We use cookies to distinguish you from other users and to provide you with a better experience on our websites. Close this message to accept cookies or find out how to manage your cookie settings.
To save content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about saving content to .
To save content items to your Kindle, first ensure coreplatform@cambridge.org
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
We consider supercritical site percolation on the
$d$
-dimensional hypercube
$Q^d$
. We show that typically all components in the percolated hypercube, besides the giant, are of size
$O(d)$
. This resolves a conjecture of Bollobás, Kohayakawa, and Łuczak from 1994.
A
$(p,q)$
-colouring of a graph
$G$
is an edge-colouring of
$G$
which assigns at least
$q$
colours to each
$p$
-clique. The problem of determining the minimum number of colours,
$f(n,p,q)$
, needed to give a
$(p,q)$
-colouring of the complete graph
$K_n$
is a natural generalization of the well-known problem of identifying the diagonal Ramsey numbers
$r_k(p)$
. The best-known general upper bound on
$f(n,p,q)$
was given by Erdős and Gyárfás in 1997 using a probabilistic argument. Since then, improved bounds in the cases where
$p=q$
have been obtained only for
$p\in \{4,5\}$
, each of which was proved by giving a deterministic construction which combined a
$(p,p-1)$
-colouring using few colours with an algebraic colouring.
In this paper, we provide a framework for proving new upper bounds on
$f(n,p,p)$
in the style of these earlier constructions. We characterize all colourings of
$p$
-cliques with
$p-1$
colours which can appear in our modified version of the
$(p,p-1)$
-colouring of Conlon, Fox, Lee, and Sudakov. This allows us to greatly reduce the amount of case-checking required in identifying
$(p,p)$
-colourings, which would otherwise make this problem intractable for large values of
$p$
. In addition, we generalize our algebraic colouring from the
$p=5$
setting and use this to give improved upper bounds on
$f(n,6,6)$
and
$f(n,8,8)$
.
In this article we introduce a simple tool to derive polynomial upper bounds for the probability of observing unusually large maximal components in some models of random graphs when considered at criticality. Specifically, we apply our method to a model of a random intersection graph, a random graph obtained through p-bond percolation on a general d-regular graph, and a model of an inhomogeneous random graph.
We provide a general purpose result for the coupling of exploration processes of random graphs, both undirected and directed, with their local weak limits when this limit is a marked Galton–Watson process. This class includes in particular the configuration model and the family of inhomogeneous random graphs with rank-1 kernel. Vertices in the graph are allowed to have attributes on a general separable metric space and can potentially influence the construction of the graph itself. The coupling holds for any fixed depth of a breadth-first exploration process.
For a graph G, let
$f(G)$
denote the maximum number of edges in a bipartite subgraph of G. Given a positive integer m and a fixed graph H, let
$f(m,H)$
denote the minimum possible cardinality of
$f(G)$
, as G ranges over all graphs on m edges that contain no copy of H. We prove bounds on
$f(m,H)$
for some bipartite graphs H and give a bound for a conjecture of Alon et al. [‘MaxCut in H-free graphs’, Combin. Probab. Comput.14 (2005), 629–647] concerning
$f(m,K_{4,s})$
.
Scale-free percolation is a stochastic model for complex networks. In this spatial random graph model, vertices
$x,y\in\mathbb{Z}^d$
are linked by an edge with probability depending on independent and identically distributed vertex weights and the Euclidean distance
$|x-y|$
. Depending on the various parameters involved, we get a rich phase diagram. We study graph distance and compare it to the Euclidean distance of the vertices. Our main attention is on a regime where graph distances are (poly-)logarithmic in the Euclidean distance. We obtain improved bounds on the logarithmic exponents. In the light tail regime, the correct exponent is identified.
The diamond is the complete graph on four vertices minus one edge; $P_n$ and $C_n$ denote the path and cycle on n vertices, respectively. We prove that the chromatic number of a $(P_6,C_4,\mbox {diamond})$-free graph G is no larger than the maximum of 3 and the clique number of G.
