Implicit elliptic differential equations

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Frigon, M. and Kaczynski, T.: Boundary value problems for systems of implicit differential equations,J. Math. Anal. Appl. 179 (1993), 317?326.

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Marano, S.A.: On a boundary value problem for the differential equationf(t, x, x?, x?)=0,J. Math. Anal. Appl. 182 (1994), 309?319.

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