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. --. --. --. --., Estimates near the boundary for solutions of elliptic partial differential equations satisfying general boundary conditions, II, Comm. Pure Appl. Math. 17 (1964), 35--72.
3. F. Bureau, Divergent integrals and partial differential equations, Comm. Pure Appl. Math. 8 (1955), 143--202.
4. S. Campanato, Sistemi parabolici del secondo ordine, non variazionali, a coefficienti discontinui, Ann. Univ. Ferrara 23 Sez. VII (N.S.) (1977), 169--187.
5. F. Chiarenza, M. Franciosi, and M. Frasca, $L^p$-estimates for linear elliptic systems with discontinuous coefficients, Atti Accad. Naz. Lincei Cl. Sci. Fis. Mat. Natur. Rend. Lincei (9) Mat. Appl. 5 (1994), 27--32.