On the resonance problem with nonlinearity which has arbitrary linear growth

This paper deals with the nonlinear two-point boundary value problem at resonance. Even nonlinearities g with an arbitrary linear growth in +∞ (resp. −∞) may be considered but only on the cost of the corresponding bound on their linear growth at −∞ (resp. +∞). It generalizes the previous results in this direction obtained by M. Schechter, J. Shapiro, and M. Snow (Trans. Amer. Math. Soc. 241 (1978), 69–78), L. Cesari and R. Kannan (Proc. Amer. Math. Soc. 88 (1983), 605–613), and S. Ahmad (Proc. Amer. Math. Soc. 93 (1984), 381–384).     

On completeness of root functions of elliptic boundary problems in a domain with conical points on the boundarySur la complétude des fonctions propres et associées d'un problème au bord elliptique dans un domaine avec points coniques sur le bord

We prove the completeness of the system of eigen and associated functions (i.e., root functions) of an elliptic boundary value problem in a domain, whose boundary is a smooth surface everywhere, except at a finite number of points, such that each point possesses a neighborhood, where the boundary is a conical surface. To cite this article: Y.V. Egorov et al., C. R. Acad. Sci. Paris, Ser. I 334 (2002) 649–654.     

Existence and regularity of solution of mixed boundary value problem of a Busemann-equation with nonlinear absorb term

The Busemann-equation is a classical equation coming from fluid dynamics. The well-posed problem and regularity of solution of Busemann-equation with nonlinear term are interesting and important. The Busemann-equation is elliptic in y>0 and is degenerate at the line y=0 in R². With a special nonlinear absorb term, we study a nonlinear degenerate elliptic equation with mixed boundary conditions in a piecewise smooth domain. By means of elliptic regularization technique, a delicate prior estimate and compact argument, we show that the solution of mixed boundary value problem of Busemann-equation is smooth in the interior and Lipschitz continuous up to the degenerate boundary on some conditions. The result is better than the result of classical boundary degenerate elliptic equation.     

Boundary Value Problems for the Elliptic Sine-Gordon Equation in a Semi-strip

We study boundary value problems posed in a semistrip for the elliptic sine-Gordon equation, which is the paradigm of an elliptic integrable PDE in two variables. We use the method introduced by one of the authors, which provides a substantial generalization of the inverse scattering transform and can be used for the analysis of boundary as opposed to initial-value problems. We first express the solution in terms of a 2×2 matrix Riemann–Hilbert problem whose “jump matrix” depends on both the Dirichlet and the Neumann boundary values. For a well posed problem one of these boundary values is an unknown function. This unknown function is characterised in terms of the so-called global relation, but in general this characterisation is nonlinear. We then concentrate on the case that the prescribed boundary conditions are zero along the unbounded sides of a semistrip and constant along the bounded side. This corresponds to a case of the so-called linearisable boundary conditions, however, a major difficulty for this problem is the existence of non-integrable singularities of the function q _y at the two corners of the semistrip; these singularities are generated by the discontinuities of the boundary condition at these corners. Motivated by the recent solution of the analogous problem for the modified Helmholtz equation, we introduce an appropriate regularisation which overcomes this difficulty. Furthermore, by mapping the basic Riemann–Hilbert problem to an equivalent modified Riemann–Hilbert problem, we show that the solution can be expressed in terms of a 2×2 matrix Riemann–Hilbert problem whose “jump matrix” depends explicitly on the width of the semistrip L, on the constant value d of the solution along the bounded side, and on the residues at the given poles of a certain spectral function denoted by h(λ). The determination of the function h remains open.     

Properties of the Principal Eigenvalues of a General Class of Non-classical Mixed Boundary Value Problems

In this paper we characterize the existence of principal eigenvalues for a general class of linear weighted second order elliptic boundary value problems subject to a very general class of mixed boundary conditions. Our theory is a substantial extension of the classical theory by P. Hess and T. Kato (1980, Comm. Partial Differential Equations5, 999-1030). In obtaining our main results we must give a number of new results on the continuous dependence of the principal eigenvalue of a second order linear elliptic boundary value problem with respect to the underlying domain and the boundary condition itself. These auxiliary results complement and in some sense complete the theory of D. Daners and E. N. Dancer (1997, J. Differential Equations138, 86-132). The main technical tool used throughout this paper is a very recent characterization of the strong maximum principle in terms of the existence of a positive strict supersolution due to H. Amann and J. López-Gómez (1998, J. Differential Equations146, 336-374).     

