New Sphere Theorems under Curvature Operator of the Second Kind

Abstract

We investigate Riemannian manifolds $(M^n,g)$ whose curvature operator of the second kind $\mathring{R}$ satisfies the condition \begin{equation*} \alpha^{-1} (\lambda_1 +\cdots +\lambda_{\alpha}) > - \theta \bar{\lambda}, \end{equation*} where $\lambda_1 \leq \cdots \leq \lambda_{(n-1)(n+2)/2}$ are the eigenvalues of $\mathring{R}$, $\bar{\lambda}$ is their average, and $\theta > -1$. Under such conditions with optimal $\theta$ depending on $n$ and $\alpha$, we prove two differentiable sphere theorems in dimensions three and four, a homological sphere theorem in higher dimensions, and a curvature characterization of K\"ahler space forms. These results generalize recent works corresponding to $\theta =0$ of Cao-Gursky-Tran, Nienhaus-Petersen-Wink, and the author. Moreover, examples are provided to demonstrate the sharpness of all results.