Strengthening Hadwiger’s conjecture, Gerards and Seymour conjectured in 1995 that every graph with no odd
$K_t$
-minor is properly
$(t-1)$
-colourable. This is known as the Odd Hadwiger’s conjecture. We prove a relaxation of the above conjecture, namely we show that every graph with no odd
$K_t$
-minor admits a vertex
$(2t-2)$
-colouring such that all monochromatic components have size at most
$\lceil \frac{1}{2}(t-2) \rceil$
. The bound on the number of colours is optimal up to a factor of
$2$
, improves previous bounds for the same problem by Kawarabayashi (2008, Combin. Probab. Comput.17 815–821), Kang and Oum (2019, Combin. Probab. Comput.28 740–754), Liu and Wood (2021, arXiv preprint, arXiv:1905.09495), and strengthens a result by van den Heuvel and Wood (2018, J. Lond. Math. Soc.98 129–148), who showed that the above conclusion holds under the more restrictive assumption that the graph is
$K_t$
-minor-free. In addition, the bound on the component-size in our result is much smaller than those of previous results, in which the dependency on
$t$
was given by a function arising from the graph minor structure theorem of Robertson and Seymour. Our short proof combines the method by van den Heuvel and Wood for
$K_t$
-minor-free graphs with some additional ideas, which make the extension to odd
$K_t$
-minor-free graphs possible.
In this paper, we study asymmetric Ramsey properties of the random graph
$G_{n,p}$
. Let
$r \in \mathbb{N}$
and
$H_1, \ldots, H_r$
be graphs. We write
$G_{n,p} \to (H_1, \ldots, H_r)$
to denote the property that whenever we colour the edges of
$G_{n,p}$
with colours from the set
$[r] \,{:\!=}\, \{1, \ldots, r\}$
there exists
$i \in [r]$
and a copy of
$H_i$
in
$G_{n,p}$
monochromatic in colour
$i$
. There has been much interest in determining the asymptotic threshold function for this property. In several papers, Rödl and Ruciński determined a threshold function for the general symmetric case; that is, when
$H_1 = \cdots = H_r$
. A conjecture of Kohayakawa and Kreuter from 1997, if true, would fully resolve the asymmetric problem. Recently, the
$1$
-statement of this conjecture was confirmed by Mousset, Nenadov and Samotij.
Building on work of Marciniszyn, Skokan, Spöhel and Steger from 2009, we reduce the
$0$
-statement of Kohayakawa and Kreuter’s conjecture to a certain deterministic subproblem. To demonstrate the potential of this approach, we show this subproblem can be resolved for almost all pairs of regular graphs. This therefore resolves the
$0$
-statement for all such pairs of graphs.
We suggest two related conjectures dealing with the existence of spanning irregular subgraphs of graphs. The first asserts that any
$d$
-regular graph on
$n$
vertices contains a spanning subgraph in which the number of vertices of each degree between
$0$
and
$d$
deviates from
$\frac{n}{d+1}$
by at most
$2$
. The second is that every graph on
$n$
vertices with minimum degree
$\delta$
contains a spanning subgraph in which the number of vertices of each degree does not exceed
$\frac{n}{\delta +1}+2$
. Both conjectures remain open, but we prove several asymptotic relaxations for graphs with a large number of vertices
$n$
. In particular we show that if
$d^3 \log n \leq o(n)$
then every
$d$
-regular graph with
$n$
vertices contains a spanning subgraph in which the number of vertices of each degree between
$0$
and
$d$
is
$(1+o(1))\frac{n}{d+1}$
. We also prove that any graph with
$n$
vertices and minimum degree
$\delta$
contains a spanning subgraph in which no degree is repeated more than
$(1+o(1))\frac{n}{\delta +1}+2$
times.
Let
$\Gamma $
be a graph of valency at least four whose automorphism group contains a minimally vertex-transitive subgroup G. It is proved that
$\Gamma $
admits a nowhere-zero
$3$
-flow if one of the following two conditions holds: (i)
$\Gamma $
is of order twice an odd number and G contains a central involution; (ii) G is a direct product of a
$2$
-subgroup and a subgroup of odd order.
We show that the
$4$
-state anti-ferromagnetic Potts model with interaction parameter
$w\in (0,1)$
on the infinite
$(d+1)$
-regular tree has a unique Gibbs measure if
$w\geq 1-\dfrac{4}{d+1_{_{\;}}}$
for all
$d\geq 4$
. This is tight since it is known that there are multiple Gibbs measures when
$0\leq w\lt 1-\dfrac{4}{d+1}$
and
$d\geq 4$
. We moreover give a new proof of the uniqueness of the Gibbs measure for the
$3$
-state Potts model on the
$(d+1)$
-regular tree for
$w\geq 1-\dfrac{3}{d+1}$
when
$d\geq 3$
and for
$w\in (0,1)$
when
$d=2$
.