Singular perturbation of an infinite interval linear state regulator problem in optimal control

The corresponding problem on a finite interval has been studied by O'Malley and Kung using two different methods, namely: (1) the two-point boundary value method (O'Malley and Kung, SIAM J. Control13 (1975), 327–337) and (2) The Riccati gain method (O'Malley and Kung, J. Differential Eqs.16 (1974), 413–427). For the infinite interval, the two-point boundary value method is no longer relevant. However, the Riccati gain method can be applied. The conditions are changed slightly from those for the finite interval case. Some conditions are eliminated and some new conditions are added.     

Existence and regularity of the solution of a mixed boundary value problem for the Keldysh equation with a nonlinear absorption term

The Keldysh equation is a more general form of the classic Tricomi equation from fluid dynamics. Its well-posedness and the regularity of its solution are interesting and important. The Keldysh equation is elliptic in y>0 and is degenerate at the line y=0 in R². Adding a special nonlinear absorption term, we study a nonlinear degenerate elliptic equation with mixed boundary conditions in a piecewise smooth domain—similar to the potential fluid shock reflection problem. By means of an elliptic regularization technique, a delicate a priori estimate and compact argument, we show that the solution of a mixed boundary value problem of the Keldysh equation is smooth in the interior and Lipschitz continuous up to the degenerate boundary under some conditions. We believe that this kind of regularity result for the solution will be rather useful.     

Elliptic regularizations for the nonlinear heat equation

The purpose of this paper is to study two elliptic regularizations for the nonlinear heat equation with nonlinear boundary conditions formulated below. Asymptotic expansions of the order zero for the solutions of these elliptic regularizations are established, including some boundary layer corrections. Under some appropriate smoothness and compatibility conditions on the data estimates for the remainder terms with respect to the C([0,T];L²(Ω)) norm are proved in order to validate these expansions.     

On one-homogeneous solutions to elliptic systems in two dimensionsSur les solutions un-homogènes des systèmes elliptiques en dimension deux

In this Note we consider a class of nonlinear second order elliptic systems in divergence form and two independent variables. We prove that all Lipschitz continuous one-homogeneous weak solutions are linear. To cite this article: D. Phillips, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 39–42.     

Solutions entropiques des problèmes non locauxEntropy solutions of nonlocal problems

We introduce a notion of entropy solution for the nonlocal problem $\text{C}\text{f+f=ψ}$ on $\text{?Ω}$, where $\text{ψ∈}\text{L}{}^1\text{(?Ω)}$ and $\text{C}$ is a nonlinear capacity operator. We prove its existence and uniqueness. This notion of solution allows also to solve a general elliptic problem with nonlinear boundary conditions. To cite this article: K. Ammar, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 751–756.     

On a nonlinear nonlocal problem of elliptic type

The solvability of a nonlinear nonlocal problem of the elliptic type that is a generalized Bitsadze–Samarskii-type problem is analyzed. Theorems on sufficient solvability conditions are stated. In particular, a nonlocal boundary value problem with p-Laplacian is studied. The results are illustrated by examples considered earlier in the linear theory (for p = 2). The examples show that, in contrast to the linear case under the same “nice” nonlocal boundary conditions, for p > 2, the problem can have one or several solutions, depending on the right-hand side.     

Remarks on a Hardy–Sobolev inequality

We compute the optimal constant for a generalized Hardy–Sobolev inequality, and using the product of two symmetrizations we present an elementary proof of the symmetries of some optimal functions. This inequality was motivated by a nonlinear elliptic equation arising in astrophysics. To cite this article: S. Secchi et al., C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

An elliptic problem with an indefinite nonlinearity and a parameter in the boundary condition

We establish the existence of solutions of nonlinear elliptic boundary value problems involving a positive parameter on the boundary. We also examine a profile of solutions of problem (1.2) when a parameter λ tends to 0.     

