## Computation of critical groups in resonance problems where the nonlinearity may not be sublinear.(English)Zbl 1088.58005

The authors compute critical groups at infinity (a notion introduced by Bartsch-Li in [T. Bartsch and S. J. Li, Nonlinear Anal., Theory Methods Appl. 28, No. 3, 419–441 (1997; Zbl 0872.58018)]) of the asymptotically quadratic functional $$F(u) ={1 \over 2}(Au,u) +G(u)$$ on a Hilbert space $$H$$, where $$A$$ is a bounded self-adjoint operator on $$H$$ and $$G\in C^ 1(H,{\mathbb R})$$ satisfies $${{| G(u)| }\over {\| u\|^ 2}} \to 0$$ as $$\| u\| \to \infty$$. They allow the nonlinearity to grow faster than sublinearly as they require the compact subdifferential $$g$$ of $$G$$ to satisfy $${{\| g(u)\| }\over {\| u\|}} \to 0$$ as $$\| u\| \to \infty$$. The case of sublinear growth at infinity $$\| g(u)\| \leq (\| u\|^ \alpha + 1)$$ for $$\alpha \in (0,1)$$ was treated by Li-Liu in [S. J. Li and J.Q. Liu, Houston J. Math. 25, No. 3, 563–582 (1999; Zbl 0981.58011)].
As applications, some existence theorems for asymptotically linear elliptic boundary value problems at resonance are proved.

### MSC:

 58E05 Abstract critical point theory (Morse theory, Lyusternik-Shnirel’man theory, etc.) in infinite-dimensional spaces 35J20 Variational methods for second-order elliptic equations 47J30 Variational methods involving nonlinear operators 49J35 Existence of solutions for minimax problems

### Keywords:

Morse theory; critical groups at infinty; resonance

### Citations:

Zbl 0872.58018; Zbl 0981.58011
Full Text:

### References:

 [1] Bartsch, T.; Li, S.J., Critical point theory for asymptotically quadratic functionals and applications to problems with resonance, Nonlinear anal., 28, 3, 419-441, (1997) · Zbl 0872.58018 [2] Chang, K.C., Infinite-dimensional Morse theory and multiple solution problems, progress in nonlinear differential equations and their applications, vol. 6, (1993), Birkhäuser Boston, MA [3] Li, S.J.; Liu, J.Q., Computations of critical groups at degenerate critical point and applications to nonlinear differential equations with resonance, Houston J. math., 25, 3, 563-582, (1999) · Zbl 0981.58011 [4] Liu, J.Q., The Morse index of a saddle point, Systems sci. math. sci., 2, 1, 32-39, (1989) · Zbl 0732.58011 [5] K. Perera, M. Schechter, Double resonance problems with respect to the Fučı́k spectrum, preprint. · Zbl 1030.35079 [6] Perera, K.; Schechter, M., Nontrivial solutions of elliptic semilinear equations at resonance, Manuscripta math., 101, 3, 301-311, (2000) · Zbl 0958.35051
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