How many 2-cells must two finite CW-complexes have to admit a common, but not finite common, covering? Leighton’s theorem says that both complexes must have 2-cells. We construct an almost (?) minimal example with two 2-cells in each complex.
The study of threshold functions has a long history in random graph theory. It is known that the thresholds for minimum degree k, k-connectivity, as well as k-robustness coincide for a binomial random graph. In this paper we consider an inhomogeneous random graph model, which is obtained by including each possible edge independently with an individual probability. Based on an intuitive concept of neighborhood density, we show two sufficient conditions guaranteeing k-connectivity and k-robustness, respectively, which are asymptotically equivalent. Our framework sheds some light on extending uniform threshold values in homogeneous random graphs to threshold landscapes in inhomogeneous random graphs.
We prove a surprising symmetry between the law of the size
$G_n$
of the greedy independent set on a uniform Cayley tree
$ \mathcal{T}_n$
of size n and that of its complement. We show that
$G_n$
has the same law as the number of vertices at even height in
$ \mathcal{T}_n$
rooted at a uniform vertex. This enables us to compute the exact law of
$G_n$
. We also give a Markovian construction of the greedy independent set, which highlights the symmetry of
$G_n$
and whose proof uses a new Markovian exploration of rooted Cayley trees that is of independent interest.
We determine all finite sets of equiangular lines spanning finite-dimensional complex unitary spaces for which the action on the lines of the set-stabiliser in the unitary group is 2-transitive with a regular normal subgroup.
We find an asymptotic enumeration formula for the number of simple
$r$
-uniform hypergraphs with a given degree sequence, when the number of edges is sufficiently large. The formula is given in terms of the solution of a system of equations. We give sufficient conditions on the degree sequence which guarantee existence of a solution to this system. Furthermore, we solve the system and give an explicit asymptotic formula when the degree sequence is close to regular. This allows us to establish several properties of the degree sequence of a random
$r$
-uniform hypergraph with a given number of edges. More specifically, we compare the degree sequence of a random
$r$
-uniform hypergraph with a given number edges to certain models involving sequences of binomial or hypergeometric random variables conditioned on their sum.
We use an inequality of Sidorenko to show a general relation between local and global subgraph counts and degree moments for locally weakly convergent sequences of sparse random graphs. This yields an optimal criterion to check when the asymptotic behaviour of graph statistics, such as the clustering coefficient and assortativity, is determined by the local weak limit.
As an application we obtain new facts for several common models of sparse random intersection graphs where the local weak limit, as we see here, is a simple random clique tree corresponding to a certain two-type Galton–Watson branching process.
Given a graph
$H$
and a positive integer
$n$
, the Turán number
$\mathrm{ex}(n,H)$
is the maximum number of edges in an
$n$
-vertex graph that does not contain
$H$
as a subgraph. A real number
$r\in (1,2)$
is called a Turán exponent if there exists a bipartite graph
$H$
such that
$\mathrm{ex}(n,H)=\Theta (n^r)$
. A long-standing conjecture of Erdős and Simonovits states that
$1+\frac{p}{q}$
is a Turán exponent for all positive integers
$p$
and
$q$
with
$q\gt p$
.
In this paper, we show that
$1+\frac{p}{q}$
is a Turán exponent for all positive integers
$p$
and
$q$
with
$q \gt p^{2}$
. Our result also addresses a conjecture of Janzer [18].
Cooperative coordination in multi-agent systems has been a topic of interest in networked control theory in recent years. In contrast to cooperative agents, Byzantine agents in a network are capable to manipulate their data arbitrarily and send bad messages to neighbors, causing serious network security issues. This paper is concerned with resilient tracking consensus over a time-varying random directed graph, which consists of cooperative agents, Byzantine agents and a single leader. The objective of resilient tracking consensus is the convergence of cooperative agents to the leader in the presence of those deleterious Byzantine agents. We assume that the number and identity of the Byzantine agents are not known to cooperative agents, and the communication edges in the graph are dynamically randomly evolving. Based upon linear system analysis and a martingale convergence theorem, we design a linear discrete-time protocol to ensure tracking consensus almost surely in a purely distributed manner. Some numerical examples are provided to verify our theoretical results.