ACS condition on minimal isoparametric hypersurfaces of positive Ricci curvature in unit spheres

Abstract We study the Ambrozio–Carlotto–Sharp (ACS) criterion on minimal isoparametric hypersurfaces $N^{n+1}\subset S^{n+2}$ with positive Ricci curvature, motivated by the Schoen–Marques–Neves conjecture on Morse index. For $g=4$ distinct principal curvatures with multiplicities $m_1,m_2$ , we prove that the pointwise ACS inequality holds if and only if $\min \{m_1,m_2\}\ge 4$ . Sufficiency is obtained via a moment-relaxation technique yielding the sharp bound $4a^2$ on the quadratic part of the integrand; necessity follows from an explicit extremal configuration in the top eigenspace of the shape operator. We also verify the ACS condition for $g=3$ with $m_1=m_2\in \{4,8\}$ . As a consequence, for any closed embedded minimal hypersurface $M^n$ in such an ambient manifold, $\operatorname {index}(M)\ge \tfrac {2}{d(d-1)}\, b_1(M)$ with $d=n+3$ .