This paper continues the development of regularity results for quasilinear measure data problems begin{align*} begin{cases} -mathrm{div}(A(x, abla u)) &= mu quad text{in} Omega, quad quad qquad u &=0 quad text{on} partial Omega, end{cases} end{align*} in Lorentz and Lorentz-Morrey spaces, where $Omega subset mathbb{R}^n$ ($n ge 2$), $mu$ is a finite Radon measure on $Omega$, and $A$ is a monotone Caratheodory vector valued operator acting between $W^{1,p}_0(Omega)$ and its dual $W^{-1,p}(Omega)$. It emphasizes that this paper studies the `very singular case $1<p le frac{3n-2}{2n-1}$ and the problem is considered under the weak assumption, where the $p$-capacity uniform thickness condition is imposed on the complement of domain $Omega$. There are two main results obtained in our study pertaining to the global gradient estimates of solutions in Lorentz and Lorentz-Morrey spaces involving the use of maximal and fractional maximal operators. The idea for writing this working paper comes directly from the recent results by others in the same research topic, where global estimates for gradient of solutions for the `very singular case still remains a challenge, specifically related to Lorentz and Lorentz-Morrey spaces.
We study interior $L^p$-regularity theory, also known as Calderon-Zygmund theory, of the equation [ int_{mathbb{R}^n} int_{mathbb{R}^n} frac{K(x,y) (u(x)-u(y)), (varphi(x)-varphi(y))}{|x-y|^{n+2s}}, dx, dy = langle f, varphi rangle quad varphi in C_c^infty(mathbb{R}^n). ] For $s in (0,1)$, $t in [s,2s]$, $p in [2,infty)$, $K$ an elliptic, symmetric, Holder continuous kernel, if $f in left (H^{t,p}_{00}(Omega)right )^ast$, then the solution $u$ belongs to $H^{2s-t,p}_{loc}(Omega)$ as long as $2s-t < 1$. The increase in differentiability is independent of the Holder coefficient of $K$. For example, our result shows that if $fin L^{p}_{loc}$ then $uin H^{2s-delta,p}_{loc}$ for any $deltain (0, s]$ as long as $2s-delta < 1$. This is different than the classical analogue of divergence-form equations ${rm div}(bar{K} abla u) = f$ (i.e. $s=1$) where a $C^gamma$-Holder continuous coefficient $bar{K}$ only allows for estimates of order $H^{1+gamma}$. In fact, it is another appearance of the differential stability effect observed in many forms by many authors for this kind of nonlocal equations -- only that in our case we do not get a small differentiability improvement, but all the way up to $min{2s-t,1}$. The proof argues by comparison with the (much simpler) equation [ int_{mathbb{R}^n} K(z,z) (-Delta)^{frac{t}{2}} u(z) , (-Delta)^{frac{2s-t}{2}} varphi(z), dz = langle g,varphirangle quad varphi in C_c^infty(mathbb{R}^n). ] and showing that as long as $K$ is Holder continuous and $s,t, 2s-t in (0,1)$ then the commutator [ int_{mathbb{R}^n} K(z,z) (-Delta)^{frac{t}{2}} u(z) , (-Delta)^{frac{2s-t}{2}} varphi(z), dz - cint_{mathbb{R}^n} int_{mathbb{R}^n} frac{K(x,y) (u(x)-u(y)), (varphi(x)-varphi(y))}{|x-y|^{n+2s}}, dx, dy ] behaves like a lower order operator.
We construct an efficient approach to deal with the global regularity estimates for a class of elliptic double-obstacle problems in Lorentz and Orlicz spaces. The motivation of this paper comes from the study on an abstract result in the viewpoint of the fractional maximal distributions and this work also extends some regularity results proved in cite{PN_dist} by using the weighted fractional maximal distributions (WFMDs). We further investigate a pointwise estimates of the gradient of weak solutions via fractional maximal operators and Riesz potential of data. Moreover, in the setting of the paper, we are led to the study of problems with nonlinearity is supposed to be partially weak BMO condition (is measurable in one fixed variable and only satisfies locally small-BMO seminorms in the remaining variables).
Given $2leq p<infty$, $sin (0, 1)$ and $tin (1, 2s)$, we establish interior $W^{t,p}$ Calderon-Zygmund estimates for solutions of nonlocal equations of the form [ int_{Omega} int_{Omega} Kleft (x,|x-y|,frac{x-y}{|x-y|}right ) frac{(u(x)-u(y))(varphi(x)-varphi(y))}{|x-y|^{n+2s}} dx dy = g[varphi], quad forall phiin C_c^{infty}(Omega) ] where $Omegasubset mathbb{R}^{n}$ is an open set. Here we assume $K$ is bounded, nonnegative and continuous in the first entry -- and ellipticity is ensured by assuming that $K$ is strictly positive in a cone. The setup is chosen so that it is applicable for nonlocal equations on manifolds, but the structure of the equation is general enough that it also applies to the certain fractional $p$-Laplace equations around points where $u in C^1$ and $| abla u| eq 0$.