Viscosity solutions to the degenerate oblique derivative problem for fully nonlinear elliptic equationsSolutions visqueuses du problème avec une dérivée oblique dégénérée pour une classe d'équations complètement non linéaires

In this paper we prove a comparison principle between the semicontinuous viscosity sub- and supersolutions of the tangential oblique derivative problem and the mixed Dirichlet–Neumann problem for fully nonlinear elliptic equations. By means of the comparison principle, the existence of a unique viscosity solution is obtained. To cite this article: P. Popivanov, N. Kutev, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 661–666.     

Analysis of curvature influence on effective boundary conditionsAnalyse de l'influence de la courbure sur les conditions aux limites équivalentes

The aim of this work is the construction of effective boundary conditions (wall laws) for elliptic problems defined in domains with curved, rough boundaries with periodic wrinkles. We present error estimates for first and second order approximations, and a numerical test. To cite this article: A. Madureira, F. Valentin, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 499–504.     

Anisotropic regularity results for Laplace and Maxwell operators in a polyhedron

As representatives of a larger class of elliptic boundary value problems of mathematical physics, we study the Dirichlet problem for the Laplace operator and the electric boundary problem for the Maxwell operator. We state regularity results in two families of weighted Sobolev spaces: A classical isotropic family, and a new anisotropic family, where the hypoellipticity along an edge of a polyhedral domain is taken into account. To cite this article: A. Buffa et al., C. R. Acad. Sci. Paris, Ser. I 336 (2003).     

Existence of weak solutions of a nonlinear parabolic equation in a noncylindrical domain

Consider the mixed boundary value problem ?_tu + L[u] = f with a squareintegrable initial value and with zero boundary values in a domain Q. L[u] is a nonlinear elliptic operator in divergence form, defined on a domain with timedependent boundary. Weak solutions in cylindrical domains are used to construct a weak solution in Q by approximating Q by a system of cylinders. It is shown that this solution is continuously dependent on the initial value.     

The entropy condition and the admissibility of shocks

The author proposed (Trans. Amer. Math. Soc.199 (1974), 89–112) the extended entropy condition (E) and solved the Riemann problem for general 2 × 2 conservation laws. The Riemann problem for 3 × 3 gas dynamics equations was treated by the author (J. Differential Equations18 (1975), 218–231). In this paper we justify condition (E) by the viscosity method in the spirit of Gelfand [Uspehi Mat. Nauk14 (1959), 87–158]. We show that a shock satisfies condition (E) if and only if the shock is admissible, that is, it is the limit of progressive wave solutions of the associated viscosity equations. For the “genuinely nonlinear” 2 × 2 conservation laws, Conley and Smoller [Comm. Pure Appl. Math.23 (1970), 867–884] proved that a shock satisfies Lax's shock inequalities [cf. Comm. Pure Appl. Math.14 (1957), 537–566] if and only if it is admissible. In this paper, we consider systems that are not necessarily genuinely nonlinear.     

Nonlinear elliptic problems with superlinear reaction and parametric concave boundary condition

We study parametric nonlinear elliptic boundary value problems driven by the p-Laplacian with convex and concave terms. The convex term appears in the reaction and the concave in the boundary condition (source). We study the existence and nonexistence of positive solutions as the parameter λ > 0 varies. For the semilinear problem (p = 2), we prove a bifurcation type result. Finally, we show the existence of nodal (sign changing) solutions.     

Fiberwise stable bundles on elliptic threefolds with relative Picard number oneFibrés vectoriels relativement stables sur variétés elliptiques de dimension trois dont le nombre relatif de Picard est un

We show that fiberwise stable vector bundles are preserved by relative Fourier–Mukai transforms between elliptic threefolds with relative Picard number one. Using these bundles we define new invariants of elliptic fibrations, and we relate the invariants of a space with those of a relative moduli space of stable sheaves on it. As a byproduct, we calculate the intersection form of a certain new example of an elliptic Calabi–Yau threefold. To cite this article: A. C?ld?raru, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 469–472.