1. S. Agmon, A. Douglis, and L. Nirenberg, “Estimates near the boundary for solutions of elliptic partial differential equations satisfying general boundary conditions I, II,” Commun. Pure Appl. Math. 12 (4), 623–727 (1959);
2. Commun. Pure Appl. Math. 17 (1), 35–92 (1964).
3. S. L. Sobolev, Cubature Formulas and Modern Analysis: An Introduction (Nauka, Moscow, 1974; Gordon and Breach, Montreux, 1992).
4. A. V. Bitsadze, “Some properties of polyharmonic functions,” Differ. Equations 24 (5), 543–548 (1988).
5. F. Gazzola, H. C. Grunau, and G. Sweers, Polyharmonic Boundary Value Problems: Positivity Preserving and Nonlinear Higher Order Elliptic Equations in Bounded Domains (Springer, Berlin, 1991).