1. Agmon, S., 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, 623–727(1959).
2. Gromov, M. L. Smoothing and inversion of differential operators. Mat. Sbornik
88 (130), 382–441 (1972) (Russian). Also in Math. USSR Sbornik
17, 381–434 (1972).
3. Gromov, M. L. and V. A. Rokhlin. Embeddings and immersions in Riemannian geometry. Usp. Mat. Nauk
25 (5) (155), 3–62 (1970) (Russian). Also in Russian Math. Surveys
25, (5) 1–57 (1970).
4. Heiskanen, W. A. and H. Moritz. Physical geodesy. San Francisco and London: W. H. Freeman and Co. 1967.
5. Hörmander, L. Linear partial differential operators. Berlin-Göttingen-Heidelberg: Springer 1963.