193 research outputs found
Vertex Ramsey problems in the hypercube
If we 2-color the vertices of a large hypercube what monochromatic
substructures are we guaranteed to find? Call a set S of vertices from Q_d, the
d-dimensional hypercube, Ramsey if any 2-coloring of the vertices of Q_n, for n
sufficiently large, contains a monochromatic copy of S. Ramsey's theorem tells
us that for any r \geq 1 every 2-coloring of a sufficiently large r-uniform
hypergraph will contain a large monochromatic clique (a complete
subhypergraph): hence any set of vertices from Q_d that all have the same
weight is Ramsey. A natural question to ask is: which sets S corresponding to
unions of cliques of different weights from Q_d are Ramsey?
The answer to this question depends on the number of cliques involved. In
particular we determine which unions of 2 or 3 cliques are Ramsey and then
show, using a probabilistic argument, that any non-trivial union of 39 or more
cliques of different weights cannot be Ramsey.
A key tool is a lemma which reduces questions concerning monochromatic
configurations in the hypercube to questions about monochromatic translates of
sets of integers.Comment: 26 pages, 3 figure
Problems in extremal graph theory
We consider a variety of problems in extremal graph and set theory.
The {\em chromatic number} of , , is the smallest integer
such that is -colorable.
The {\it square} of , written , is the supergraph of in which also
vertices within distance 2 of each other in are adjacent.
A graph is a {\it minor} of if
can be obtained from a subgraph of by contracting edges.
We show that the upper bound for
conjectured by Wegner (1977) for planar graphs
holds when is a -minor-free graph.
We also show that is equal to the bound
only when contains a complete graph of that order.
One of the central problems of extremal hypergraph theory is
finding the maximum number of edges in a hypergraph
that does not contain a specific forbidden structure.
We consider as a forbidden structure a fixed number of members
that have empty common intersection
as well as small union.
We obtain a sharp upper bound on the size of uniform hypergraphs
that do not contain this structure,
when the number of vertices is sufficiently large.
Our result is strong enough to imply the same sharp upper bound
for several other interesting forbidden structures
such as the so-called strong simplices and clusters.
The {\em -dimensional hypercube}, ,
is the graph whose vertex set is and
whose edge set consists of the vertex pairs
differing in exactly one coordinate.
The generalized Tur\'an problem asks for the maximum number
of edges in a subgraph of a graph that does not contain
a forbidden subgraph .
We consider the Tur\'an problem where is and
is a cycle of length with .
Confirming a conjecture of Erd{\H o}s (1984),
we show that the ratio of the size of such a subgraph of
over the number of edges of is ,
i.e. in the limit this ratio approaches 0
as approaches infinity
Random subcube intersection graphs I: cliques and covering
We study random subcube intersection graphs, that is, graphs obtained by
selecting a random collection of subcubes of a fixed hypercube to serve
as the vertices of the graph, and setting an edge between a pair of subcubes if
their intersection is non-empty. Our motivation for considering such graphs is
to model `random compatibility' between vertices in a large network. For both
of the models considered in this paper, we determine the thresholds for
covering the underlying hypercube and for the appearance of s-cliques. In
addition we pose some open problems.Comment: 38 pages, 1 figur
An extremal theorem in the hypercube
The hypercube Q_n is the graph whose vertex set is {0,1}^n and where two
vertices are adjacent if they differ in exactly one coordinate. For any
subgraph H of the cube, let ex(Q_n, H) be the maximum number of edges in a
subgraph of Q_n which does not contain a copy of H. We find a wide class of
subgraphs H, including all previously known examples, for which ex(Q_n, H) =
o(e(Q_n)). In particular, our method gives a unified approach to proving that
ex(Q_n, C_{2t}) = o(e(Q_n)) for all t >= 4 other than 5.Comment: 6 page
Ramsey numbers of cubes versus cliques
The cube graph Q_n is the skeleton of the n-dimensional cube. It is an
n-regular graph on 2^n vertices. The Ramsey number r(Q_n, K_s) is the minimum N
such that every graph of order N contains the cube graph Q_n or an independent
set of order s. Burr and Erdos in 1983 asked whether the simple lower bound
r(Q_n, K_s) >= (s-1)(2^n - 1)+1 is tight for s fixed and n sufficiently large.
We make progress on this problem, obtaining the first upper bound which is
within a constant factor of the lower bound.Comment: 26 page
Exact Ramsey numbers of odd cycles via nonlinear optimisation
For a graph G, the k-colour Ramsey number R k(G) is the least integer N such that every k-colouring of the edges of the complete graph K N contains a monochromatic copy of G. Let C n denote the cycle on n vertices. We show that for fixed k≥2 and n odd and sufficiently large, R k(C n)=2 k−1(n−1)+1. This resolves a conjecture of Bondy and Erdős for large n. The proof is analytic in nature, the first step of which is to use the regularity method to relate this problem in Ramsey theory to one in nonlinear optimisation. This allows us to prove a stability-type generalisation of the above and establish a correspondence between extremal k-colourings for this problem and perfect matchings in the k-dimensional hypercube Q k
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