no. S4
Least Squares Approximations to First Order Elliptic Systems
Publications des séminaires de mathématiques et informatique de Rennes, no. S4 (1977), article no. 11, 14 p. 
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     author = {Nitsche, J. A.},
     title = {Least {Squares} {Approximations} to {First} {Order} {Elliptic} {Systems}},
     journal = {Publications des s\'eminaires de math\'ematiques et informatique de Rennes},
     eid = {11},
     pages = {1--14},
     publisher = {D\'epartement de Math\'ematiques et Informatique, Universit\'e de Rennes},
     number = {S4},
     year = {1977},
     language = {en},
     url = {http://archive.numdam.org/item/PSMIR_1977___S4_A11_0/}
}
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Nitsche, J. A. Least Squares Approximations to First Order Elliptic Systems. Publications des séminaires de mathématiques et informatique de Rennes, no. S4 (1977), article  no. 11, 14 p. http://archive.numdam.org/item/PSMIR_1977___S4_A11_0/

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[2] Nitsche, J.A. A projection method for Dirichlet-Problems using subspaces with nearly zero boundary conditions in "The mathematical foundations of the finite element method with application to partial differential equations", K. Aziz and I. Babuska eds., Acad. Press, New York and London, 1972, 603-627 | MR | Zbl

[3] Vekua, I.N. Generalized Analytic Functions. Pergarnon Press, Oxford (1962). | MR | Zbl