33 research outputs found
On the general position subset selection problem
Let be the maximum integer such that every set of points in
the plane with at most collinear contains a subset of points
with no three collinear. First we prove that if then
. Second we prove that if
then , which implies all previously known lower bounds on and
improves them when is not fixed. A more general problem is to consider
subsets with at most collinear points in a point set with at most
collinear. We also prove analogous results in this setting
General Position Subsets and Independent Hyperplanes in d-Space
Erd\H{o}s asked what is the maximum number such that every set of
points in the plane with no four on a line contains points in
general position. We consider variants of this question for -dimensional
point sets and generalize previously known bounds. In particular, we prove the
following two results for fixed :
- Every set of hyperplanes in contains a subset
of size at least , for some
constant , such that no cell of the arrangement of is bounded by
hyperplanes of only.
- Every set of points in , for some constant
, contains a subset of cohyperplanar points or points in
general position.
Two-dimensional versions of the above results were respectively proved by
Ackerman et al. [Electronic J. Combinatorics, 2014] and by Payne and Wood [SIAM
J. Discrete Math., 2013].Comment: 8 page
Sets with few distinct distances do not have heavy lines
Let be a set of points in the plane that determines at most
distinct distances. We show that no line can contain more than points of . We also show a similar result for rectangular
distances, equivalent to distances in the Minkowski plane, where the distance
between a pair of points is the area of the axis-parallel rectangle that they
span
On Cartesian Products which Determine Few Distinct Distances
Every set of points P determines Ω(|P|/log|P|) distances. A close version of this was initially conjectured by Erdős in 1946 and rather recently proved by Guth and Katz. We show that when near this lower bound, a point set P of the form A×A must satisfy |A−A|≪|A|2−2/7log1/7|A| This improves recent results of Hanson and Roche-Newton
Bisector energy and few distinct distances
We introduce the bisector energy of an -point set in ,
defined as the number of quadruples from such that and
determine the same perpendicular bisector as and . If no line or circle
contains points of , then we prove that the bisector energy is
. We also prove the
lower bound , which matches our upper bound when is
large. We use our upper bound on the bisector energy to obtain two rather
different results:
(i) If determines distinct distances, then for any
, either there exists a line or circle that contains
points of , or there exist
distinct lines that contain points of . This result
provides new information on a conjecture of Erd\H{o}s regarding the structure
of point sets with few distinct distances.
(ii) If no line or circle contains points of , then the number of
distinct perpendicular bisectors determined by is
. This appears to
be the first higher-dimensional example in a framework for studying the
expansion properties of polynomials and rational functions over ,
initiated by Elekes and R\'onyai.Comment: 18 pages, 2 figure
Convexity, Elementary Methods, and Distances
This paper considers an extremal version of the Erd\H{o}s distinct distances
problem. For a point set , let denote the
set of all Euclidean distances determined by . Our main result is the
following: if and , then there exists with such that .
This is one part of a more general result, which says that, if the growth of
is restricted, it must be the case that has some additive
structure. More specifically, for any two integers , we have the following
information: if then there exists with and These results are higher dimensional analogues of a result of Hanson, who
considered the two-dimensional case