Equivariant min-max hypersurface in $G$-manifolds with positive Ricci curvature

Abstract

In this paper, we consider a connected orientable closed Riemannian manifold $M^{n+1}$ with positive Ricci curvature. Suppose $G$ is a compact Lie group acting by isometries on $M$ with $3\leq {\rm codim}(G\cdot p)\leq 7$ for all $p\in M$. Then we show the equivariant min-max $G$-hypersurface $\Sigma$ corresponding to the fundamental class $[M]$ is a multiplicity one minimal $G$-hypersurface with a $G$-invariant unit normal and $G$-equivariant index one. As an application, we are able to establish a genus bound for $\Sigma$, a control on the singular points of $\Sigma/G$, and an upper bound for the (first) $G$-width of $M$ provided $n+1=3$ and the actions of $G$ are orientation preserving.