# Counting Quadrics and Delaunay Triangulations and a new Convex Hull Theorem

2 GEOMETRICA - Geometric computing
CRISAM - Inria Sophia Antipolis - Méditerranée
Abstract : Given a set $\cal S$ of $n$ points in three dimensions, we study the maximum numbers of quadrics spanned by subsets of points in $\cal S$ in various ways. We prove that the set of empty or enclosing ellipsoids has $\Theta(n^4)$ complexity. The same bound applies to empty general cylinders, while for empty circular cylinders a gap remains between the $\Omega(n^3)$ lower bound and the $O(n^4)$ upper bound. We also take interest in pairs of empty homothetic ellipsoids, with complexity $\Theta(n^6)$, while the specialized versions yield $\Theta(n^5)$ for pairs of general homothetic cylinders, and $\Omega(n^4)$ and $O(n^5)$ for pairs of parallel {circular} cylinders, respectively. This implies that the number of combinatorially distinct Delaunay triangulations obtained by orthogonal projections of $\cal S$ on a two-dimensional plane is $\Omega(n^4)$ and $O(n^5)$. Our lower bounds are derived from a generic geometric construction and its variants. The upper bounds result from tailored linearization schemes, in conjunction with a new result on convex polytopes which is of independent interest: In even dimensions~$d$, the convex hull of a set of $n$ points, where one half lies in a subspace of odd dimension~\mbox{$\delta > \frac{d}{2}$}, and the second half is the (multi-dimensional) projection of the first half on another subspace of dimension~$\delta$, has complexity only $\bigO{n^{\frac{d}{2}-1}}$.
Document type :
Reports
Complete list of metadata

https://hal.inria.fr/inria-00343651
Contributor : Olivier Devillers Connect in order to contact the contributor
Submitted on : Tuesday, December 2, 2008 - 2:10:01 PM
Last modification on : Friday, February 4, 2022 - 3:18:55 AM
Long-term archiving on: : Thursday, October 11, 2012 - 12:20:45 PM

### File

RR-6748.pdf
Files produced by the author(s)

### Identifiers

• HAL Id : inria-00343651, version 1

### Citation

Oswin Aichholzer, Olivier Devillers, Franz Aurenhammer, Thomas Hackl, Monique Teillaud, et al.. Counting Quadrics and Delaunay Triangulations and a new Convex Hull Theorem. [Research Report] RR-6748, INRIA. 2008. ⟨inria-00343651⟩

Record views