In this paper, we make some observations on the work of Di Fazio concerning $W^{1,p}$ estimates, $1<p<\mathrm{\infty }$, for solutions of elliptic equations $\text{div}A\nabla u=\text{div}f$ , on a domain $\mathrm{\Omega }$ with Dirichlet data $0$ whenever $A\in VMO\left(\mathrm{\Omega }\right)$ and $f\in L^p\left(\mathrm{\Omega }\right)$. We weaken the assumptions allowing real and complex non-symmetric operators and $C^1$ boundary. We also consider the corresponding inhomogeneous Neumann problem for which we prove the similar result. The main tool is an appropriate representation for the Green (and Neumann) function on the upper half space....

We consider the following singularly perturbed elliptic problem $$\begin{array}{cc}\hfill {ϵ}^2\Delta u-u+f\left(u\right)& =0,u\&gt;0\text{in}\Omega ,\hfill \\ \hfill ϵ\frac{\partial u}{\partial \nu }+\lambda u& =0\text{on}\partial \Omega ,\hfill \end{array}$$ where $f$ satisfies some growth conditions, $0\le \lambda \le +\infty$, and $\Omega \subset {ℝ}^N$ ($N\&gt;1$) is a smooth and bounded domain. The cases $\lambda =0$ (Neumann problem) and $\lambda =+\infty$ (Dirichlet problem) have been studied by many authors in recent years. We show that, there exists a generic constant ${\lambda }_{*}\&gt;1$ such that, as $ϵ\to 0$, the least energy solution has a spike near the boundary if $\lambda \le {\lambda }_{*}$, and has an interior spike near the innermost part of the domain if $\lambda \&gt;{\lambda }_{*}$. Central...

Let $L$ be a second order, uniformly elliptic, non variational operator with coefficients which are bounded and measurable in ${\mathbb{R}}^d$ ($d\ge 3$) and continuous in ${\mathbb{R}}^d\setminus \left\{0\right\}$. Then, if $\mathrm{\Omega }\subset {\mathbb{R}}^d$ is a bounded domain, we prove that $L\left(W^{2,p}\left(\mathrm{\Omega }\right)\right)$ is dense in $L^p\left(\mathrm{\Omega }\right)$ for any $p\in \left(1,d/2\right]$.

In this paper we consider two-dimensional quasilinear equations of the form $\text{div}\left(a\left(\left|\nabla u\right|\right)\nabla u\right)+f\left(u\right)=0$ and study the properties of the solutions u with bounded and non-vanishing gradient. Under a weak assumption involving the growth of the argument of $\nabla u$(notice that $\text{arg}\left(\nabla u\right)$ is a well-defined real function since $\left|\nabla u\right|>0$ on ${\mathbb{R}}^2$) we prove that $u$ is one-dimensional, i.e., $u=u\left(\nu \cdot x\right)$ for some unit vector $\nu$. As a consequence of our result we obtain that any solution $u$ having one positive derivative is one-dimensional. This result provides...

In this paper we deal with the Hölder regularity up to the boundary of the solutions to a nonhomogeneous Dirichlet problem for second order discontinuous elliptic systems with nonlinearity $q>1$ and with natural growth. The aim of the paper is to clarify that the solutions of the above problem are always global Hölder continuous in the case of the dimension $n=q$ without any kind of regularity assumptions on the coefficients. As a consequence of this sharp result, the singular sets are always...