PORTUGALIAE MATHEMATICA Vol. 61, No. 4, pp. 439459 (2004) 

Semilinear problems with a nonsymmetric linear part having an infinite dimensional kernelJ. Berkovits and C. FabryDepartment of Mathematical Sciences, University of Oulu,P.O.Box 3000, FIN90014 Oulu  FINLAND Email: jberkovi@sun3.oulu.fi Institut de Mathématique Pures et Appliquée, Université Catholique de Louvain, Chemin du Cyclotron 2, B1348 LouvainlaNeuve  BELGIUM Email: fabry@math.ucl.ac.be Abstract: We consider semilinear equations, where the linear part $L$ is nonsymmetric and has a possibly infinite dimensional kernel. We shall show that, under certain monotonicity conditions for the nonlinearity, a generalized LeraySchauder degree can be defined for these problems. In order to build the degree theory, we introduce, for the nonlinearity $N$, monotonicity properties with respect to a linear map $T$, e.g. $T$pseudomonotonicity or maps of class $(S_+)_{T}$. As applications, we obtain new existence results for semilinear equations, in particular in resonance situations. In this latter case, we modify the standard inequalities of LandesmanLazer type by replacing the identity map $I$ by a linear homeomorphism ${\mathcal J}$, which will then appear in the monotonicity conditions. Keywords: nonsymmetric\,linear\,operators; topological\,degree; resonance; LandesmanLazer conditions. Classification (MSC2000): 47J25, 47H11. Full text of the article:
Electronic version published on: 7 Mar 2008.
© 2004 Sociedade Portuguesa de Matemática
