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# Zeitschrift für Analysis und ihre Anwendungen

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**Volume 18, Issue 3, 1999, pp. 585–610**

**DOI: 10.4171/ZAA/900**

Published online: 1999-09-30

Existence Theorems for Boundary Value Problems for Strongly Nonlinear Elliptic Systems

Hong Thai Nguyen^{[1]}(1) Szczecin University, Poland

Let $L$ be a linear elliptic, a pseudomonotone or a generalized monotone operator (in the sense of F. E. Browder and I. V. Skrypnik), and let $F$ be the nonlinear Nemytskij superposition operator generated by a vector-valued function $f$. We give two general existence theorems for solutions of boundary value problems for the equation $Lx = Fx$. These theorems are based on a new functional-theoretic approach to the pair $(L, F)$, on the one hand, and on recent results on the operator $F$, on the other hand. We treat the above mentioned problems in the case of strong non-linearity $F$, i.e. in the case of lack of compactness of the operator $L - F$. In particular, we do not impose the usual growth conditions on the nonlinear function $f$; this allows us to treat elliptic systems with rapidly growing coefficients or exponential non-linearities. Concerning solutions, we consider existence in the classical weak sense, in the so-called $L_{\infty}$-weakened sense in both Sobolev and Sobolev-Orlicz spaces, and in a generalized weak sense in Sobolev-type spaces which are modelled by means of Banach $L_{\infty}$-modules. Finally, we illustrate the abstract results by some applied problems occuring in nonlinear mechanics.

*Keywords: *Strongly nonlinear boundary value problems, existence theorems, solution in the usual weak sense, solution in the $L_{\infty}$-weakened sense, elliptic operators, pseudomonotone operators, generalized monotone operators, complementary systems, Sobolev spaces, Sobolev-Orlicz spaces, exact embedding theorems

Nguyen Hong Thai: Existence Theorems for Boundary Value Problems for Strongly Nonlinear Elliptic Systems. *Z. Anal. Anwend.* 18 (1999), 585-610. doi: 10.4171/ZAA/900