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The classic game of Battleship involves two players taking turns attempting to guess the positions of a fleet of vertically or horizontally positioned enemy ships hidden on a
$10\times 10$
grid. One variant of this game, also referred to as Battleship Solitaire, Bimaru or Yubotu, considers the game with the inclusion of X-ray data, represented by knowledge of how many spots are occupied in each row and column in the enemy board. This paper considers the Battleship puzzle problem: the problem of reconstructing an enemy fleet from its X-ray data. We generate non-unique solutions to Battleship puzzles via certain reflection transformations akin to Ryser interchanges. Furthermore, we demonstrate that solutions of Battleship puzzles may be reliably obtained by searching for solutions of the associated classical binary discrete tomography problem which minimise the discrete Laplacian. We reformulate this optimisation problem as a quadratic unconstrained binary optimisation problem and approximate solutions via a simulated annealer, emphasising the future practical applicability of quantum annealers to solving discrete tomography problems with predefined structure.
We first establish a lower bound on the size and spectral radius of a graph G to guarantee that G contains a fractional perfect matching. Then, we determine an upper bound on the distance spectral radius of a graph G to ensure that G has a fractional perfect matching. Furthermore, we construct some extremal graphs to show all the bounds are best possible.
Under the assumption that sequences of graphs equipped with resistances, associated measures, walks and local times converge in a suitable Gromov-Hausdorff topology, we establish asymptotic bounds on the distribution of the
$\varepsilon$
-blanket times of the random walks in the sequence. The precise nature of these bounds ensures convergence of the
$\varepsilon$
-blanket times of the random walks if the
$\varepsilon$
-blanket time of the limiting diffusion is continuous at
$\varepsilon$
with probability 1. This result enables us to prove annealed convergence in various examples of critical random graphs, including critical Galton-Watson trees and the Erdős-Rényi random graph in the critical window. We highlight that proving continuity of the
$\varepsilon$
-blanket time of the limiting diffusion relies on the scale invariance of a finite measure that gives rise to realizations of the limiting compact random metric space, and therefore we expect our results to hold for other examples of random graphs with a similar scale invariance property.
The book graph
$B_n ^{(k)}$
consists of
$n$
copies of
$K_{k+1}$
joined along a common
$K_k$
. In the prequel to this paper, we studied the diagonal Ramsey number
$r(B_n ^{(k)}, B_n ^{(k)})$
. Here we consider the natural off-diagonal variant
$r(B_{cn} ^{(k)}, B_n^{(k)})$
for fixed
$c \in (0,1]$
. In this more general setting, we show that an interesting dichotomy emerges: for very small
$c$
, a simple
$k$
-partite construction dictates the Ramsey function and all nearly-extremal colourings are close to being
$k$
-partite, while, for
$c$
bounded away from
$0$
, random colourings of an appropriate density are asymptotically optimal and all nearly-extremal colourings are quasirandom. Our investigations also open up a range of questions about what happens for intermediate values of
$c$
.
We give an example of an FIID vertex-labeling of
${\mathbb T}_3$
whose marginals are uniform on
$[0,1]$
, and if we delete the edges between those vertices whose labels are different, then some of the remaining clusters are infinite. We also show that no such process can be finitary.
Let
$G=(V,E)$
be a countable graph. The Bunkbed graph of
$G$
is the product graph
$G \times K_2$
, which has vertex set
$V\times \{0,1\}$
with “horizontal” edges inherited from
$G$
and additional “vertical” edges connecting
$(w,0)$
and
$(w,1)$
for each
$w \in V$
. Kasteleyn’s Bunkbed conjecture states that for each
$u,v \in V$
and
$p\in [0,1]$
, the vertex
$(u,0)$
is at least as likely to be connected to
$(v,0)$
as to
$(v,1)$
under Bernoulli-
$p$
bond percolation on the bunkbed graph. We prove that the conjecture holds in the
$p \uparrow 1$
limit in the sense that for each finite graph
$G$
there exists
$\varepsilon (G)\gt 0$
such that the bunkbed conjecture holds for
$p \geqslant 1-\varepsilon (G)$
.
An old conjecture of Erdős and McKay states that if all homogeneous sets in an
$n$
-vertex graph are of order
$O(\!\log n)$
then the graph contains induced subgraphs of each size from
$\{0,1,\ldots, \Omega \big(n^2\big)\}$
. We prove a bipartite analogue of the conjecture: if all balanced homogeneous sets in an
$n \times n$
bipartite graph are of order
$O(\!\log n)$
, then the graph contains induced subgraphs of each size from
$\{0,1,\ldots, \Omega \big(n^2\big)\}$
.
