In this paper, we pursue a systematic treatment of the regularity theory fo
r products and bilinear forms of solutions of the homogeneous wave equation
. We discuss necessary and sufficient conditions for the validity of biline
ar estimates, based on L-2 norms in space and time, of derivatives of produ
cts of solutions. Also, we give necessary conditions and formulate some con
jectures for similar estimates based on (LtLxr)-L-q, norms. (C) 2000 Editio
ns scientifiques et medicales Elsevier SAS.