Boundedness of Bi-parameter Littlewood-Paley operators on product Hardy   space
  Let $n_1,n_2\ge 1, \lambda_1>1$ and $\lambda_2>1$. For any $x=(x_1,x_2) \in \mathbb {R}^n\times\mathbb{R}^m$, let $g$ and $g_{\vec{\lambda}}^*$ be the bi-parameter Littlewood-Paley square functions defined by \begin{align*} g(f)(x)= \Big(\int_0^{\infty}\int_0^{\infty}|\theta_{t_1,t_2} f(x_1,x_2)|^2 \frac{dt_1}{t_1} \frac{dt_2}{t_2} \Big)^{1/2}, \hbox{and} \end{align*} $$ g_{\vec{\lambda}}^*(f)(x) = \Big(\iint_{\mathbb{R}^{m+1}_{+}} \iint_{\mathbb{R}^{n+1}_{+}} \prod_{i=1}^2\Big(\frac{t_1}{t_i + |x_i - y_i|}\Big)^{n_i \lambda_i} |\theta_{t_1,t_2} f(y_1,y_2)|^2 \frac{dy_1 dt_1}{t_1^{n+1}} \frac{dy_2 dt_2}{t_2^{m+1}} \Big)^{1/2}, $$ \noindent where $\theta_{t_1,t_2} f(x_1, x_2) = \iint_{\mathbb{R}^n\times\mathbb{R}^m} s_{t_1,t_2}(x_1,x_2,y_1,y_2)f(y_1,y_2) dy_1dy_2$. It is known that the $L^2$ boundedness of bi-parameter $g$ and $g_{\vec{\lambda}}^*$ have been established recently by Martikainen, and Cao, Xue, respectively. In this paper, under certain structure conditions assumed on the kernel $s_{t_1,t_2},$ we show that both $g$ and $g_{\vec{\lambda}}^*$ are bounded from product Hardy space $H^1(\mathbb{R}^n\times\mathbb{R}^m)$ to $L^1(\mathbb{R}^n\times\mathbb{R}^m)$. As consequences, the $L^p$ boundedness of $g$ and $g_{\vec{\lambda}}^*$ will be obtained for $1<p<2$. 
Zhengyang Li and Qingying Xue
math.CA