On Stability and Isoperimetry of Constant Mean Curvature Spheres of $\mathbb H^n\times\mathbb R$ and $\mathbb S^n\times\mathbb R.$

Abstract: We approach the one-parameter family of rotational constant mean curvature (CMC) spheres of $\mathbb H^n\times\mathbb R$ and $\mathbb S^n\times\mathbb R$ focusing on their stability and isoperimetry properties. We prove that all rotational CMC spheres of $\mathbb H^n\times\mathbb R$ are stable, and that the ones in $\mathbb S^n\times\mathbb R$ with sufficiently small (resp.~large) mean curvature are unstable (resp.~stable). We also show that there exists a one-parameter family of stable CMC rotational spheres in $\mathbb S^n\times\mathbb R$ which are not isoperimetric (i.e., they do not bound isoperimetric regions). We establish the uniqueness of the regions enclosed by the rotational CMC spheres of $\mathbb H^n\times\mathbb R$ as solutions to the isoperimetric problem, filling in a gap in the original proof given by Hsiang and Hsiang. We establish, as well, a sharp upper bound for the volume of the spherical regions of $\mathbb S^n\times\mathbb R$ which are unique solutions to the isoperimetric problem. In essence, all these results come from the fact that the rotational CMC spheres of $\mathbb H^n\times\mathbb R$, and those of $\mathbb S^n\times\mathbb R$ with sufficiently large mean curvature, are nested.