1. Sagraloff, B., Normal solvability of linear partial differential operators in
$$C^\infty
(\Omega )$$, Geom.
Approaches Differ. Equat. Lect. Not. Math., 2006, vol. 810,
pp. 290–305.
2. Hörmander, L., The Analysis of Linear
Partial Differential Operators I. Distribution Theory and Fourier Analysis,
Berlin–Heidelberg–New York–Tokyo: Springer-Verlag, 1983. Translated under the title:
Analiz lineinykh differentsial’nykh operatorov, Moscow:
Mir, 1986.
3. Duistermaat, J.J. and Hörmander, L., Fourier integral operators. II,
Acta Math., 1972, vol. 128, no. 3–4,
pp. 183–269.
4. Hörmander, L., Propagation of singularities and semiglobal existence
theorems for (pseudo)differential operators of principal type, Ann.
Math., 1978, vol. 108 (3), no. 2, pp. 569–609.
5. Hounie, J., A note on global solvability of vector fields, Proc. Am. Math. Soc., 1985, vol. 94, no. 1,
pp. 61–64.