1. [1] S. Agmon, A. Douglis and L. Nirenberg: Estimates near the boundary for solutions of elliptic partial differential equations satisfying general boundary conditions I, Comm. Pure Appl. Math., 12 (1959), 623-727.
2. [2] K. Akô and T. Kusano: On bounded solutions of second order elliptic differential equations, J. Fac. Sci. Univ. Tokyo. Sect. I, 11 (1964), 29-37.
3. [3] M. M. Belova: On bounded solutions of nonlinear differential equations of second order, Matem. Sbornik, 56 (99) (1962), 469-503. (in Russian)
4. [4] A. Douglis and L. Nirenberg: Interior estimates for elliptic systems of partial differential equations, Comm. Pure Appl. Math., 8 (1955), 503-538.
5. [5] R. Fiorenza: Sui problemi di derivata obliqua per le equazioni ellittiche, Ricerche di Mat., 8 (1959), 83-110.