds with the property that any two manifolds in the same family are homeomorphic. While the construction gives strong evidence that any two of these manifolds of are not diffeomorphic, they cannot be distinguished by Seiberg-Witten invariants. Whether these manifolds are, or are not, diffeomorphic seems to be a very difficult question to answer. For one of these constructions, each member of the family is symplectic with the further property that each contains nullhomologous tori with the property that infinitely many log transformations on these tori yield nonsymplectic 4-manifolds. This is detected by calculations of Seiberg-Witten invariants. The surgery in question can be performed on any 4-manifold which contains as a codimension 0 submanifold a punctured surface bundle over a punctured surface and a nontrivial loop in the base which has trivial monodromy. A starting point for another class of examples in this paper is a family of examples which show that the Parshin-Arakelov theorem for holomorphic Lefschetz fibrations is false in the symplectic category. Such families are constructed by means of knot surgery on ellipitic surfaces. It is shown that for a fixed homeomorphism type X (of a simply connected elliptic surface) and a fixed integer g?3, there are infinitely many genus g Lefschetz fibrations on nondiffeomorphic 4-manifolds, all homeomorphic to X.     

Sur le théorème de la stabilité non asymptotique dans la méthode directe de Lyapunov

This paper provides a new theorem for nonasymptotic stability of autonomous dynamical systems by Lyapunov's direct method in the class of the indefinite functions. To cite this article: B. Kalitine, C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

Convex subsets of 2n and bounded truth-table reducibility

Let 2ⁿ be the set of n-tuples of 0's and 1's, partially ordered componentwise. A characterization is given of the possible decompositions of arbitrary subsets of 2ⁿ as disjoint unions of sets which are convex in this ordering; this result is used to obtain a decomposition theorem for Boolean functions in terms of monotone functions. The second half of the paper contains applications to recursion theory; in particular, canonical forms for certain minimum-norm bounded-truth-table reductions are obtained.     

Characterisation of unbounded multiplier maps

This paper generalizes Wendel's theorem to cover an arbitrary closed linear map in L₁(G), G a locally compact group.     

On Saint Venant's compatibility conditions and Poincaré's lemma

Saint Venant's theorem constitutes a classical characterization of smooth matrix fields as linearized strain tensor fields. This theorem has been extended to matrix fields with components in $L^2$ by the second author and P. Ciarlet, Jr. in 2005. One objective of this Note is to further extend this characterization to matrix fields whose components are only in $H^{\mathrm{?1}}$. Another objective is to demonstrate that Saint Venant's theorem is in fact nothing but the matrix analog of Poincaré's lemma. To cite this article: C. Amrouche et al., C. R. Acad. Sci. Paris, Ser. I 342 (2006).     

On Darboux continuity and continuity

Lipinski's (Colloq. Math.19 (1968), 251–253) theorem for a Darboux function to be continuous is extended and a question raised by him is partially answered.     

Monodromie du problème de Cauchy ramifiéMonodromy of the ramified Cauchy problem

In this Note, we study the monodromy of the ramified Cauchy problem for operators with multiple characteristics of constant multiplicity. More precisely, we give an estimation of the eigenvalues of the solution's monodromy, first with the assumptions of the theorem of Hamada–Leray–Wagschal, then with the assumptions of the theorem of Leichtnam. To cite this article: R. Camalès, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 639–642.