In this paper, we consider the following problemwhereλ> 0 is a parameter, and p =N+2/N-2. We will prove that when 3≤N≤5there exists 0 <λ* < +∞such that (*) has exactly two positive solutions forλ∈(0,λ*), and no solution forλ>λ*, a unique solution forλ=λ*, (λ*,u*) is a turning point in C2,α(RN)∩W2,2(RN). We will also give some analysis on the set of positive solutions.
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