Characterization of a two-parameter matrix valued BMO by commutator with the Hilbert transform

Source abstract

In this paper, we prove that the space of two parameter matrix-valued BMO functions can be char\-ac\-terized by considering iterated commutators with the Hilbert transform. Specifically, we prove that $$\| B \|_{BMO} \!\lesssim \! \| [[M_B, H_1],H_2] \|_{L^2(\mathbb{R} ^2;\mathbb{C} ^d) \rightarrow L^2(\mathbb{R} ^2;\mathbb{C} ^d)} \!\lesssim \! \| B \|_{BMO}.$$ The upper estimate relies on Petermichl's representation of the Hilbert transform as an average of dyadic shifts and the boundedness of certain paraproduct operators, while the lower bound follows Ferguson and Lacey's proof for the scalar case.