Extremal parameters, multiplicity and nonexistence for a singular elliptic problem
We consider the following class of singular elliptic problems \[ \begin{cases} -Δu = -\dfrac{u}{|u|^{β+1}}χ_{\{|u|>0\}} +λ|u|^{p-1}u, & \text{in }Ω,\\[1mm] u=0, & \text{on }\partialΩ, \end{cases} \] where $Ω\subset\mathbb R^N$ is a bounded smooth domain, $N\geq3$, $0<β,p<1$, and $λ>0$. The energy functional is not of class $C^1$ in $H_0^1(Ω)$, and the standard critical point theory does not apply. We prove that a nontrivial nonnegative weak solution exists if and only if $λ$ exceeds a critical value $\barλ$, which we identify and locate strictly between two Rayleigh-type parameters $0<λ_*<\barλ<λ^*<+\infty$. This gives a sharp existence threshold. We also prove that for every $λ$ in a neighborhood on the left of $λ^*$, there exist at least two distinct nontrivial nonnegative weak solutions. The first is a minimizer on the Nehari manifold, with an energy changing sign at $λ^*$. The second has a strictly positive energy. For $λ<λ^*$ sufficiently close to $λ^*$, both solutions have positive energy. This contrasts with previous multiplicity results for the same problem, in which the two solutions obtained have energies of opposite signs.