On the Morse index of free-boundary CMC hypersurfaces in the upper hemisphere

Abstract

We prove results for free-boundary hypersurfaces in the upper unit hemisphere $\mathbb{S}^{n+1}_{+}$ of $\mathbb{R}^{n+2}$. First we show that if the norm squared of the second fundamental form is constant, the Morse index of a free-boundary minimal hypersurface $\Sigma\subset \mathbb{S}^{n+1}_{+}$ equals: $1$ if $\Sigma$ is a totally geodesic equator, $n+1$ if $\Sigma$ is half of the Clifford torus, or it is at least $2(n+1)$ when $\Sigma$ is not totally geodesic. Next we prove an estimate for the first eigenvalue $\lambda_1$ of the second variation's Jacobi operator, and show that $\lambda_1 \leq -2n$ if $\Sigma$ is not totally geodesic, with equality iff $\Sigma$ is half of the minimal Clifford torus. Furthermore, $\lambda_1 = -n$ iff $\Sigma$ is totally geodesic. Finally, if $\Sigma$ is not totally umbilical the Morse index is at least $n+1$, with equality precisely when $\Sigma$ is the upper $\mathrm{H}$-torus. For totally umbilical hypersurfaces the Morse index is $1$. We also prove an upper bound for the first eigenvalue of free-boundary $\mathrm{CMC}$ hypersurfaces, where equality corresponds to totally umbilical hypersurfaces.