On the bilinear square Fourier multiplier operators and related multilinear square functions (1604.05579v1)

Abstract: Let $n\ge 1$ and $\mathfrak{T}{m}$ be the bilinear square Fourier multiplier operator associated with a symbol $m$, which is defined by $$ \mathfrak{T}{m}(f_1,f_2)(x) = \biggl( \int_{0}^\infty\Big|\int_{(\mathbb{R}^n)^2} e^{2\pi ix\cdot (\xi_1 +\xi_2) }m(t\xi_1,t\xi_2) \hat{f}{1}(\xi_1)\hat{f}{2}(\xi_2)d\xi_1 d\xi_2\Big|^2\frac{dt}{t } \biggr)^{\frac 12}. $$ Let $s$ be an integer with $s\in[n+1,2n]$ and $p_0$ be a number satisfying $2n/s\le p_0\le 2$. Suppose that $\nu_{\vec{\omega}}=\prod_{i=1}^2\omega_i^{p/ p_i}$ and each $\omega_i$ is a nonnegative function on $\mathbb{R}^n$. In this paper, we show that $\mathfrak{T}{m}$ is bounded from $L^{p_1}(\omega_1)\times L^{p_2}(\omega_2)$ to $L^p(\nu{\vec{\omega}})$ if $p_0< p_1, p_2<\infty$ with $1/p=1/p_1+ 1/p_2$. Moreover, if $p_0>2n/s$ and $p_1=p_0$ or $p_2=p_0$, then $\mathfrak{T}{m}$ is bounded from $L^{p_1}(\omega_1)\times L^{p_2}(\omega_2)$ to $L^{p,\infty}(\nu{\vec{\omega}})$. The weighted end-point $L\log L$ type estimate and strong estimate for the commutators of $\mathfrak{T}_{m}$ are also given. These were done by considering the boundedness of some related multilinear square functions associated with mild regularity kernels and essentially improving some basic lemmas which have been used before.