Hardy space estimates for bi-parameter Littlewood-Paley square functions

Suppose that g(f) are bi-parameter Littlewood-Paley square functions which were introduced by H. Martikainen. It is known that the L^{\mathrm{2}}({ℝ}^n\times {ℝ}^m) boundedness and the H^{\mathrm{1}}({ℝ}^n\times {ℝ}^m)-L^{\mathrm{1}}({ℝ}^n\times {ℝ}^m) boundedness of g(f) have been proved by H. Martikainen and by Z. Li and Q. Xue, respectively. In this paper, we apply the vector-valued theory, the atomic decomposition of product Hardy spaces, and Journe's covering lemma to show that g(f) are bounded from H^p({ℝ}^n\times {ℝ}^m) to L^p({ℝ}^n\times {ℝ}^m) with p smaller than 1.