On hypoellipticity and solvability of linear partial differential operators with multiple characteristics
This paper is devoted to the problem of constructing a ``characteristic manifold'' that takes into account lower order terms and at the same time describes the hypoellipticity or the local solvability of the operator in the same way as the classical characteristic does (for elliptic or principal type operators). This strategy is applied to some examples of operators with multiple characteristics.
scientific article; zbMATH DE number 469234