48 research outputs found
A priori bounds for a class of nonlinear elliptic equations and applications to physical problems
Upper and lower bounds for the solutions of a nonlinear Dirichlet problem are given and isoperimetric inequalities for the maximal pressure of an ideal charged gas are constructed. The method used here is based on a geometrical result for two-dimensional abstract surface
"Boundary blowup" type sub-solutions to semilinear elliptic equations with Hardy potential
Semilinear elliptic equations which give rise to solutions blowing up at the
boundary are perturbed by a Hardy potential. The size of this potential effects
the existence of a certain type of solutions (large solutions): if the
potential is too small, then no large solution exists. The presence of the
Hardy potential requires a new definition of large solutions, following the
pattern of the associated linear problem. Nonexistence and existence results
for different types of solutions will be given. Our considerations are based on
a Phragmen-Lindelof type theorem which enables us to classify the solutions and
sub-solutions according to their behavior near the boundary. Nonexistence
follows from this principle together with the Keller-Osserman upper bound. The
existence proofs rely on sub- and super-solution techniques and on estimates
for the Hardy constant derived in Marcus, Mizel and Pinchover.Comment: 23 pages, 3 figure
Reaction-diffusion problems on time-dependent Riemannian manifolds: stability of periodic solutions
We investigate the stability of time-periodic solutions of semilinear
parabolic problems with Neumann boundary conditions. Such problems are posed on
compact submanifolds evolving periodically in time. The discussion is based on
the principal eigenvalue of periodic parabolic operators. The study is
motivated by biological models on the effect of growth and curvature on
patterns formation. The Ricci curvature plays an important role
Blowup in diffusion equations: A survey
AbstractThis paper deals with quasilinear reaction-diffusion equations for which a solution local in time exists. If the solution ceases to exist for some finite time, we say that it blows up. In contrast to linear equations blowup can occur even if the data are smooth and well-defined for all times. Depending on the equation either the solution or some of its derivatives become singular. We shall concentrate on those cases where the solution becomes unbounded in finite time. This can occur in quasilinear equations if the heat source is strong enough. There exist many theoretical studies on the question on the occurrence of blowup. In this paper we shall recount some of the most interesting criteria and most important methods for analyzing blowup. The asymptotic behavior of solutions near their singularities is only completely understood in the special case where the source is a power. A better knowledge would be useful also for their numerical treatment. Thus, not surprisingly, the numerical analysis of this type of problems is still at a rather early stage. The goal of this paper is to collect some of the known results and algorithms and to direct the attention to some open problems
Optimization problems for weighted Sobolev constants
In this paper, we study a variational problem under a constraint on the mass. Using a penalty method we prove the existence of an optimal shape. It will be shown that the minimizers are Hölder continuous and that for a large class they are even Lipschitz continuous. Necessary conditions in form of a variational inequality in the interior of the optimal domain and a condition on the free boundary are derive
On the positive, "radial” solutions of a semilinear elliptic equation in N
We discuss various kinds of existence and non existence results for positive solutions of Emden-Fowler type equations in the hyperbolic space. The main tools are perturbation analysis, variational methods, Pohozeav type identities and reduction to Matukuma equation
The Brézis–Nirenberg Problem on S3
AbstractIn this paper we study existence and nonexistence of solutions to the Brézis–Nirenberg problem for different values of λ in geodesic spheres on S3. The picture differs considerably from the one in the Euclidean space. It is shown that large spheres containing the hemisphere have two different type of radial solutions for negative values of λ. Numerical results indicate that for λ very small the solutions have a maximum near the boundary, whereas for larger values of λ the maximum is at the origin. The techniques used are: Pohozaev type identities, concentration-compactness lemma and numerical methods