151 research outputs found
A Center Transversal Theorem for Hyperplanes and Applications to Graph Drawing
Motivated by an open problem from graph drawing, we study several
partitioning problems for line and hyperplane arrangements. We prove a
ham-sandwich cut theorem: given two sets of n lines in R^2, there is a line l
such that in both line sets, for both halfplanes delimited by l, there are
n^{1/2} lines which pairwise intersect in that halfplane, and this bound is
tight; a centerpoint theorem: for any set of n lines there is a point such that
for any halfplane containing that point there are (n/3)^{1/2} of the lines
which pairwise intersect in that halfplane. We generalize those results in
higher dimension and obtain a center transversal theorem, a same-type lemma,
and a positive portion Erdos-Szekeres theorem for hyperplane arrangements. This
is done by formulating a generalization of the center transversal theorem which
applies to set functions that are much more general than measures. Back to
Graph Drawing (and in the plane), we completely solve the open problem that
motivated our search: there is no set of n labelled lines that are universal
for all n-vertex labelled planar graphs. As a side note, we prove that every
set of n (unlabelled) lines is universal for all n-vertex (unlabelled) planar
graphs
Semi-algebraic Ramsey numbers
Given a finite point set , a -ary semi-algebraic
relation on is the set of -tuples of points in , which is
determined by a finite number of polynomial equations and inequalities in
real variables. The description complexity of such a relation is at most if
the number of polynomials and their degrees are all bounded by . The Ramsey
number is the minimum such that any -element point set
in equipped with a -ary semi-algebraic relation , such
that has complexity at most , contains members such that every
-tuple induced by them is in , or members such that every -tuple
induced by them is not in .
We give a new upper bound for for and fixed.
In particular, we show that for fixed integers , establishing a subexponential upper bound on .
This improves the previous bound of due to Conlon, Fox, Pach,
Sudakov, and Suk, where is a very large constant depending on and
. As an application, we give new estimates for a recently studied
Ramsey-type problem on hyperplane arrangements in . We also study
multi-color Ramsey numbers for triangles in our semi-algebraic setting,
achieving some partial results
Geometry of contextuality from Grothendieck's coset space
The geometry of cosets in the subgroups H of the two-generator free group G
=\textless{} a, b \textgreater{} nicely fits, via Grothendieck's dessins
d'enfants, the geometry of commutation for quantum observables. Dessins
stabilize point-line incidence geometries that reflect the commutation of
(generalized) Pauli operators [Information 5, 209 (2014); 1310.4267 and
1404.6986 (quant-ph)]. Now we find that the non-existence of a dessin for which
the commutator (a, b) = a^ (--1) b^( --1) ab precisely corresponds to the
commutator of quantum observables [A, B] = AB -- BA on all lines of the
geometry is a signature of quantum contextuality. This occurs first at index |G
: H| = 9 in Mermin's square and at index 10 in Mermin's pentagram, as expected.
Commuting sets of n-qubit observables with n \textgreater{} 3 are found to be
contextual as well as most generalized polygons. A geometrical contextuality
measure is introduced.Comment: 13 pages, Quant. Inf. Pro
Discrete Geometry (hybrid meeting)
A number of important recent developments in various branches of
discrete geometry were presented at the workshop, which took place in
hybrid format due to a pandemic situation. The presentations
illustrated both the diversity of the area and its strong connections
to other fields of mathematics such as topology, combinatorics,
algebraic geometry or functional analysis. The open questions abound
and many of the results presented were obtained by young researchers,
confirming the great vitality of discrete geometry
Higher-order Erdos--Szekeres theorems
Let P=(p_1,p_2,...,p_N) be a sequence of points in the plane, where
p_i=(x_i,y_i) and x_1<x_2<...<x_N. A famous 1935 Erdos--Szekeres theorem
asserts that every such P contains a monotone subsequence S of
points. Another, equally famous theorem from the same paper implies that every
such P contains a convex or concave subsequence of points.
Monotonicity is a property determined by pairs of points, and convexity
concerns triples of points. We propose a generalization making both of these
theorems members of an infinite family of Ramsey-type results. First we define
a (k+1)-tuple to be positive if it lies on the graph of a
function whose kth derivative is everywhere nonnegative, and similarly for a
negative (k+1)-tuple. Then we say that is kth-order monotone if
its (k+1)-tuples are all positive or all negative.
We investigate quantitative bound for the corresponding Ramsey-type result
(i.e., how large kth-order monotone subsequence can be guaranteed in every
N-point P). We obtain an lower bound ((k-1)-times
iterated logarithm). This is based on a quantitative Ramsey-type theorem for
what we call transitive colorings of the complete (k+1)-uniform hypergraph; it
also provides a unified view of the two classical Erdos--Szekeres results
mentioned above.
For k=3, we construct a geometric example providing an upper
bound, tight up to a multiplicative constant. As a consequence, we obtain
similar upper bounds for a Ramsey-type theorem for order-type homogeneous
subsets in R^3, as well as for a Ramsey-type theorem for hyperplanes in R^4
recently used by Dujmovic and Langerman.Comment: Contains a counter example of Gunter Rote which gives a reply for the
problem number 5 in the previous versions of this pape
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