Let $b_1,b_2\in \mathrm{BMO}\left({ℝ}^n\right)$ and $T_{\sigma }$ be a bilinear Fourier multiplier operator with associated multiplier $\sigma$ satisfying the Sobolev regularity that ${sup}_{\kappa \in \mathbb{Z}}{\parallel {\sigma }_{\kappa }\parallel }_{W^{s_1,s_2}\left({ℝ}^{2n}\right)}<\infty$ for some $s_1,s_2\in (n/2,n]$. In this paper, the behavior on $L^{p_1}\left({ℝ}^n\right)\times L^{p_2}\left({ℝ}^n\right)$ $(p_1,p_2\in (1,\infty ))$, on $H^1\left({ℝ}^n\right)\times L^{p_2}\left({ℝ}^n\right)$ $(p_2\in [2,\infty ))$, and on $H^1\left({ℝ}^n\right)\times H^1\left({ℝ}^n\right)$, is considered for the commutator $T_{\sigma ,\overrightarrow{b}}$ defined by $$\begin{array}{ccc}\hfill T_{\sigma ,\overrightarrow{b}}(f_1,f_2)\left(x\right)=& b_1\left(x\right)T_{\sigma }(f_1,f_2)\left(x\right)-T_{\sigma }(b_1{f}_1,f_2)\left(x\right)\hfill & \hfill +b_2\left(x\right)T_{\sigma }(f_1,f_2)\left(x\right)-T_{\sigma }(f_1,b_2{f}_2)\left(x\right).\end{array}$$ By kernel estimates of the bilinear Fourier multiplier operators and employing some techniques in the theory of bilinear singular integral operators, it is proved that these mapping properties are very similar to those...

In this paper, we are going to characterize the space $\mathrm{BMO}\left({ℝ}^n\right)$ through variable Lebesgue spaces and Morrey spaces. There have been many attempts to characterize the space $\mathrm{BMO}\left({ℝ}^n\right)$ by using various function spaces. For example, Ho obtained a characterization of $\mathrm{BMO}\left({ℝ}^n\right)$ with respect to rearrangement invariant spaces. However, variable Lebesgue spaces and Morrey spaces do not appear in the characterization. One of the reasons is that these spaces are not rearrangement invariant. We also obtain an analogue...

Let $M_{\beta }$ be the fractional maximal function. The commutator generated by $M_{\beta }$ and a suitable function $b$ is defined by $[M_{\beta },b]f=M_{\beta }\left(bf\right)-b{M}_{\beta }\left(f\right)$. Denote by $𝒫\left({ℝ}^n\right)$ the set of all measurable functions $p(·):{ℝ}^n\to [1,\infty )$ such that $$1<p_{-}:=\underset{x\in {ℝ}^n}{\mathrm{ess}\inf }p\left(x\right)\text{and}{p}_{+}:=\underset{x\in {ℝ}^n}{\mathrm{ess}\sup }p\left(x\right)<\infty ,$$ and by $ℬ\left({ℝ}^n\right)$ the set of all $p(·)\in 𝒫\left({ℝ}^n\right)$ such that the Hardy-Littlewood maximal function $M$ is bounded on $L^{p(·)}\left({ℝ}^n\right)$. In this paper, the authors give some characterizations of $b$ for which $[M_{\beta },b]$ is bounded from $L^{p(·)}\left({ℝ}^n\right)$ into $L^{q(·)}\left({ℝ}^n\right)$, when $p(·)\in 𝒫\left({ℝ}^n\right)$, $0<\beta <n/p_{+}$ and $1/q(·)=1/p(·)-\beta /n$ with $q(·)(n-\beta )/n\in ℬ\left({ℝ}^n\right)$.

We prove an existence and uniqueness theorem for the Dirichlet problem for the equation $\text{div}\left(a\left(x\right)\nabla u\right)=\text{div}f$ in an open cube $\mathrm{\Omega }\subset {\mathbb{R}}^N$, when $f$ belongs to some $L^p\left(\mathrm{\Omega }\right)$, with $p$ close to 2. Here we assume that the coefficient $a$ belongs to the space BMO($\mathrm{\Omega }$) of functions of bounded mean oscillation and verifies the condition $a\left(x\right)\ge {\lambda }_0>0$ for a.e. $x\in \mathrm{\Omega }$.

Given a Borel function defined on a bounded open set with Lipschitz boundary and $\varphi \in L^1(\partial \Omega ,{\mathscr{H}}^{n-1})$, we prove an explicit representation formula for the lower semicontinuous envelope of Mumford-Shah type functionals with the obstacle constraint $u^{+}\ge \psi$ ${\mathscr{H}}^{n-1}$ a.e. on and the Dirichlet boundary condition $u=\varphi$ on $\partial \Omega$.

In the paper we find conditions on the pair $({\omega }_1,{\omega }_2)$ which ensure the boundedness of the maximal operator and the Calderón-Zygmund singular integral operators from one generalized Morrey space ${ℳ}_{p,{\omega }_1}$ to another ${ℳ}_{p,{\omega }_2}$, $1<p<\infty$, and from the space ${ℳ}_{1,{\omega }_1}$ to the weak space $W{ℳ}_{1,{\omega }_2}$. As applications, we get some estimates for uniformly elliptic operators on generalized Morrey spaces.