New Lower Bounds for ϵ-nets

International audienceFollowing groundbreaking work by Haussler and Welzl (1987), the use of small-nets has become a standard technique for solving algorithmic and extremal problems in geometry and learning theory. Two significant recent developments are: (i) an upper bound on the size of the smallest-nets for set systems, as a function of their so-called shallow-cell complexity (Chan, Grant, Könemann, and Sharpe); and (ii) the construction of a set system whose members can be obtained by intersecting a point set in R^4 by a family of half-spaces such that the size of any-net for them is Ω(1/ϵ log 1/ϵ) (Pach and Tardos).The present paper completes both of these avenues of research. We (i) give a lower bound, matching the result of Chan et al., and (ii) generalize the construction of Pach and Tardos to half-spaces in R^d, for any d ≥ 4, to show that the general upper bound, O(d/ϵ log 1/ϵ), of Haussler ans Welzl for the size of the smallest ϵ-nets is tight