1. Ovcyannikov Theorem and Hyperdifferential Operators;Treves,1969
2. On the theory of linear partial differential equations with analytic coefficients;Treves;Trans. Amer. Math. Soc.,1969
3. Pseudo-differential operators;Nirenberg,1970
4. Linear Partial Differential Operators;Hörmander,1963
5. Pseudo-differential operators and hypoelliptic equations;Hörmander,1968