For a graph G, let f(G) denote the maximum number of edges in a cut of G. For an integer m and for 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. In this paper we study this function for various graphs H. In particular we show that for any graph H obtained by connecting a single vertex to all vertices of a fixed nontrivial forest, there is a $c(H) >0$ such that $f(m,H) \geq \frac{m}{2} + c(H) m^{4/5}$, and that this is tight up to the value of $c(H)$. We also prove that for any even cycle $C_{2k}$ there is a $c(k)>0$ such that $f(m,C_{2k}) \geq \frac{m}{2} + c(k) m^{(2k+1)/(2k+2)}$, and that this is tight, up to the value of $c(k)$, for $2k\in \{4,6,10\}$. The proofs combine combinatorial, probabilistic and spectral techniques.

This paper is partly an overview of the subject (Sections 1–4), and partly a research paper containing proofs for new results (Sections 5–7).

We introduce a family of one-dimensional geometric growth models, constructed iteratively by locally optimizing the trade-offs between two competing metrics, and show that this family is equivalent to a family of preferential attachment random graph models with upper cut-offs. This is the first explanation of how preferential attachment can arise from a more basic underlying mechanism of local competition. We rigorously determine the degree distribution for the family of random graph models, showing that it obeys a power law up to a finite threshold and decays exponentially above this threshold.

We also rigorously analyse a generalized version of our graph process, with two natural parameters, one corresponding to the cut-off and the other a ‘fertility’ parameter. We prove that the general model has a power-law degree distribution up to a cut-off, and establish monotonicity of the power as a function of the two parameters. Limiting cases of the general model include the standard preferential attachment model without cut-off and the uniform attachment model.

In 1978, Bollobás and Eldridge [5] made the following two conjectures.

1. (C1) There exists an absolute constant $c>0$ such that, if k is a positive integer and $G_1$ and $G_2$ are graphs of order n such that $\Delta(G_1),\Delta(G_2)\leq n-k$ and $e(G_1),e(G_2)\leq ck n$, then the graphs $G_1$ and $G_2$ pack.

2. (C2) For all $0<\alpha<1/2$ and $0<c<\sqrt{1/8}$, there exists an $n_0=n_0(\alpha,c)$ such that, if $G_1$ and $G_2$ are graphs of order $n>n_0$ satisfying $e(G_1)\leq \alpha n$ and $e(G_2)\leq c\sqrt{n^3/ \alpha}$, then the graphs $G_1$ and $G_2$ pack.

Motivated by the result that an ‘approximate’ evaluation of the Jones polynomial of a braid at a 5th root of unity can be used to simulate the quantum part of any algorithm in the quantum complexity class BQP, and results relating BQP to the counting class GapP, we introduce a form of additive approximation which can be used to simulate a function in BQP. We show that all functions in the classes #P and GapP have such an approximation scheme under certain natural normalizations. However, we are unable to determine whether the particular functions we are motivated by, such as the above evaluation of the Jones polynomial, can be approximated in this way. We close with some open problems motivated by this work.

If all nonzero eigenvalues of the (normalized) Laplacian of a graph $G$ are close to 1, then $G$ is $t$-Turán in the sense that any subgraph of $G$ containing no $K_{t+1}$ contains at most $(1-1/t + o(1) ) e(G)$ edges where $e(G)$ denotes the number of edges in G.

Let $P(n)$ and $C(n)$ denote, respectively, the maximum possible numbers of Hamiltonian paths and Hamiltonian cycles in a tournament on n vertices. The study of $P(n)$ was suggested by Szele [14], who showed in an early application of the probabilistic method that $P(n) \geq n!2^{-n+1}$, and conjectured that $\lim ( {P(n)}/ {n!} )^{1/n}= 1/2.$ This was proved by Alon [2], who observed that the conjecture follows from a suitable bound on $C(n)$, and showed $C(n) <O(n^{3/2}(n-1)!2^{-n}).$ Here we improve this to $C(n)<O\big(n^{3/2-\xi}(n-1)!2^{-n}\big),$ with $\xi = 0.2507$… Our approach is mainly based on entropy considerations.

