2 research outputs found
Epsilon-Nets for Halfspaces Revisited
Given a set of points in , we show that, for any
, there exists an -net of for halfspace
ranges, of size . We give five proofs of this result, which
are arguably simpler than previous proofs \cite{msw-hnlls-90, cv-iaags-07,
pr-nepen-08}. We also consider several related variants of this result,
including the case of points and pseudo-disks in the plane
Tighter Estimates for epsilon-nets for Disks
The geometric hitting set problem is one of the basic geometric combinatorial
optimization problems: given a set of points, and a set of
geometric objects in the plane, the goal is to compute a small-sized subset of
that hits all objects in . In 1994, Bronniman and Goodrich
made an important connection of this problem to the size of fundamental
combinatorial structures called -nets, showing that small-sized
-nets imply approximation algorithms with correspondingly small
approximation ratios. Very recently, Agarwal and Pan showed that their scheme
can be implemented in near-linear time for disks in the plane. Altogether this
gives -factor approximation algorithms in time for hitting
sets for disks in the plane.
This constant factor depends on the sizes of -nets for disks;
unfortunately, the current state-of-the-art bounds are large -- at least
and most likely larger than . Thus the approximation
factor of the Agarwal and Pan algorithm ends up being more than . The best
lower-bound is , which follows from the Pach-Woeginger construction
for halfspaces in two dimensions. Thus there is a large gap between the
best-known upper and lower bounds. Besides being of independent interest,
finding precise bounds is important since this immediately implies an improved
linear-time algorithm for the hitting-set problem.
The main goal of this paper is to improve the upper-bound to
for disks in the plane. The proof is constructive, giving a simple algorithm
that uses only Delaunay triangulations. We have implemented the algorithm,
which is available as a public open-source module. Experimental results show
that the sizes of -nets for a variety of data-sets is lower, around