Infinite families of manifolds of positive $k^{\rm th}$-intermediate Ricci curvature with $k$ small

Abstract

Positive $k^{\rm th}$-intermediate Ricci curvature on a Riemannian $n$-manifold, to be denoted by $\mathrm{Ric}_k > 0$, is a condition that interpolates between positive sectional and positive Ricci curvature (when $k =1$ and $k=n-1$ respectively). In this work, we produce many examples of manifolds of $\mathrm{Ric}_k > 0$ with $k$ small by examining symmetric and normal homogeneous spaces, along with certain metric deformations of fat homogeneous bundles. As a consequence, we show that every dimension $n\geq 7$ congruent to $3\,\mathrm{mod}\ 4$ supports infinitely many closed simply connected manifolds of pairwise distinct homotopy type, all of which admit homogeneous metrics of $\mathrm{Ric}_k > 0$ for some $k<n/2$. We also prove that each dimension $n\geq 4$ congruent to $0$ or $1\,\mathrm{mod}\ 4$ supports closed manifolds which carry metrics of $\mathrm{Ric}_k > 0$ with $k\leq n/2$, but do not admit metrics of positive sectional curvature.