We study a nonlinear elliptic differential inclusion involving monotone and nonmonotone terms. Our formulation incorporates variational-hemivariational inequalities. The problem is strongly nonlinear in the sense that on the nonmonotone multivalued term we do not impose any growth condition and instead we use a generalized sign condition. Our approach is based on methods and techniques from multivalued analysis and from the theory of nonlinear operators of monotone type.

We prove that a system of conservation laws on $ \mathbb R^N$ is locally well posed in the `critical' Besov space $B^{\frac{N}{2} +1}_{2,1}$. The time of local existence depends only on the size of the inhomogeneous part of the initial data. We also obtain a blow-up criterion of the local solution. For the conservation system with a dissipation term added we prove global existence of solutions under the assumption of smallness of the homogeneous part of the arbitrary-sized $B^{\frac{N}{2} -1}_{2,1}$ norm of the initial data. For the proof of these results we essentially use the Littlewood-Paley decomposition of functions to derive the energy type of estimate.

In this paper we prove some existence and regularity results for solutions to a class of nonlinear parabolic equations whose prototype is $$\left\{\begin{array}{lll} \displaystyle\frac{\partial u}{\partial t}-\Delta_p u=f(x,t) &\mbox{ in }Q,\cr u(x,0)=0 &\mbox{ in } \Omega,\cr u(x,t)=0 &\mbox{ on } \Gamma, \end{array}\right. $$ \noindent where $\Omega$ is a bounded open subset of ${{\mathbb R}^ N} $, $N\ge 2$, $Q$ is the cylinder $\Omega \times ]0,T[$, $T>0$, $\Gamma$ the lateral surface $\partial\Omega\times ]0,T[$, $\bigtriangleup _p$ is the so-called $p-$Laplace operator, $ p>1 $ and $f$ belongs to some space $ L^r (0,T;L^q(\Omega )),$ $r\geq 1$, $q\geq 1$.

We establish the existence of nontrivial solutions to systems of singular Poisson equations in unbounded domains, under some invariance conditions and singular subcritical growth. The proofs rely on a concentration-compactness argument and on a generalized linking theorem due to Krysewski and Szulkin.

Let $\Omega\subset\mathbb{R}^{n}$, $n\geq2$, be a bounded domain withboundary $\partial\Omega$, and $Q=\Omega\times(0,T]$ be a cylinder of height $Ts < \infty$. We study local weak solutions of the parabolic equation \[ Lu\equiv\frac{\partial u}{\partial t}-div\left( \left| \nabla u\right| ^{p(z)-2}\nabla u\right)=0,\quad z=(x,t)\in\Omega\times(0,T), \] with variable exponent of nonlinearity $p$. We assume that $p(z)\in C(\Omega)$ and is such that \[ \frac{2n}{n+2}s < \alpha\leq p(z)\leq\beta < \infty,\quad z\in Q, \] \[ \left| p(z_{1})-p(z_{2})\right| \leq\omega(|z_{1}-z_{2}|)\quad \forall \,(z_{1},\,z_{2})\in\overline{Q}, \] \[ \overline{\lim_{\tau\rightarrow0}}\quad \omega(\tau)\ln\frac{1}{\tau}s <\infty. \] We prove that the weak solution is bounded and establish Meyer's type estimates: there exists a positive constant $\varepsilon>0$ such that for every subdomain $Q^{\prime}$, $\overline{Q^{\prime}}\subset Q$, \[ \int_{Q^{\prime}}\left| \nabla u\right| ^{p(z)(1+ \varepsilon )}dz s <\infty. \]