The -t-Net Problem Noga Alon Department of Mathematics, Princeton University, Princeton, NJ 08544, USA Schools of Mathematics and Computer Science, Tel Aviv University, Tel Aviv 69978, Israel
[email protected] Bruno Jartoux Department of Computer Science, Ben-Gurion University of the Negev, Be’er-Sheva, Israel
[email protected] Chaya Keller Department of Computer Science, Ariel University, Ariel, Israel
[email protected] Shakhar Smorodinsky Department of Mathematics, Ben-Gurion University of the Negev, Be’er-Sheva, Israel
[email protected] Yelena Yuditsky Department of Mathematics, Ben-Gurion University of the Negev, Be’er-Sheva, Israel
[email protected] Abstract We study a natural generalization of the classical -net problem (Haussler–Welzl 1987), which we t call the -t-net problem: Given a hypergraph on n vertices and parameters t and ≥ n , find a minimum-sized family S of t-element subsets of vertices such that each hyperedge of size at least n contains a set in S. When t = 1, this corresponds to the -net problem. We prove that any sufficiently large hypergraph with VC-dimension d admits an -t-net of (1+log t)d 1 size O( log ). For some families of geometrically-defined hypergraphs (such as the dual 1 hypergraph of regions with linear union complexity), we prove the existence of O( )-sized -t-nets. We also present an explicit construction of -t-nets (including -nets) for hypergraphs with bounded VC-dimension. In comparison to previous constructions for the special case of -nets (i.e., for t = 1), it does not rely on advanced derandomization techniques.