Given a graphon
$W$
and a finite simple graph
$H$
, with vertex set
$V(H)$
, denote by
$X_n(H, W)$
the number of copies of
$H$
in a
$W$
-random graph on
$n$
vertices. The asymptotic distribution of
$X_n(H, W)$
was recently obtained by Hladký, Pelekis, and Šileikis [17] in the case where
$H$
is a clique. In this paper, we extend this result to any fixed graph
$H$
. Towards this we introduce a notion of
$H$
-regularity of graphons and show that if the graphon
$W$
is not
$H$
-regular, then
$X_n(H, W)$
has Gaussian fluctuations with scaling
$n^{|V(H)|-\frac{1}{2}}$
. On the other hand, if
$W$
is
$H$
-regular, then the fluctuations are of order
$n^{|V(H)|-1}$
and the limiting distribution of
$X_n(H, W)$
can have both Gaussian and non-Gaussian components, where the non-Gaussian component is a (possibly) infinite weighted sum of centred chi-squared random variables with the weights determined by the spectral properties of a graphon derived from
$W$
. Our proofs use the asymptotic theory of generalised
$U$
-statistics developed by Janson and Nowicki [22]. We also investigate the structure of
$H$
-regular graphons for which either the Gaussian or the non-Gaussian component of the limiting distribution (but not both) is degenerate. Interestingly, there are also
$H$
-regular graphons
$W$
for which both the Gaussian or the non-Gaussian components are degenerate, that is,
$X_n(H, W)$
has a degenerate limit even under the scaling
$n^{|V(H)|-1}$
. We give an example of this degeneracy with
$H=K_{1, 3}$
(the 3-star) and also establish non-degeneracy in a few examples. This naturally leads to interesting open questions on higher order degeneracies.
For a graph G and a family of graphs
$\mathcal {F}$
, the Turán number
${\mathrm {ex}}(G,\mathcal {F})$
is the maximum number of edges an
$\mathcal {F}$
-free subgraph of G can have. We prove that
${\mathrm {ex}}(G,\mathcal {F})\ge {\mathrm {ex}}(K_r, \mathcal {F})$
if the chromatic number of G is r and
$\mathcal {F}$
is a family of connected graphs. This result answers a question raised by Briggs and Cox [‘Inverting the Turán problem’, Discrete Math.342(7) (2019), 1865–1884] about the inverse Turán number for all connected graphs.
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
$f(n,8,8)$
.
A recent paper of Balogh, Li and Treglown [3] initiated the study of Dirac-type problems for ordered graphs. In this paper, we prove a number of results in this area. In particular, we determine asymptotically the minimum degree threshold for forcing
(i) a perfect H-tiling in an ordered graph, for any fixed ordered graph H of interval chromatic number at least
$3$
;
(ii) an H-tiling in an ordered graph G covering a fixed proportion of the vertices of G (for any fixed ordered graph H);
(iii) an H-cover in an ordered graph (for any fixed ordered graph H).
The first two of these results resolve questions of Balogh, Li and Treglown, whilst (iii) resolves a question of Falgas-Ravry. Note that (i) combined with a result from [3] completely determines the asymptotic minimum degree threshold for forcing a perfect H-tiling. Additionally, we prove a result that, combined with a theorem of Balogh, Li and Treglown, asymptotically determines the minimum degree threshold for forcing an almost perfect H-tiling in an ordered graph (for any fixed ordered graph H). Our work therefore provides ordered graph analogues of the seminal tiling theorems of Kühn and Osthus [Combinatorica 2009] and of Komlós [Combinatorica 2000]. Each of our results exhibits some curious, and perhaps unexpected, behaviour. Our solution to (i) makes use of a novel absorbing argument.
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
$f(m,K_{4,s})$
.
One of the central questions in Ramsey theory asks how small the largest clique and independent set in a graph on N vertices can be. By the celebrated result of Erdős from 1947, a random graph on N vertices with edge probability
$1/2$
contains no clique or independent set larger than
$2\log _2 N$
, with high probability. Finding explicit constructions of graphs with similar Ramsey-type properties is a famous open problem. A natural approach is to construct such graphs using algebraic tools.
Say that an r-uniform hypergraph
$\mathcal {H}$
is algebraic of complexity
$(n,d,m)$
if the vertices of
$\mathcal {H}$
are elements of
$\mathbb {F}^{n}$
for some field
$\mathbb {F}$
, and there exist m polynomials
$f_1,\dots ,f_m:(\mathbb {F}^{n})^{r}\rightarrow \mathbb {F}$
of degree at most d such that the edges of
$\mathcal {H}$
are determined by the zero-patterns of
$f_1,\dots ,f_m$
. The aim of this paper is to show that if an algebraic graph (or hypergraph) of complexity
$(n,d,m)$
has good Ramsey properties, then at least one of the parameters
$n,d,m$
must be large.
In 2001, Rónyai, Babai and Ganapathy considered the bipartite variant of the Ramsey problem and proved that if G is an algebraic graph of complexity
$(n,d,m)$
on N vertices, then either G or its complement contains a complete balanced bipartite graph of size
$\Omega _{n,d,m}(N^{1/(n+1)})$
. We extend this result by showing that such G contains either a clique or an independent set of size
$N^{\Omega (1/ndm)}$
and prove similar results for algebraic hypergraphs of constant complexity. We also obtain a polynomial regularity lemma for r-uniform algebraic hypergraphs that are defined by a single polynomial that might be of independent interest. Our proofs combine algebraic, geometric and combinatorial tools.
We derive three critical exponents for Bernoulli site percolation on the uniform infinite planar triangulation (UIPT). First, we compute explicitly the probability that the root cluster is infinite. As a consequence, we show that the off-critical exponent for site percolation on the UIPT is
$\beta = 1/2$
. Then we establish an integral formula for the generating function of the number of vertices in the root cluster. We use this formula to prove that, at criticality, the probability that the root cluster has at least n vertices decays like
$n^{-1/7}$
. Finally, we also derive an expression for the law of the perimeter of the root cluster and use it to establish that, at criticality, the probability that the perimeter of the root cluster is equal to n decays like
$n^{-4/3}$
. Among these three exponents, only the last one was previously known. Our main tools are the so-called gasket decomposition of percolation clusters, generic properties of random Boltzmann maps, and analytic combinatorics.
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.