Boundedness criteria for bilinear Fourier multipliers via shifted square function estimates

Abstract: We prove a sharp criterion for the boundedness of bilinear Fourier multiplier operators associated with symbols obtained by summing all dyadic dilations of a given bounded function $m_0$ compactly supported away from the origin. Our result admits the best possible behavior with respect to a modulation of the function $m_0$ and is intimately connected with optimal bounds for the family of shifted square functions. As an application, we obtain estimates for bilinear singular integral operators with rough homogeneous kernels whose restriction to the unit sphere belongs to the Orlicz space $L(\log L)^\alpha$. This improves an earlier result of the first and third authors, where such estimates were established for rough kernels belonging to the space $L^q$, $q>1$, on the unit sphere.