3,722 research outputs found
On Grids in Point-Line Arrangements in the Plane
The famous Szemer\'{e}di-Trotter theorem states that any arrangement of
points and lines in the plane determines incidences, and this
bound is tight. In this paper, we prove the following Tur\'an-type result for
point-line incidence. Let and be two sets of
lines in the plane and let be the set of intersection points
between and . We say that forms a \emph{natural grid} if , and
does not contain the intersection point of some two lines in
for For fixed , we show that any arrangement
of points and lines in the plane that does not contain a natural
grid determines incidences, where
. We also provide a construction of points
and lines in the plane that does not contain a natural grid
and determines at least incidences.Comment: 13 pages, 5 figure
Dense point sets have sparse Delaunay triangulations
The spread of a finite set of points is the ratio between the longest and
shortest pairwise distances. We prove that the Delaunay triangulation of any
set of n points in R^3 with spread D has complexity O(D^3). This bound is tight
in the worst case for all D = O(sqrt{n}). In particular, the Delaunay
triangulation of any dense point set has linear complexity. We also generalize
this upper bound to regular triangulations of k-ply systems of balls, unions of
several dense point sets, and uniform samples of smooth surfaces. On the other
hand, for any n and D=O(n), we construct a regular triangulation of complexity
Omega(nD) whose n vertices have spread D.Comment: 31 pages, 11 figures. Full version of SODA 2002 paper. Also available
at http://www.cs.uiuc.edu/~jeffe/pubs/screw.htm
An Improved Bound for Weak Epsilon-Nets in the Plane
We show that for any finite set of points in the plane and
there exist
points in , for arbitrary small , that pierce every
convex set with . This is the first improvement
of the bound of that was
obtained in 1992 by Alon, B\'{a}r\'{a}ny, F\"{u}redi and Kleitman for general
point sets in the plane.Comment: A preliminary version to appear in the proceedings of FOCS 201
Discrete Geometry
The workshop on Discrete Geometry was attended by 53 participants, many of them young researchers. In 13 survey talks an overview of recent developments in Discrete Geometry was given. These talks were supplemented by 16 shorter talks in the afternoon, an open problem session and two special sessions. Mathematics Subject Classification (2000): 52Cxx. Abstract regular polytopes: recent developments. (Peter McMullen) Counting crossing-free configurations in the plane. (Micha Sharir) Geometry in additive combinatorics. (József Solymosi) Rigid components: geometric problems, combinatorial solutions. (Ileana Streinu) • Forbidden patterns. (János Pach) • Projected polytopes, Gale diagrams, and polyhedral surfaces. (Günter M. Ziegler) • What is known about unit cubes? (Chuanming Zong) There were 16 shorter talks in the afternoon, an open problem session chaired by Jesús De Loera, and two special sessions: on geometric transversal theory (organized by Eli Goodman) and on a new release of the geometric software Cinderella (Jürgen Richter-Gebert). On the one hand, the contributions witnessed the progress the field provided in recent years, on the other hand, they also showed how many basic (and seemingly simple) questions are still far from being resolved. The program left enough time to use the stimulating atmosphere of the Oberwolfach facilities for fruitful interaction between the participants
A Szemeredi-Trotter type theorem in
We show that points and two-dimensional algebraic surfaces in
can have at most
incidences, provided that the
algebraic surfaces behave like pseudoflats with degrees of freedom, and
that . As a special case, we obtain a
Szemer\'edi-Trotter type theorem for 2--planes in , provided
and the planes intersect transversely. As a further special case, we
obtain a Szemer\'edi-Trotter type theorem for complex lines in
with no restrictions on and (this theorem was originally proved by
T\'oth using a different method). As a third special case, we obtain a
Szemer\'edi-Trotter type theorem for complex unit circles in . We
obtain our results by combining several tools, including a two-level analogue
of the discrete polynomial partitioning theorem and the crossing lemma.Comment: 50 pages. V3: final version. To appear in Discrete and Computational
Geometr
The visible perimeter of an arrangement of disks
Given a collection of n opaque unit disks in the plane, we want to find a
stacking order for them that maximizes their visible perimeter---the total
length of all pieces of their boundaries visible from above. We prove that if
the centers of the disks form a dense point set, i.e., the ratio of their
maximum to their minimum distance is O(n^1/2), then there is a stacking order
for which the visible perimeter is Omega(n^2/3). We also show that this bound
cannot be improved in the case of a sufficiently small n^1/2 by n^1/2 uniform
grid. On the other hand, if the set of centers is dense and the maximum
distance between them is small, then the visible perimeter is O(n^3/4) with
respect to any stacking order. This latter bound cannot be improved either.
Finally, we address the case where no more than c disks can have a point in
common. These results partially answer some questions of Cabello, Haverkort,
van Kreveld, and Speckmann.Comment: 12 pages, 5 figure
Debris Disks: Seeing Dust, Thinking of Planetesimals and Planets
Debris disks are optically thin, almost gas-free dusty disks observed around
a significant fraction of main-sequence stars older than about 10 Myr. Since
the circumstellar dust is short-lived, the very existence of these disks is
considered as evidence that dust-producing planetesimals are still present in
mature systems, in which planets have formed - or failed to form - a long time
ago. It is inferred that these planetesimals orbit their host stars at asteroid
to Kuiper-belt distances and continually supply fresh dust through mutual
collisions. This review outlines observational techniques and results on debris
disks, summarizes their essential physics and theoretical models, and then
places them into the general context of planetary systems, uncovering
interrelations between the disks, dust parent bodies, and planets. It is shown
that debris disks can serve as tracers of planetesimals and planets and shed
light on the planetesimal and planet formation processes that operated in these
systems in the past.Comment: Review paper, accepted for publication in "Research in Astronomy and
Astrophysics
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