1. P.Popivanov. Local solvability of ψ do having double characteristics. Math. Sb. 100, (1976), 217–241.
2. Regularity and Solvability of Pseudo-differential Operators with Double Characteristics
3. Yu. V. Egorov. Linear differential operators of principal type. Monographs in Conteporary Math, Springer, (1986).
4. L. Hörmander. The Analysis of linear partial differential operators, I-IV, Springer, Berlin, (1983-1985).
5. N.Dencker. Operators of subprincipal type, arXiv:1507.05594 v2 [math.AP], (2015), 1–26.