Kakeya problem and projection problem for $k$-geodesics in Grassmannians

Abstract

The Kakeya problem in $\mathbb{R}^n$ is about estimating the size of union of $k$-planes; the projection problem in $\mathbb{R}^n$ is about estimating the size of projection of a set onto every $k$-plane ($1\le k\le n-1$). The $k=1$ case has been studied on general manifolds in which $1$-planes become geodesics, while $k\ge 2$ cases were still only considered in $\mathbb{R}^n$. We formulate these problems on homogeneous spaces, where $k$-planes are replaced by $k$-dimensional totally geodesic submanifolds. After formulating the problem, we prove a sharp estimate for Grassmannians.