Multiple solutions for resonant semilinear elliptic problems in RN

We prove the existence of multiple nontrivial solutions for the semilinear elliptic problem −Δu=h(λu+g(u)) in RN, u∈D1,2, where h∈L1∩Lα for α>N/2, N⩾3, g is a C1(R,R) function that has at most linear growth at infinity, g(0)=0, and λ is an eigenvalue of the corresponding linear problem −Δu=λhu in RN, u∈D1,2. Existence of multiple solutions, for certain values of g′(0), is obtained by imposing a generalized Landesman-Lazer type condition. We use the saddle point theorem of Ambrosetti and Rabinowitz and the mountain pass theorem, as well as a Morse-index result of Ambrosetti [A. Ambrosetti, Differential Equations with Multiple Solutions and Nonlinear Functional Analysis, Equadiff 82, Lecture Notes in Math., vol. 1017, Springer-Verlag, Berlin, 1983] and a Leray-Schauder index theorem for mountain pass type critical points due to Hofer [H. Hofer, A note on the Topological Degree at a critical Point of Mountain Pass Type, Proc. Amer. Math. Soc. 90 (1984) 309-315]. The results of this paper are based upon multiplicity results for resonant problems on bounded domains in [E. Landesman, S. Robinson, A. Rumbos, Multiple solutions of semilinear elliptic problems at resonance, Nonlinear Anal. 24 (1995) 1049-1059] and [S. Robinson, Multiple solutions for semilinear elliptic boundary value problems at resonance, Electron. J. Differential Equations 1995 (1995) 1-14], and complement a previous existence result by the authors in [G. López Garza, A. Rumbos, Resonance and strong resonance for semilinear elliptic equations in RN, Electron. J. Differential Equations 2003 (2003) 1-22] for resonant problems in RN in which g was assumed to be bounded.