On Spheres with $k$ Points Inside

Abstract: We generalize a classical result by Boris Delaunay that introduced Delaunay triangulations. In particular, we prove that for a locally finite and coarsely dense generic point set $A$ in $\mathbb{R}^d$, every generic point of $\mathbb{R}^d$ belongs to exactly $\binom{d+k}{d}$ simplices whose vertices belong to $A$ and whose circumspheres enclose exactly $k$ points of $A$. We extend this result to the cases in which the points are weighted, and when $A$ contains only finitely many points in $\mathbb{R}^d$ or in $\mathbb{S}^d$. Furthermore, we use the result to give a new geometric proof for the fact that volumes of hypersimplices are Eulerian numbers.