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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. (4) 12 (1959), 623-727. MR 0125307 (23:A2610)

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N. G. Meyers and J. Serrin, , Proc. Nat. Acad. Sci. U.S.A. (1) 51 (1964), 1055-1056. MR 0164252 (29:1551)

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C. B. Morrey, Quasi-convexity and the lower semicontinuity of multiple integrals, Pacific J. Math. (1) 2 (1952), 25-53. MR 0054865 (14:992a)

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-, Multiple integral problems in the calculus of variations and related topics, Ann. Scuola Norm. Sup. Pisa (1) 14 (1960), 1-61. MR 0115117 (22:5919)

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L. Van Hove, Sur l'extension de la condition de Legendre du calcul des variations aux integrals multiples a plusiers fonctions inconnues, Nederl. Akad. Wetensch. (1) 50 (1947), 18-23. MR 0020223 (8:522a)