1. R.ARTINO, “Gevrey classes and hypoelliptic boundary value problem”, Thesis, Rutgers University.
2. J.BARROS NETO, The parametrix of a regular hypoelliptic boundary value problems, Ann. della Sc. Normale Superiori di Pisa, Serie III, vol. XXVI, Fasc. 1, (1972), 247–268.
3. J.BARROS NETO, On regular hypoelliptic boundary problems, Journ. Math. Anal. and Appl., vol. 41, No2, 1973, pp. 508–530.
4. L.HÖRMANDER, On the regularity of the solutions of boundary problems, Acta Math. 99 (1958), 225–264.
5. L.HÖRMANDER, Linear Partial differential operators, Springer Verlag, Berlin, 1963.