We study existence and boundedness of solutions for the quasilinear elliptic equation $-\Delta_{m} u=\lambda(1+u)^p$ in a bounded domain $\Omega$ with homogeneous Dirichlet boundary conditions. The assumptions on both the parameters $\lambda$ and $p$ are fundamental. Strange critical exponents appear when boundedness of solutions is concerned. In our proofs we use techniques from calculus of variations, from critical-point theory, and from the theory of ordinary differential equations.

The aim of this paper is to analyze a model of a degenerate nonlinear system arising from immiscible, compressible, two-phase, three-dimensional flows occurring in porous media. A degenerate weighted formulation is introduced to take into account the compressibility and the degeneracy. Two existence results of such a degenerate weak solution are introduced. The first result concerns the existence of solutions under a reasonable assumption on the capillary pressure. This condition allows the degeneracy only where one of the two phases is exclusively present. The second result establishes, for suitable initial data, the existence of solutions when the degeneracy occurs where one or the other phase is exclusively present. Nevertheless, for suitable initial data, a classical weak solution is obtained when the degeneracy is not too strong and occurs only where the injected phase is exclusively present.

The present paper is concerned with a Cauchy problem for a semilinear heat equation, \[ \left\{ \begin{array}{ll} u_t = \Delta u + u^p & \quad \mbox{ in } {{\bf R}}^N \times (0,\infty), \hspace{3.5cm}\\ u(x,0) = u_0(x) \geq 0 & \quad \mbox{ in } {{\bf R}}^N. \end{array} \right. \tag*{(P)} \] A solution $ u $ of (P) is said to exhibit type-II blowup at $ t = T < + \infty $ if \[ \limsup_{ t \nearrow T } (T-t)^{ 1/(p-1) } |u(t)|_\infty = + \infty \] with the supremum norm $ | \cdot |_\infty $ in $ {{\bf R}}^N $. We show the existence of type-II-blowup solutions. Though our proof is based on the argument due to [9, 10], it is simpler than theirs by taking account of the number of intersections with the singular steady state of (P).

We study here a model of a conservation law with a flux function with discontinuous coefficients, namely the equation $u_t + (k(x)g(u)+f(u))_x=0$. We prove the existence and the uniqueness of an entropy solution in $L^{\infty}(\mathbb R_+ \times \mathbb R)$ for $u_0$, the initial condition, in $L^{\infty}(\mathbb R)$. We provide some physical background for the study of this equation. In particular, $g$ is assumed to be neither convex nor concave and $k$ is a discontinuous function.

In this paper we study the existence of solutions to zero-Dirichlet-boundary-value problems for the quasilinear elliptic equation ${\rm (QE)_c}$ $- \Delta_p u - p \nabla \theta(x) \cdot \nabla u |\nabla u|^{p-2} = \lambda a(x) |u|^{p-2}u + K(x)|u|^{p^*-2}u$ in an unbounded domain $\Omega \subset {\bf R}^N$ with smooth boundary $\partial \Omega$. By using Brézis-Nirenberg's results, we prove that ${\rm (QE)_c}$ admits at least one nontrivial weak solution for positive $\lambda$ in a suitable interval.

A thermoelastic system with the Gurtin-Pipkin model for heat conduction with large memory is considered. If the control time is large enough, exact controllability with boundary controls is established for both the displacement and the temperature.

Recently, a class of weak solutions called "stratified" solutions have been introduced for systems of conservation laws in 1d which have a linear degenerate field and a "good symmetrizer," like the Euler system of entropic gaz dynamics ([5], [16]). These solutions can have an unbounded local variation and they generalize the "large-amplitude oscillatory solutions" previously studied by W.E, A. Heibig, and D. Serre ([13], [7], [22]). In this paper we show that, in the case of the Lagragian Euler equations of gaz dynamics, these stratified solutions are the limit of solutions of a vanishing viscosity perturbation of the system.