Un isomorphisme de type Deligne–Riemann–Roch

We prove an Adams–Riemann–Roch theorem for projective morphisms between regular schemes, in the sense of the program of P. Deligne on the functorial Riemann–Roch theorem and we deduce some geometric consequences. To cite this article: D. Eriksson, C. R. Acad. Sci. Paris, Ser. I 347 (2009).     

Asymptotique des approximants de Hermite–Padé quadratiques de la fonction exponentielle et problèmes de Riemann–Hilbert

We describe the asymptotic behavior of the polynomials $\text{p,}\text{q,}\text{r}$ of degree n in type I Hermite–Padé approximation to the exponential function, i.e., $\text{p(z)}\text{e}{}^{\mathrm{?z}}\text{+q(z)+r(z)}\text{e}{}^{\mathrm{z}}\text{=}\text{O}\text{(z}{}^{\mathrm{3n+2}}\text{)}$ as z→0. A steepest descent method for Riemann–Hilbert problems, due to Deift and Zhou, is used to obtain strong uniform asymptotics for the scaled polynomials $\text{p(3nz),}\text{q(3nz),}\text{r(3nz)}$ in every domain of the complex plane. An important role is played by a three-sheeted Riemann surface and certain measures and functions defined on it. To cite this article: A. Kuijlaars et al., C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

Une démonstration simple de la fidélité de la représentation de Lawrence–Krammer–Paris

The theorem of linearity of the Artin–Tits groups of spherical type and the theorem of injectivity of any Artin–Tits monoid in its group are essentially based on the faithfulness of the Lawrence–Krammer–Paris representation restricted to the monoid. We prove this faithfulness using neither the normal forms of the elements of the monoid nor the closed subsets of the associated root system; only very elementary notions are needed.     

Asymptotic behavior of the generalized Korteweg–de Vries–Burgers equation

Cauchy’s problem for a generalization of the KdV–Burgers equation is considered in Sobolev spaces H¹(\mathbbR){H^1(\mathbb{R})} and H²(\mathbbR){H^2(\mathbb{R})}. We study its local and global solvability and the asymptotic behavior of solutions (in terms of the global attractors). The parabolic regularization technique is used in this paper which allows us to extend the strong regularity properties and estimates of solutions of the fourth order parabolic approximations onto their third order limit—the generalized Korteweg–de Vries–Burgers (KdVB) equation. For initial data in H²(\mathbbR){H^2(\mathbb{R})} we study the notion of viscosity solutions to KdVB, while for the larger H¹(\mathbbR){H^1(\mathbb{R})} phase space we introduce weak solutions to that problem. Finally, thanks to our general assumptions on the nonlinear term f guaranteeing that the global attractor is usually nontrivial (i.e., not reduced to a single stationary solution), we study an upper semicontinuity property of the family of global attractors corresponding to parabolic regularizations when the regularization parameter e{\epsilon} tends to 0⁺ (which corresponds the passage to the KdVB equation).     

Travelling waves for a non-local Korteweg–de Vries–Burgers equation

We study travelling wave solutions of a Korteweg–de Vries–Burgers equation with a non-local diffusion term. This model equation arises in the analysis of a shallow water flow by performing formal asymptotic expansions associated to the triple-deck regularisation (which is an extension of classical boundary layer theory). The resulting non-local operator is a fractional derivative of order between 1 and 2. Travelling wave solutions are typically analysed in relation to shock formation in the full shallow water problem. We show rigorously the existence of these waves. In absence of the dispersive term, the existence of travelling waves and their monotonicity was established previously by two of the authors. In contrast, travelling waves of the non-local KdV–Burgers equation are not in general monotone, as is the case for the corresponding classical KdV–Burgers equation. This requires a more complicated existence proof compared to the previous work. Moreover, the travelling wave problem for the classical KdV–Burgers equation is usually analysed via a phase-plane analysis, which is not applicable here due to the presence of the non-local diffusion operator. Instead, we apply fractional calculus results available in the literature and a Lyapunov functional. In addition we discuss the monotonicity of the waves in terms of a control parameter and prove their dynamic stability in case they are monotone.     

Vecteurs de Perron–Frobenius et produits Gamma

This Note presents formulas to express the coordinates of Perron–Frobenius vectors of Cartan matrices (finite or affine) as products of Gamma values, for each finite irreducible root system of rank r.     

