The solution gap of the Brezis-Nirenberg problem on the hyperbolic space

Abstract: We consider the positive solutions of the nonlinear eigenvalue problem $-\Delta_{\mathbb{H}^n} u = \lambda u + u^p, $ with $p=\frac{n+2}{n-2}$ and $u \in H_0^1(\Omega),$ where $\Omega$ is a geodesic ball of radius $\theta_1$ on $\mathbb{H}^n.$ For radial solutions, this equation can be written as an ODE having $n$ as a parameter. In this setting, the problem can be extended to consider real values of $n.$ We show that if $2<n\<4$ this problem has a unique positive solution if and only if $\lambda\in \left(n(n-2)/4 +L^*\,,\, \lambda_1\right).$ Here $L^*$ is the first positive value of $L = -\ell(\ell+1)$ for which a suitably defined associated Legendre function $P_{\ell}^{-\alpha}(\cosh\theta) \>0$ if $0 < \theta<\theta_1$ and $P_{\ell}^{-\alpha}(\cosh\theta_1)=0,$ with $\alpha = (2-n)/2.$