1. J. Nečas, Les méthodes in théorie deséquations elliptiques (Masson-Acad., Paris, 1967).
2. S. A. Nazarov, “Selfadjoint Elliptic Boundary Value Problems. The Polynomial Property and Formally Positive Operators,” in Problems of Mathematical Analysis, Vol. 16 (St. Petersburg. Gos. Univ., St. Petersburg, 1997), pp. 167–192.
3. S. A. Nazarov, “The Polynomial Property of Selfadjoint Elliptic Boundary Value Problems and an Algebraic Description of Their Attributes,” UspekhiMat. Nauk 54(5), 77–142 (1999) [RussianMath. Surveys 54 (5), 947–1014 (1999)].
4. J.-L. Lions and E. Madgenes, Problemes aux limites non homogenes et applications (Dunod, Paris, 1970; Mir, Moscow, 1971).
5. O. A. Ladyzhenskaya, Boundary Value Problems of Mathematical Physics (Nauka, Moscow, 1973) [in Russian].