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On the local structure of the Euler-Lagrange mapping. Pгoc. of the ČSSR-GDR-Polish Confeгence on Diffeгential Geometry and its Applications, Septembeг 1980, Nové Město na Moravě, edited by O. Kowalski, to appear.
MR 2402207
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Variational formulation of non-linear differential equations. I, II, Bull. Classe Sciences Acad. R. de Belgique 55 (1969), 137-165, 262-268.
MR 0256235