This paper is devoted to the computation of the index of a critical point for nonlinear operators with strong coefficient growth. These operators are associated with boundary value problems of the type \begin{center} \displaystyle ∑_{|α|=1}\mathscr{D}
α{ρ
2(u)\mathscr{D}
αu+a
α(x, \mathscr{D}
1u)}=λ a
0(x, u, \mathscr{D}
1u), \ x∈Ω, u(x)=0, \ x∈∂Ω, \end{center} where Ω=\bm{R}
n is open, bounded and such that ∂Ω∈ \bm{C}
2, while ρ:\bm{R}→ \bm{R}
+ can have exponential growth. An index formula is given for such densely defined operators acting from the Sobolev space W
01, m(Ω) into its dual space. We consider different sets of assumptions for m>2 (the case of a real Banach space) and m=2 (the case of a real Hilbert space). The computation of the index is important for various problems concerning nonlinear equations: solvability, estimates for the number of solutions, branching of solutions, etc. The results of this paper are based upon recent results of the authors involving the computation of the index of a critical point for densely defined abstract operators of type (S
+). The latter are based in turn upon a new degree theory for densely defined (S
+)-mappings, which has also been developed by the authors in a recent paper. Applications of the index formula to the relevant bifurcation problems are also included.
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