A solution to Lu's second gap conjecture

Abstract

Let $M^n\to {\mathbb{S}}^{n+q}(1)$ be a closed connected minimal immersion, where $n\ge 3$, and set $Q=S+{\lambda }_2$, with $S=|h{|}^2$ and ${\lambda }_2$ the second largest eigenvalue of Lu's fundamental matrix. We determine the sharp codimension range for Lu's second-gap conjecture. For every $2\le q\le n$, there exists ${\gamma }_{n,q}>0$ such that, if $Q$ is constant and $Q>n$, then $Q\ge n+{\gamma }_{n,q}$. Conversely, for every $q\ge n+1$, we construct closed connected homogeneous minimal embeddings, followed when necessary by totally geodesic inclusions, with constant scalar curvature and constant $Q$-values dense in $(n,2n)$. Thus, in every dimension $n\ge 3$, Lu's conjecture holds precisely for $q\le n$. Combined with the theorem of Peng-Terng for hypersurfaces and the recent resolution of the two-dimensional case, this gives a complete resolution of Lu's second-gap conjecture: for every $n\ge 2$, the conjecture holds exactly when $q\le n$ and fails when $q\ge n+1$. In codimension two we further obtain the explicit admissible gap ${\gamma }_{n,2}=\exp (-10^{16}{n}^2)$.

arXiv comment56 pages. Comments and suggestions are welcome