A new gap for complete hypersurfaces with constant mean curvature in space forms

Abstract

Let $M$ be an $n$-dimensional closed hypersurface with constant mean curvature and constant scalar curvature in an unit sphere. Denote by $H$ and $S$ the mean curvature and the squared length of the second fundamental form respectively. We prove that if $S>\alpha (n,H)$, where $n\ge 4$ and $H\ne 0$, then $S>\alpha (n,H)+B_n\frac{n{H}^2}{n-1}$. Here \[ α(n, H) = n + \frac{n^3}{2 (n - 1)} H^2 - \frac{n (n - 2)}{2 (n - 1)}\sqrt{n^2 H^4 + 4 (n - 1) H^2}, \] $B_n=\frac{1}{5}$ for $4\le n\le 20$, and $B_n=\frac{49}{250}$ for $n>20$. Moreover, we obtain a gap theorem for complete hypersurfaces with constant mean curvature and constant scalar curvature in space forms.

13 pages