On Lu's second gap conjecture in higher codimension

Abstract

Let $M^n\to\Sph^{n+q}(1)$, $n\ge3$ and $q\ge2$, be a closed minimal immersion. Set $S=|h|^2$ and $Q=S+λ_2$, where $h$ is the second fundamental form and $λ_2$ is the second largest eigenvalue of Lu's fundamental matrix. We establish two complementary results concerning Lu's second-gap conjecture. First, for every $n\ge3$, we exhibit closed connected homogeneous minimal embeddings of $\Sph^1\times\Sph^{n-1}$ into $\Sph^{2n+1}(1)$ with constant $S$ and constant $Q$, whose $Q$-values are dense in $(n,2n)$. Totally geodesic inclusions yield the same density in every codimension $q\ge n+1$. Thus Lu's conjecture fails in these codimensions even under constant scalar curvature. Second, for every $n\ge3$, we prove that there exists $γ_n>0$, depending only on $n$, such that no closed connected minimal immersion into $\Sph^{n+2}(1)$ with constant $Q$ satisfies $n<Q<n+γ_n$. Hence Lu's second-gap conjecture holds in codimension two for all $n\ge3$.