[Conjecture]任意闭的可见黎曼流形都存在一个度量，使得截面曲率小于等于-1.
Conjecture: Any closed visibility manifold $M^n$ must admit a metric $g$ of negative sectional curvature $\text{sec}_g\leq −1$.
Reference:
Jianguo Cao and Xiaoyang Chen, Minimal volume and simplicial norm of visibility n-manifolds and compact 3-manifolds. arXiv:0812.3353v4.