JumbleG is a Maker–Breaker game. Maker and Breaker take turns in choosing edges from the complete graph $K_n$. Maker's aim is to choose what we call an $\epsilon$-regular graph (that is, the minimum degree is at least $(\frac12-\epsilon) n$ and, for every pair of disjoint subsets $S,T\subset V$ of cardinalities at least $\epsilon n$, the number of edges $e(S,T)$ between $S$ and $T$ satisfies $\bigl|\frac{e(S,T)}{|S|\,|T|}-\frac12\bigr|\leq \epsilon$.) In this paper we show that Maker can create an $\epsilon$-regular graph, for $\epsilon\geq 2(\log n/n)^{1/3}$. We also consider a similar game, JumbleG2, where Maker's aim is to create a graph with minimum degree at least $\bigl(\frac12-\epsilon\bigr)n$ and maximum co-degree at most $\bigl(\frac14+\epsilon\bigr)n$, and show that Maker has a winning strategy for $\epsilon> 3 (\log n/n)^{1/2}$. Thus, in both games Maker can create a pseudo-random graph of density $\frac12$. This guarantees Maker's win in several other positional games, also discussed here.

Let ${\cal H}$ be a 3-uniform hypergraph on an $n$-element vertex set $V$. The neighbourhood of $a,b\in V$ is $N(ab):= \{x: abx\in E({\cal H})\} $. Such a 3-graph has independent neighbourhoods if no $N(ab)$ contains an edge of ${\cal H}$. This is equivalent to ${\cal H}$ not containing a copy of $\mathbb{F} :=\{ abx$, $aby$, $abz$, $xyz\}$.

In this paper we prove an analogue of the Andrásfai–Erdös–Sós theorem for triangle-free graphs with minimum degree exceeding $2n/5$. It is shown that any $\mathbb{F}$-free 3-graph with minimum degree exceeding $(\frac{4}{9}-\frac{1}{125})\binom{n}{2}$ is bipartite, (for $n> n_0$), i.e., the vertices of ${\cal H}$ can be split into two parts so that every triple meets both parts.

This is, in fact, a Turán-type result. It solves a problem of Erdös and T.Sós, and answers a question of Mubayi and Rödl that

\[\ex(n,\mathbb{F}_{3,2})=\max\limits_{\alpha}(n-\alpha)\left({\alpha\atop2}\right)\].

Here the right-hand side is $\frac{4}{9}\binom{n}{3}+O(n^2)$. Moreover $e({\cal H})={\rm ex}(n,\mathbb{F})$ is possible only if $V({\cal H})$ can be partitioned into two sets $A$ and $B$ so that each triple of ${\cal H}$ intersects $A$ in exactly two vertices and $B$ in one.

We extend a result by Füredi and Komlós and show that the first eigenvalue of a random graph is asymptotically normal, both for $G_{n,p}$ and $G_{n,m}$, provided $np\geq n^\delta$ or $m/n\geq n^\delta$ for some $\delta>0$. The asymptotic variance is of order $p$ for $G_{n,p}$, and $n^{-1}$ for $G_{n,m}$. This gives a (partial) solution to a problem raised by Krivelevich and Sudakov.

The formula for the asymptotic mean involves a mysterious power series.

For an arbitrary n-dimensional convex body, at least almost n Steiner symmetrizations are required in order to symmetrize the body into an isomorphic ellipsoid. We say that a body $T \subset \mathbb{R}^n$ is ‘quickly symmetrizable with function $c(\varepsilon)$’ if for any $\varepsilon > 0$ there exist only $\lfloor \varepsilon n \rfloor$ symmetrizations that transform T into a body which is $c(\varepsilon)$-isomorphic to an ellipsoid. In this note we ask, given a body $K \subset \mathbb{R}^n$, whether it is possible to remove a small portion of its volume and obtain a body $T \subset K$ which is quickly symmetrizable. We show that this question, for $c(\varepsilon)$ polynomially depending on $\frac{1}{\varepsilon}$, is equivalent to the slicing problem.