Constructive Approximation in de Branges–Rovnyak Spaces

In most classical holomorphic function spaces on the unit disk in which the polynomials are dense, a function f can be approximated in norm by its dilates \(f_r(z):=f(rz)~(r<1)\). We show that this is not the case for the de Branges–Rovnyak spaces \(\mathcal{H}(b)\). More precisely, we exhibit a space \(\mathcal{H}(b)\) in which the polynomials are dense and a function \(f\in \mathcal{H}(b)\) such that \(\lim _{r\rightarrow 1^-}\Vert f_r\Vert _{\mathcal{H}(b)}=\infty \). On the positive side, we prove the following approximation theorem for Toeplitz operators on general de Branges–Rovnyak spaces \(\mathcal{H}(b)\). If \((h_n)\) is a sequence in \(H^\infty \) such that \(\Vert h_n\Vert _{H^\infty }\le 1\) and \(h_n(0)\rightarrow 1\), then \(\Vert T_{\overline{h}_n}f-f\Vert _{\mathcal{H}(b)}\rightarrow 0\) for all \(f\in \mathcal{H}(b)\). Using this result, we give the first constructive proof that, if b is a nonextreme point of the unit ball of \(H^\infty \), then the polynomials are dense in \(\mathcal{H}(b)\).     

Sur un théorème de Cauchy–Kowalewski–Nagumo global dans des espaces de Gevrey projectifs

We propose a new approach, based on a combination of the contraction principle and Picard successive approximations, for the study of a global Cauchy problem associated to partial differential operator $D_t^m?{\sum }_{(j\text{,}\alpha )\in B}{a}_{j\alpha }{D}_t^j{D}_x^{\alpha }$ with coefficients $a_{j\alpha }$ continuous or holomorphic with respect to t in projective Gevrey spaces. We extend the result of a previous Note to the case of non Kowalewskian operators. To cite this article: D. Gourdin, T. Gramchev, C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

Fusion de Dempster–Shafer dans les chaînes triplet partiellement de Markov

Hidden Markov Chains (HMC), Pairwise Markov Chains (PMC), and Triplet Markov Chains (TMC), allow one to estimate a hidden process X from an observed process Y. More recently, TMC have been generalized to Triplet Partially Markov chain (TPMC), where the estimation of X from Y remains workable. Otherwise, when introducing a Dempster–Shafer mass function instead of prior Markov distribution in classical HMC, the result of its Dempster–Shafer fusion with a distribution provided $Y=y$, which generalizes the posterior distribution of X, is a TMC. The aim of this Note is to generalize the latter result replacing HMC with multisensor TPMC. To cite this article: W. Pieczynski, C. R. Acad. Sci. Paris, Ser. I 339 (2004).     

Nonautonomous soliton solutions of the modified Korteweg–de Vries–sine-Gordon equation

Multisoliton solutions of the modified Korteweg–de Vries–sine-Gordon (mKdV–SG) equation with time-dependent coefficients are considered. Cases describing changes in the shape of soliton solutions (kinks and breathers) observed in gradual transitions between the mKdV, SG, and mKdV–SG equations are numerically studied.     

Lʼindice des champs de vecteurs sur les courbes de Cohen–Macaulay

In this paper a new method for computing the topological index of a vector field at Cohen–Macaulay curves is described. It is based on properties of regular meromorphic differential forms which are used for computing the homological index of vectors fields introduced by X. Gómez-Mont. In particular, we show how to compute the index at quasihomogeneous Gorenstein curves and complete intersections, at monomial curves, at Cohen–Macaulay space curves, and others. In contrast to previous articles on this subject we do not use the technique of spectral sequences, or computer algebra systems for symbolic calculations.     

Propagation de fronts dans les équations de Fisher–KPP avec diffusion fractionnaire

We study in this Note the Fisher–KPP equation where the Laplacian is replaced by the generator of a Feller semigroup with slowly decaying kernel, an important example being the fractional Laplacian. Contrary to what happens in the standard Laplacian case, where the stable state invades the unstable one at constant speed, we prove here that invasion holds at an exponential in time velocity. These results provide a mathematically rigorous justification of numerous heuristics about this model. To cite this article: X. Cabré, J.-M. Roquejoffre, C. R. Acad. Sci. Paris, Ser. I 347 (2009).     

Bifurcations in the Generalized Korteweg–de Vries Equation

We study the generalized Korteweg–de Vries (KdV) equation and the Korteweg–de Vries–Burgers (KdVB) equation with periodic in the spatial variable boundary conditions. For various values of parameters, in a sufficiently small neighborhood of the zero equilibrium state we construct asymptotics of periodic solutions and invariant tori. Separately we consider the case when the stability spectrum of the zero solution contains a countable number of roots of the characteristic equation. In this case we state a special nonlinear boundary-value problem which plays the role of a normal form and determines the dynamics of the initial problem.     

Une estimation de type Aronson–Bénilan

We consider the equation u_t=Δϕ(u), where ϕ∈C³(0,∞) is increasing. Under the condition ν·sϕ″(s)/ϕ′(s)⩾γ for some γ>0 and ν∈{−1;1}, we prove the estimate ν·du/dt⩾−u/γt. This result improves the estimates given by M.G. Crandall and M. Pierre (in J. Funct. Anal. 45 (1982) 194–212) for this equation. To cite this article: E. Chasseigne, C. R. Acad. Sci. Paris, Ser. I 336 (2003).