We shall pack circuits of arbitrary lengths into the complete graph KN. More precisely, if N is odd and [sum ]ti=1mi = (N2), mi [ges ] 3, then the edges of KN can be written as an edge-disjoint union of circuits of lengths m1,…,mt. Since the degrees of the vertices in any such packing must be even, this result cannot hold for even N. For N even, we prove that if [sum ]ti=1mi [les ] (N2) − N−2 then we can write some subgraph of KN as an edge-disjoint union of circuits of lengths m1,…,mt. In particular, KN minus a 1-factor can be written as a union of such circuits when [sum ]ti=1mi = (N2) − N−2. We shall also show that these results are best possible.

We strengthen the Cohen–Körner–Simonyi upper bound on counting ‘very different sequences’.

Algorithmic aspects of a chip-firing game on a graph introduced by Biggs are studied. This variant of the chip-firing game, called the dollar game, has the properties that every starting configuration leads to a so-called critical configuration. The set of critical configurations has many interesting properties. In this paper it is proved that the number of steps needed to reach a critical configuration is polynomial in the number of edges of the graph and the number of chips in the starting configuration, but not necessarily in the size of the input. An alternative algorithm is also described and analysed.

Let [Mscr ] be the class of simple matroids which do not contain the 5-point line U2,5, the Fano plane F7, the non-Fano plane F−7, or the matroid P7 as minors. Let h(n) be the maximum number of points in a rank-n matroid in [Mscr ]. We show that h(2) = 4, h(3) = 7, and h(n) = (n+12) for n [ges ] 4, and we also find all the maximum-sized matroids for each rank.

We investigate a graph function which is related to the local density, the maximal cut and the least eigenvalue of a graph. In particular it enables us to prove the following assertions.

Let p [ges ] 3 be an integer, c ∈ (0, 1/2) and G be a Kp-free graph on n vertices with e [les ] cn2 edges. There exists a positive constant α = α (c, p) such that:

(a) some [lfloor ]n/2[rfloor ]-subset of V (G) induces at most (c-4 − α) n2 edges (this answers a question of Paul Erdős);

(b) G can be made bipartite by the omission of at most (c-2 − α) n2 edges.

We consider an extension of the Monotone Subsequence lemma of Erdős and Szekeres in higher dimensions. Let v1,…,vn ∈ ℝd be a sequence of real vectors. For a subset I ⊆ [n] and vector [srarr ]c ∈ {0,1}d we say that I is [srarr ]c-free if there are no i < j ∈ I, such that, for every k = 1,…,d, vik < vik if and only if [srarr ]ck = 0. We construct sequences of vectors with the property that the largest [srarr ]c-free subset is small for every choice of [srarr ]c. In particular, for d = 2 the largest [srarr ]c-free subset is O(n⅝) for all the four possible [srarr ]c. The smallest possible value remains far from being determined.

We also consider and resolve a simpler variant of the problem.