In this paper, asymptotical estimates of the form Rn(1+o(1)) for various classes of planar valency-restricted Eulerian maps are established. It follows, in particular, that ‘almost all’ (as n → ∞) n-edged planar Eulerian maps have n/3 (1+o(1)) vertices. A brief survey of known asymptotical results (a table of values of R) for various classes of planar maps is also presented.

In this paper we consider the following problem, given a graph H, what is the structure of a typical, i.e. random, H-free graph? We completely solve this problem for all graphs H containing a critical vertex. While this result subsumes a sequence of known results, its short proof is self contained.

A Steiner quadruple system SQS(v) of order v is a family ℬ of 4-element subsets of a v-element set V such that each 3-element subset of V is contained in precisely one B ∈ ℬ. We prove that if T ∩ B ≠ ø for all B ∈ ℬ (i.e., if T is a transversal), then |T| ≥ v/2, and if T is a transversal of cardinality exactly v/2, then V \ T is a transversal as well (i.e., T is a blocking set). Also, in respect of the so-called ‘doubling construction’ that produces SQS(2v) from two copies of SQS(v), we give a necessary and sufficient condition for this operation to yield a Steiner quadruple system with blocking sets.

We consider the set of polynomials of degree n over a finite field and put the uniform probability measure on this set. Any such polynomial factors uniquely into a product of its irreducible factors. To each polynomial we associate a step function on the interval [0,1] such that the size of each jump corresponds to the number of factors of a certain degree in the factorization of the random polynomial. We normalize these random functions and show that the resulting random process converges weakly to Brownian motion as n → ∞. This result complements earlier work by the author on the order statistics of the degree sequence of the factors of a random polynomial.

A graph G is called n-minimizable if it can be reduced, by deleting a set of its edges, to a minimally n-connected graph. It is shown that, if n-connected graphs G and H differ only by finitely many vertices and edges, then G is n-minimizable if and only if H is n-minimizable (Theorem 4.12). In the main result, conditions are given that a tree decomposition of an n-connected graph G must satisfy in order to guarantee that the n-minimizability of each of the members of this decomposition implies the n-minimizability of the graph G (Theorem 6.5).

This paper gives precise isoperimetric inequalities for infinite graphs on which a group acts with finite quotient. Decay estimates are obtained for the iterated kernels of the associated random walks.

Let be the hypergraph whose points are the subsets X of [n] := {1,…,n} with l≤ |X| ≤ u, l < u, and whose edges are intervals in the Boolean lattice of the form I = {C ⊆[n] : X⊆C⊆Y} where |X| = l, |Y| = u, X ⊆ Y.We study the matching number i.e. the the maximum number of pairwise disjoint edges, and the covering number i.e. the minimum number of points which cover all edges. We prove that max and that for every ε > 0 the inequalities hold, where for the lower bounds we suppose that n is not too small. The corresponding fractional numbers can be determined exactly. Moreover, we show by construction that

A dominating set for a graph G is a set D of vertices of G such that every vertex of G not in D is adjacent to a vertex of D. We prove that any graph G of minimum degree at least three contains a dominating set D of size at most 3|V(G)|/8. A star S is a graph consisting of a centre x and a set of edges from x to S — x. Clearly, a dominating set D for a graph G corresponds to a set of |D| stars which cover V(G). Thus, we show that the vertices of any graph G of minimum degree 3 can be covered by at most 3|V(G)|/8 vertex disjoint stars. We also show that any connected cubic graph G can be covered by [|V(G)|/9] vertex disjoint paths. Both these results are sharp.

There is a close relationship between biased graph games and random graph processes. In this paper, we develop the analogy and give further interesting instances.

It is shown that an oriented graph of order n whose every indegree and outdegree is at least cn is hamiltonian if c ≥ ½ − 2−15 but need not be if c < ⅜.

A definition is adopted for convexity of a set of directed lines in the plane. Following this, the duals of a number of standard problems of geometric probability are formulated. Problems considered in detail are the duals of Sylvester's problem, chord length distributions and Ambartzumian's combinatorial geometry. The paper suggests some questions for further work.

We describe a very simple method of randomly permuting the cube {0, 1}n such that the sample space is very small, but, given any m distinct points in {0, 1}n, the images of those points under the random permutation are approximately uniformly distributed over all sequences of m distinct points.

Let S be a closed surface with boundary ∂S and let G be a graph. Let K ⊆ G be a subgraph embedded in S such that ∂S ⊆ K. An embedding extension of K to G is an embedding of G in S that coincides on K with the given embedding of K. Minimal obstructions for the existence of embedding extensions are classified in cases when S is the disk or the cylinder. Linear time algorithms are presented that either find an embedding extension, or return an obstruction to the existence of extensions. These results are to be used as the corner stones in the design of linear time algorithms for the embeddability of graphs in an arbitrary surface and for solving more general embedding extension problems.

We show that the random insertion method for the traveling salesman problem (TSP) may produce a tour Ω(log log n/log log log n) times longer than the optimal tour. The lower bound holds even in the Euclidean Plane. This is in contrast to the fact that the random insertion method performs extremely well in practice. In passing, we show that other insertion methods may produce tours Ω(log n/log log n) times longer than the optimal one. No non-constant lower bounds were previously known.

We study the asymptotic properties of a “uniform” random graph process in which the minimum degree of U(n, M) grows at least as fast as ⌊M/n⌋. We show that if M — n → → ∞, almost surely U(n, M) consists of one giant component and some number of small unicyclic components. We go on to study the distribution of cycles in unicyclic components as they emerge at the beginning of the process and disappear when captured by the giant one.