4,282 research outputs found
Piercing axis-parallel boxes
Let \F be a finite family of axis-parallel boxes in such that \F
contains no pairwise disjoint boxes. We prove that if \F contains a
subfamily \M of pairwise disjoint boxes with the property that for every
F\in \F and M\in \M with , either contains a
corner of or contains corners of , then \F can be
pierced by points. One consequence of this result is that if and
the ratio between any of the side lengths of any box is bounded by a constant,
then \F can be pierced by points. We further show that if for each two
intersecting boxes in \F a corner of one is contained in the other, then \F
can be pierced by at most points, and in the special case
where \F contains only cubes this bound improves to
Piercing axis-parallel boxes
Let F be a finite family of axis-parallel boxes in Rd such that F contains no k + 1 pairwise disjoint boxes. We prove that if F contains a subfamily M of k pairwise disjoint boxes with the property that for every F E F and M E M with F ∩ M ≠ 6= Ø, either F contains a corner of M or M contains 2d-1 corners of F, then F can be pierced by O(k) points. One consequence of this result is that if d = 2 and the ratio between any of the side lengths of any box is bounded by a constant, then F can be pierced by O(k) points. We further show that if for each two intersecting boxes in F a corner of one is contained in the other, then F can be pierced by at
most O(k log log(k)) points, and in the special case where F contains only cubes this bound improves to O(k).Supported by NSF grant DMS-1550991 and US Army Research Office Grant W911NF-16-1-040
Selection Lemmas for various geometric objects
Selection lemmas are classical results in discrete geometry that have been
well studied and have applications in many geometric problems like weak epsilon
nets and slimming Delaunay triangulations. Selection lemma type results
typically show that there exists a point that is contained in many objects that
are induced (spanned) by an underlying point set.
In the first selection lemma, we consider the set of all the objects induced
(spanned) by a point set . This question has been widely explored for
simplices in , with tight bounds in . In our paper,
we prove first selection lemma for other classes of geometric objects. We also
consider the strong variant of this problem where we add the constraint that
the piercing point comes from . We prove an exact result on the strong and
the weak variant of the first selection lemma for axis-parallel rectangles,
special subclasses of axis-parallel rectangles like quadrants and slabs, disks
(for centrally symmetric point sets). We also show non-trivial bounds on the
first selection lemma for axis-parallel boxes and hyperspheres in
.
In the second selection lemma, we consider an arbitrary sized subset of
the set of all objects induced by . We study this problem for axis-parallel
rectangles and show that there exists an point in the plane that is contained
in rectangles. This is an improvement over the previous
bound by Smorodinsky and Sharir when is almost quadratic
On Partitions of Two-Dimensional Discrete Boxes
Let and be finite sets and consider a partition of the \emph{discrete
box} into \emph{sub-boxes} of the form where and . We say that such a partition has the
-piercing property for positive integers and if every
\emph{line} of the form intersects at least sub-boxes and
every line of the form intersects at least sub-boxes.
We show that a partition of that has the -piercing
property must consist of at least sub-boxes. This bound is nearly sharp (up
to one additive unit) for every and .
As a corollary we get that the same bound holds for the minimum number of
vertices of a graph whose edges can be colored red and blue such that every
vertex is part of red -clique and a blue -clique.Comment: 10 pages, 2 figure
Approximation Algorithm for Line Segment Coverage for Wireless Sensor Network
The coverage problem in wireless sensor networks deals with the problem of
covering a region or parts of it with sensors. In this paper, we address the
problem of covering a set of line segments in sensor networks. A line segment `
is said to be covered if it intersects the sensing regions of at least one
sensor distributed in that region. We show that the problem of finding the
minimum number of sensors needed to cover each member in a given set of line
segments in a rectangular area is NP-hard. Next, we propose a constant factor
approximation algorithm for the problem of covering a set of axis-parallel line
segments. We also show that a PTAS exists for this problem.Comment: 16 pages, 5 figures
Polylogarithmic Approximation for Generalized Minimum Manhattan Networks
Given a set of terminals, which are points in -dimensional Euclidean
space, the minimum Manhattan network problem (MMN) asks for a minimum-length
rectilinear network that connects each pair of terminals by a Manhattan path,
that is, a path consisting of axis-parallel segments whose total length equals
the pair's Manhattan distance. Even for , the problem is NP-hard, but
constant-factor approximations are known. For , the problem is
APX-hard; it is known to admit, for any \eps > 0, an
O(n^\eps)-approximation.
In the generalized minimum Manhattan network problem (GMMN), we are given a
set of terminal pairs, and the goal is to find a minimum-length
rectilinear network such that each pair in is connected by a Manhattan
path. GMMN is a generalization of both MMN and the well-known rectilinear
Steiner arborescence problem (RSA). So far, only special cases of GMMN have
been considered.
We present an -approximation algorithm for GMMN (and, hence,
MMN) in dimensions and an -approximation algorithm for 2D.
We show that an existing -approximation algorithm for RSA in 2D
generalizes easily to dimensions.Comment: 14 pages, 5 figures; added appendix and figure
From a (p,2)-Theorem to a Tight (p,q)-Theorem
A family F of sets is said to satisfy the (p,q)-property if among any p sets of F some q have a non-empty intersection. The celebrated (p,q)-theorem of Alon and Kleitman asserts that any family of compact convex sets in R^d that satisfies the (p,q)-property for some q >= d+1, can be pierced by a fixed number (independent on the size of the family) f_d(p,q) of points. The minimum such piercing number is denoted by {HD}_d(p,q). Already in 1957, Hadwiger and Debrunner showed that whenever q > (d-1)/d p+1 the piercing number is {HD}_d(p,q)=p-q+1; no exact values of {HD}_d(p,q) were found ever since.
While for an arbitrary family of compact convex sets in R^d, d >= 2, a (p,2)-property does not imply a bounded piercing number, such bounds were proved for numerous specific families. The best-studied among them is axis-parallel boxes in R^d, and specifically, axis-parallel rectangles in the plane. Wegner (1965) and (independently) Dol\u27nikov (1972) used a (p,2)-theorem for axis-parallel rectangles to show that {HD}_{rect}(p,q)=p-q+1 holds for all q>sqrt{2p}. These are the only values of q for which {HD}_{rect}(p,q) is known exactly.
In this paper we present a general method which allows using a (p,2)-theorem as a bootstrapping to obtain a tight (p,q)-theorem, for families with Helly number 2, even without assuming that the sets in the family are convex or compact. To demonstrate the strength of this method, we show that {HD}_{d-box}(p,q)=p-q+1 holds for all q > c\u27 log^{d-1} p, and in particular, {HD}_{rect}(p,q)=p-q+1 holds for all q >= 7 log_2 p (compared to q >= sqrt{2p}, obtained by Wegner and Dol\u27nikov more than 40 years ago).
In addition, for several classes of families, we present improved (p,2)-theorems, some of which can be used as a bootstrapping to obtain tight (p,q)-theorems. In particular, we show that any family F of compact convex sets in R^d with Helly number 2 admits a (p,2)-theorem with piercing number O(p^{2d-1}), and thus, satisfies {HD}_{F}(p,q)=p-q+1 for all q>cp^{1-1/(2d-1)}, for a universal constant c
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