Multilinear fractional maximal and integral operators with homogeneous kernels, Hardy--Littlewood--Sobolev and Olsen-type inequalities

Abstract: Let $m\in \mathbb{N}$ and $0<\alpha<mn$.In this paper, we will use the idea of Hedberg to reprove that the multilinear operators $\mathcal{T}{\Omega,\alpha;m}$ and $\mathcal{M}{\Omega,\alpha;m}$ are bounded from $L^{p_1}(\mathbb R^n)\times L^{p_2}(\mathbb R^n)\times\cdots\times L^{p_m}(\mathbb R^n)$ into $L^q(\mathbb R^n)$ provided that $\vec{\Omega}=(\Omega_1,\Omega_2,\dots,\Omega_m)\in L^s(\mathbf{S}^{n-1})$, $s'<p_1,p_2,\dots,p_m<n/{\alpha}$, \begin{equation*} \frac{\,1\,}{p}=\frac{1}{p_1}+\frac{1}{p_2}+\cdots+\frac{1}{p_m} \quad \mbox{and} \quad \frac{\,1\,}{q}=\frac{\,1\,}{p}-\frac{\alpha}{n}. \qquad () \end{equation} We also prove that under the assumptions that $\vec{\Omega}=(\Omega_1,\Omega_2,\dots,\Omega_m)\in L^s(\mathbf{S}^{n-1})$, $s'\leq p_1,p_2,\dots,p_m<n/{\alpha}$ and $()$, the multilinear operators $\mathcal{T}{\Omega,\alpha;m}$ and $\mathcal{M}{\Omega,\alpha;m}$ are bounded from $L^{p_1}(\mathbb R^n)\times L^{p_2}(\mathbb R^n)\times \cdots\times L^{p_m}(\mathbb R^n)$ into $L^{q,\infty}(\mathbb R^n)$, which are completely new. Moreover, we will use the idea of Adams to show that $\mathcal{T}{\Omega,\alpha;m}$ and $\mathcal{M}{\Omega,\alpha;m}$ are bounded from $L^{p_1,\kappa}(\mathbb R^n)\times L^{p_2,\kappa}(\mathbb R^n)\times \cdots\times L^{p_m,\kappa}(\mathbb R^n)$ into $L^{q,\kappa}(\mathbb R^n)$ whenever $s'<p_1,p_2,\dots,p_m<n/{\alpha}$, $0<\kappa<1$, \begin{equation} \frac{\,1\,}{p}=\frac{1}{p_1}+\frac{1}{p_2}+\cdots+\frac{1}{p_m} \quad \mbox{and} \quad \frac{\,1\,}{q}=\frac{\,1\,}{p}-\frac{\alpha}{n(1-\kappa)},\qquad () \end{equation*} and also bounded from $L^{p_1,\kappa}(\mathbb R^n)\times L^{p_2,\kappa}(\mathbb R^n)\times \cdots\times L^{p_m,\kappa}(\mathbb R^n)$ into $WL^{q,\kappa}(\mathbb R^n)$ whenever $s'\leq p_1,p_2,\dots,p_m<n/{\alpha}$, $0<\kappa<1$ and $()$.