Let $\Omega $ be an open set of $\mathbb{R}^{n}$ and let $f:\Omega \times \mathbb{R}\times \mathbb{R}^{n}$ be a nonnegative continuous function, convex with respect to $\xi \in \mathbb{R}^{n}$. Following the well known theory originated by Serrin Serrin [14] in 1961, we deal with the lower semicontinuity of the integral $F\left( u,\Omega \right) =\int_{\Omega }f\left( x,u(x),Du(x)\right) \,dx$ with respect to the $L_{\text{loc} }^{1}\left( \Omega \right) $ strong convergence. Only recently it has been discovered that dependence of $f\left( x,s,\xi \right) $ on the $x$ variable plays a crucial role in the lower semicontinuity. In this paper we propose a mild assumption on $x$ that allows us to consider discontinuous integrands too. More precisely, we assume that $f\left( x,s,\xi \right) $ is a nonnegative Carathéodory function, convex with respect to $\xi $ , continuous in $\left( s,\xi \right) $ and such that $f(\cdot ,s,\xi )\in W_{\text{loc}}^{1,1}\left( \Omega \right) $ for every $s\in \mathbb{R}$ and $\xi \in \mathbb{R}^{n}$, with the $L^{1}$ norm of $f_{x}(\cdot ,s,\xi )$ locally bounded. We also discuss some other conditions on $x$; in particular we prove that Hölder continuity of $f$ with respect to $x$ is not sufficient for lower semicontinuity, even in the one dimensional case, thus giving an answer to a problem posed by the authors in [12]. Finally, we investigate the lower semicontinuity of the integral $F\left( u,\Omega \right) $, with respect to the strong norm topology of $L_{\text{ loc}}^{1}\left( \Omega \right) $, in the vector-valued case, i.e., when $ f:\Omega \times \mathbb{R}^{m}\times \mathbb{R}^{m\times n}\rightarrow \mathbb{R}$ for some $n\geq 1$ and $m>1$.

The uniqueness of weak solutions of the primitive equations with Dirichlet boundary conditions at the bottom is an open problem even in the two dimensional case. The aim of this paper is to prove the uniqueness of weak solutions when we replace the Dirichlet boundary condition at the bottom by a friction condition. With this boundary condition at the bottom, we establish an additional regularity result for the vertical derivative of the horizontal velocity which allows us to conclude the uniqueness of weak solutions.

We show existence and multiple existence results of subharmonic solutions for the system of second order differential equation $\ddot{u}(t)+ G'(u(t))=f(t)$ for $t\in \mathbb R$, where $N \in \mathbb N$, $f \in C(\mathbb R, \mathbb R^N)$ is a $T$-periodic function, $G\in C^{2}(\mathbb R^{N},\mathbb R)$ is a nonconvex functional.

We study the stability of standing waves $e^{i \omega t}\phi_{\omega}(x)$ for a nonlinear Schrödinger equation with an attractive power nonlinearity $|u|^{p-1}u$ and a potential $V(x)$ in $\mathbb R^n$. Here, $\omega\in \mathbb R$ and $\phi_{\omega}(x)$ is a ground state of the stationary problem. Under suitable assumptions on $V(x)$, we show that $e^{i \omega t}\phi_{\omega}(x)$ is stable for $p <1+4/n$ and sufficiently large $\omega$, or for $1 <p <2^*-1$ and $\omega$ close to $-\lambda_1$, where $\lambda_1$ is the lowest eigenvalue of the operator $-\Delta+V(x)$. We give an improvement of previous results such as Rose and Weinstein [19], or Grillakis, Shatah and Strauss [11], for unbounded potentials $V(x)$ which cannot be treated by the standard perturbation argument.