Extension of Multilinear Fractional Integral Operators to Linear Operators on Lebesgue Spaces with Mixed Norms

Abstract

In [C. E. Kenig and E. M. Stein, Multilinear estimates and fractional integration, Math. Res. Lett., 6(1):1-15, 1999], the following type of multilinear fractional integral \[ \int_{\mathbb{R}^{mn}} \frac{f_1(l_1(x_1,\ldots,x_m,x))\cdots f_{m+1}(l_{m+1}(x_1,\ldots,x_m,x))}{(|x_1|+\ldots+|x_m|)^λ} dx_1\ldots dx_m \] was studied, where $l_i$ are linear maps from ${\mathbb{R}}^{(m+1)n}$ to ${\mathbb{R}}^n$ satisfying certain conditions. They proved the boundedness of such multilinear fractional integral from $L^{p_1}\times \dots \times L^{p_{m+1}}$ to $L^q$ when the indices satisfy the homogeneity condition. In this paper, we show that the above multilinear fractional integral extends to a linear operator for functions in the mixed-norm Lebesgue space $L^{\vec{p}}$ which contains $L^{p_1}\times \dots \times L^{p_{m+1}}$ as a subset. Under less restrictions on the linear maps $l_i$, we give a complete characterization of the indices $\vec{p}$, $q$ and $\lambda$ for which such an operator is bounded from $L^{\vec{p}}$ to $L^q$. And for $m=1$ or $n=1$, we give necessary and sufficient conditions on $(l_1,\dots ,l_{m+1})$, $\vec{p}=(p_1,\dots ,p_{m+1})$, $q$ and $\lambda$ such that the operator is bounded.

74 pages