We study uniqueness of weak solutions for elliptic equations of the following type \[ -\partial_{x_{i}}\left( a_{i}(x,u)\left\vert \partial_{x_{i}}u\right\vert ^{p_{i}-2}\partial_{x_{i}}u\right) +b(x,u) =f(x), \] in a bounded domain $\Omega\subset{\mathbb{R}}^{n}$ with Lipschitz continuous boundary $\Gamma=\partial\Omega$. We consider in particular mixed boundary conditions, i.e., Dirichlet condition on one part of the boundary and Neumann condition on the other part. We study also uniqueness of weak solutions for the parabolic equations \[ \left\{ \begin{array}[c]{cc} \partial_{t}u=\partial_{x_{i}}\left( a_{i}(x,t,u)\left\vert \partial_{x_{i} }u\right\vert ^{p_{i}-2}\partial_{x_{i}}u\right) +f & \text{in}\ \Omega \times(0,T),\\ u=0 & \text{on\ }\Gamma\times(0,T)=\partial\Omega\times(0,T),\\ u(x,0)=u_{0} & x\in\Omega. \end{array} \right. \] It is assumed that the constant exponents $p_{i}$ satisfy $1 <p_{i} <\infty$ and the coefficients $a_{i}\,$ are such that $0 <\lambda\leq\lambda_{i}\leq a_{i}(x,u) <\infty,\ \forall i,a.e. \; x\in\Omega$, (a.e. $t\in(0,T)$), $\forall u\in{\mathbb{R}}$. We indicate also conditions which guarantee existence of solutions.

A local property of the mountain-pass critical point obtained by Struwe's monotonicity trick is shown. Given one parameter family of functionals $\{ I_\lambda\}$ provided with the structural assumption of monotonicity, we assume that each $I_\lambda$ satisfies the bounded Palais-Smale condition, its mountain-pass critical value $c_{\lambda}$ is differentiable at $\lambda=\lambda_0$, and $\mbox{Cr}(I, c_{\lambda_0})=\{v\mid I'_{\lambda_0}(v)=0, \ I_{\lambda_0}(v)=c_{\lambda_0} \}$ is compact. Then, there is $v\in \mbox{Cr}(I, c_{\lambda_0})$, either a local minimum or of mountain pass type. Application to the mean field equation is provided.

It is known that each solution of the telegraph equation \begin{equation} u^{\prime \prime}(t)+2au^{\prime}(t)+A^2u(t)=0, \tag*{(0.1)} \end{equation} $(A=A^* $on$\: \mathcal {H}, a>0)$ is approximately equal to some solution of the abstract heat equation, \begin{equation} 2av^\prime(t) + A^2v(t)=0. \tag*{(0.2)} \end{equation} It is shown how to find $v(0)$, in terms of $u(0)$ and $u^\prime(0)$, so that one can say that a given solution of (0.1) is like a specific solution of (0.2).

We obtain some regularity conditions for solutions of the 3D generalized Navier-Stokes equations with fractional powers of the Laplacian, in terms of the velocity, the vorticity, and the pressure in Besov space, Triebel-Lizorkin space, and Lorentz space, respectively. We also present a regularity condition for the 3D Lagrangian averaged Euler equations.

This paper is devoted to the prescribed scalar curvature problem under minimal boundary mean curvature condition on the standard $n$-dimensional half Sphere, with $n\geq 3.$ Using tools related to the theory of critical points at infinity, we provide some topological conditions, on the level sets of a given positive function on $S_+^n$, under which we prove some existence results.

We investigate the problem of existence and symmetry of maximizers for \[ S(\alpha,4\pi)=\sup_{\|u\|=1 }{\int_B\left(e^{4\pi u^2}-1\right)|x|^{\alpha}dx,} \] where $B$ is the unit disk in $\mathbb{R}^2$ and $\alpha >0$, proposed by Secchi and Serra in [11]. Through the notion of spherical symmetrization with respect to a measure, we prove that supremum is attained for $\alpha$ small. Furthermore, we prove that $S(\alpha,4\pi)$ is attained by a radial function.

We use a direct approach for proving an isomorphism result for a general higher order abstract ordinary differential equation in a UMD Banach space. In fact, it gives maximal $L_p$-regularity property. As a consequence, we get some interpolation theorem (about intermediate derivatives). A situation of a higher order equation generated by one operator is also treated. Finally, we present some application to elliptic PDEs.

In this paper, we shall establish sufficient conditions for the existence of integral solutions and extremal integral solutions for some non-densely defined impulsive semilinear functional differential equations in separable Banach spaces. We shall rely on a fixed point theorem for the sum of completely continuous and contraction operators. The question of controllability of these equations are also considered.

One of the important open questions in the theory of free--surface ideal fluid flows is the dynamic stability of traveling wave solutions. In a spectral stability analysis, the first variation of the governing Euler equations is required which raises both theoretical and numerical issues. With Zakharov and Craig and Sulem's formulation of the Euler equations in mind, this paper addresses the question of analyticity properties of first (and higher) variations of the Dirichlet--Neumann operator. This analysis will have consequences not only for theoretical investigations, but also for numerical simulations of spectral stability of traveling water waves.

In this paper, we study the existence of asymptotically almost periodic solutions to some classes of second-order partial functional-differential equations with unbounded delay. Our results are, subsequently, applied to studying the existence of asymptotically almost periodic solutions to some integrodifferential equations, which arise in the theory of heat conduction within fading memory materials.