Anisotropic Bilinear L2 Estimates Related to the 3D Wave Equation

We first review the L² bilinear generalizations of the L⁴ estimate of Strichartz for solutions of the homogeneous 3D wave equation and give a short proof based solely on an estimate for the volume of intersection of two thickened spheres. We then go on to prove a number of new results, the main theme being how additional anisotropic Fourier restrictions influence the estimates. Moreover we prove some refinements, which are able to simultaneously detect both concentrations and nonconcentrations in Fourier space.