Formal and Convergent Solutions of Singular Partial Differential Equations. Gunter Bengel, Raymond Gérard (1982) Manuscripta mathematica
Fundamental operator-functions of degenerate differential and difference-differential operators in Banach spaces which have a Noether operator in the principal part. Falaleev, M.V., Grazhdantseva, E.Yu. (2005) Sibirskij Matematicheskij Zhurnal
Fundamental solution for operators preserving a quadratic form Zofia Szmydt, Bogdan Ziemian (1983) Annales Polonici Mathematici