Ellipticity conditions for the Lax operator of the KP equations

The Lax pseudo-differential operator plays a key role in studying the gener al set of Kadomtsev-Petviashvili equations, although it is normally treated in a formal way, without worrying about a complete characterization of its mathematical properties. The aim of this paper is therefore to investigate the ellipticity condition. For this purpose, after a careful evaluation of the kernel with the associated symbol, the majorization ensuring elliptici ty is studied in detail. This leads to non-trivial restrictions on the admi ssible set of potentials in the Lax operator. When their time evolution is also considered, the ellipticity conditions turn out to involve derivatives of the logarithm of the tau -function.