In this paper, we consider the best constant and its typical lower bound of the Poincaré inequality for diffusion operators on $\mathbb R$. We are interested in the critical case such that these constants are equal. Our goal is to show that they are equal if and only if a diffusion operator is the Ornstein-Uhlenbeck operator with a suitable property. Hence, the Ornstein-Uhlenbeck operator with this property plays a critical role in the Poincaré inequality.

Consider the mean field equation with parameter $\lambda=8\pi$ in a bounded smooth domain $\Omega$. Denote by $E_{8\pi}(\Omega)$ the infimum of the associated functional $I_{8\pi}(\Omega)$. We prove that if $|\Omega|=\pi$, then $E_{8\pi}(\Omega)\ge E_{8\pi}(B_1)$ and equality holds if and only if $\Omega$ is a ball. We also give a sufficient condition for the existence of a minimizer for $I_{8\pi}(\Omega)$.

We analyze the well-posedness of the initial-value problem for the semilinear equation in Marcinkiewicz spaces $L^{(p,\infty)}$. Mild solutions are obtained in spaces with the right homogeneity to allow the existence of self-similar solutions. As a consequence of our results we prove that the class $C([0,T);L^{p}(\Omega)),\ 0 < T\leq\infty, \ p={\frac{n(\rho-1)}{2\gamma }},$ $\Omega=R^{n},$ has uniqueness of solutions (including large solutions) obtained in [19], [20] and [8]. The asymptotic stability of solutions is obtained, and as a consequence, a criterion for self-similarity persistence at large times is obtained.

In this paper we study a nonlinear Dirichlet problem driven by the p-Laplacian and a right-hand side nonlinearity which exhibits an asymmetric behavior near $+ \infty$ and $- \infty$. Using variational techniques based on the mountain pass theorem and the second deformation theorem, we prove the existence of at least two nontrivial $C^1$- solutions, one of which is strictly positive.

This work deals with the study of a nonlinear degenerate pseudoparabolic problem. Arising from the modelling of sedimentary basin formation, the equation degenerates in order to take implicitly into account a constraint on the time derivative of the unknown. An existence result of a solution with an adapted compactness result and qualitative properties of the solutions are proposed (localization, finite speed of propagation, etc.).

In this paper, we are interested in sufficient conditions on partial differential operators with several Fuchsian variables. We give a result of existence and uniqueness of global solutions of the associated Goursat problem, in holomorphic function spaces. Our techniques are those of C. Wagschal [35] based on formal norms of Leray-Waelbroeck [17].