11,216 research outputs found
Tur\'an and Ramsey Properties of Subcube Intersection Graphs
The discrete cube is a fundamental combinatorial structure. A
subcube of is a subset of of its points formed by fixing
coordinates and allowing the remaining to vary freely. The subcube
structure of the discrete cube is surprisingly complicated and there are many
open questions relating to it.
This paper is concerned with patterns of intersections among subcubes of the
discrete cube. Two sample questions along these lines are as follows: given a
family of subcubes in which no of them have non-empty intersection, how
many pairwise intersections can we have? How many subcubes can we have if among
them there are no which have non-empty intersection and no which are
pairwise disjoint? These questions are naturally expressed as Tur\'an and
Ramsey type questions in intersection graphs of subcubes where the intersection
graph of a family of sets has one vertex for each set in the family with two
vertices being adjacent if the corresponding subsets intersect.
Tur\'an and Ramsey type problems are at the heart of extremal combinatorics
and so these problems are mathematically natural. However, a second motivation
is a connection with some questions in social choice theory arising from a
simple model of agreement in a society. Specifically, if we have to make a
binary choice on each of separate issues then it is reasonable to assume
that the set of choices which are acceptable to an individual will be
represented by a subcube. Consequently, the pattern of intersections within a
family of subcubes will have implications for the level of agreement within a
society.
We pose a number of questions and conjectures relating directly to the
Tur\'an and Ramsey problems as well as raising some further directions for
study of subcube intersection graphs.Comment: 18 page
Integer colorings with forbidden rainbow sums
For a set of positive integers , an -coloring of is
rainbow sum-free if it contains no rainbow Schur triple. In this paper we
initiate the study of the rainbow Erd\H{o}s-Rothchild problem in the context of
sum-free sets, which asks for the subsets of with the maximum number of
rainbow sum-free -colorings. We show that for , the interval is
optimal, while for , the set is optimal. We
also prove a stability theorem for . The proofs rely on the hypergraph
container method, and some ad-hoc stability analysis.Comment: 20 page
Optimal transportation, topology and uniqueness
The Monge-Kantorovich transportation problem involves optimizing with respect
to a given a cost function. Uniqueness is a fundamental open question about
which little is known when the cost function is smooth and the landscapes
containing the goods to be transported possess (non-trivial) topology. This
question turns out to be closely linked to a delicate problem (# 111) of
Birkhoff [14]: give a necessary and sufficient condition on the support of a
joint probability to guarantee extremality among all measures which share its
marginals. Fifty years of progress on Birkhoff's question culminate in Hestir
and Williams' necessary condition which is nearly sufficient for extremality;
we relax their subtle measurability hypotheses separating necessity from
sufficiency slightly, yet demonstrate by example that to be sufficient
certainly requires some measurability. Their condition amounts to the vanishing
of the measure \gamma outside a countable alternating sequence of graphs and
antigraphs in which no two graphs (or two antigraphs) have domains that
overlap, and where the domain of each graph / antigraph in the sequence
contains the range of the succeeding antigraph (respectively, graph). Such
sequences are called numbered limb systems. We then explain how this
characterization can be used to resolve the uniqueness of Kantorovich solutions
for optimal transportation on a manifold with the topology of the sphere.Comment: 36 pages, 6 figure
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