The question of the maximum number $\mbox{ex}(m,n,C_{2k})$ of edges in an m by n bipartite graph without a cycle of length 2k is addressed in this note. For each $k \geq 2$, it is shown that $\mbox{ex}(m,n,C_{2k}) \leq \begin{cases} (2k-3)\bigl[(mn)^{\frac{k+1}{2k}} + m + n\bigr] & \mbox{ if }k \mbox{ is odd,}\\[2pt] (2k-3)\bigl[m^{\frac{k+2}{2k}}\, n^{\frac{1}{2}} + m + n\bigr] & \mbox{ if }k \mbox{ is even.}\\ \end{cases}$

A set of n triangles sharing a common edge is called a book with n pages and is denoted by $B_{n}$. It is known that the Ramsey number $r ( B_{n} ) $ satisfies $r ( B_{n} ) = ( 4+o ( 1 ) ) n.$ We show that every red–blue edge colouring of $K_{ \lfloor ( 4-\varepsilon ) n \rfloor }$ with no monochromatic $B_{n}$ exhibits quasi-random properties when $\varepsilon$ tends to 0. This implies that there is a constant $c>0$ such that for every red–blue edge colouring of $K_{r ( B_{n} ) }$ there is a monochromatic $B_{n}$ whose vertices span at least $ \lfloor cn^{2} \rfloor $ edges of the same colour as the book.

As an application we find the Ramsey number for a class of graphs.

The dimension of a graph, that is, the dimension of its incidence poset, has become a major bridge between posets and graphs. Although allowing a nice characterization of planarity, this dimension behaves badly with respect to homomorphisms.

We introduce the universal dimension of a graph G as the maximum dimension of a graph having a homomorphism to G. The universal dimension, which is clearly homomorphism monotone, is related to the existence of some balanced bicolouration of the vertices with respect to some realizer.

Nontrivial new results related to the original graph dimension are subsequently deduced from our study of universal dimension, including chromatic properties, extremal properties and a disproof of two conjectures of Felsner and Trotter.

A solid diagram of volume n is a packing of n unit cubes into a corner so that the heights of vertical stacks of cubes do not increase in either of two horizontal directions away from the corner. An asymptotic distribution of the dimensions – heights, depths, and widths – of the diagram chosen uniformly at random among all such diagrams is studied. For each k, the planar base of k tallest stacks is shown to be Plancherel distributed in the limit $n\to\infty$.

Building on the methods developed in joint work with Béla Bollobás and Svante Janson, we study the phase transition in four ‘scale-free’ random graph models, obtaining upper and lower bounds on the size of the giant component when there is one. In particular, we determine the extremely slow rate of growth of the giant component just above the phase transition. We greatly reduce the significant gaps between the existing upper and lower bounds, giving bounds that match to within a factor $1+o(1)$ in the exponent.

In all cases the method used is to couple the neighbourhood expansion process in the graph on n vertices with a continuous-type branching process that is independent of n. It can be shown (requiring some separate argument for each case) that with probability tending to 1 as $n\to\infty$ the size of the giant component divided by n is within $o(1)$ of the survival probability $\sigma$ of the branching process. This survival probability is given in terms of the maximal solution $\phi$ to certain non-linear integral equations, which can be written in the form $\phi={\bf F}(\phi)$ for a certain operator ${\bf F}$. Upper and lower bounds are found by constructing trial functions $\phi_0$, $\phi_1$ with ${\bf F}(\phi_0)\leq \phi_0$ and ${\bf F}(\phi_1)\geq \phi_1$ holding pointwise; basic properties of branching processes then imply that $\phi_1\leq \phi\leq \phi_0$, giving upper and lower bounds on $\sigma$.