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ECONOMICAL COVERS WITH GEOMETRIC APPLICATIONS

Published online by Cambridge University Press:  06 March 2003

NOGA ALON
Affiliation:
Department of Mathematics, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel. noga@math.tau.ac.il
BÉLA BOLLOBÁS
Affiliation:
Department of Mathematics, University of Memphis, Memphis, TN 38152-6429, USA and Trinity College, Cambridge CB2 1TQ. bollobas@msci.memphis.edu
JEONG HAN KIM
Affiliation:
Microsoft Research, Redmond, WA 98052, USA. jehkim@microsoft.com
VAN H. VU
Affiliation:
Department of Mathematics, University of California at San Diego, La Jolla, CA 92093, USA. vanvu@ucsd.edu, www.math.ucsd/∼vanvu
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Abstract

A cover of a hypergraph is a collection of edges whose union contains all vertices. Let $H = (V, E)$ be a $k$-uniform, $D$-regular hypergraph on $n$ vertices, in which no two vertices are contained in more than $o(D / e^{2k} \log D)$ edges as $D$ tends to infinity. Our results include the fact that if $k = o(\log D)$, then there is a cover of $(1 + o(1)) n / k$ edges, extending the known result that this holds for fixed $k$. On the other hand, if $k \geq 4 \log D $ then there are $k$-uniform, $D$-regular hypergraphs on $n$ vertices in which no two vertices are contained in more than one edge, and yet the smallest cover has at least $\Omega (\frac {n}{k} \log (\frac {k}{\log D} ))$ edges. Several extensions and variants are also obtained, as well as the following geometric application. The minimum number of lines required to separate $n$ random points in the unit square is, almost surely, $\Theta (n^{2/3} / (\log n)^{1/3}).$

2000 Mathematical Subject Classification: 05C65, 05D15, 60D05.

Type
Research Article
Copyright
2003 London Mathematical Society

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