Colouring 4-cycle systems with equitably coloured blocks

AbstractA colouring of a 4-cycle system (V,B) is a surjective mapping φ:V→Γ. The elements of Γ are colours and, for each i∈Γ, the set Ci=φ−1(i) is a colour class. If |Γ|=m, we have an m-colouring of (V,B). For every B∈B, let φ(B)={φ(x)|x∈B}. We say that a block B is equitably coloured if either |φ(B)∩Ci|=0 or |φ(B)∩Ci|=2 for every i∈Γ. Let F(n) be the set of integers m such that there exists an m-coloured 4-cycle system of order n with every block equitably coloured. We prove that: •minF(n)=3 for every n≡1(mod8), n⩾17, F(9)=∅,•{m|3⩽m⩽n+3116}⊆F(n), n≡1(mod16), n⩾17,•{m|3⩽m⩽n+2316}⊆F(n), n≡9(mod16), n⩾25,•for every sufficiently large n≡1(mod8), there is an integer m̄ such that maxF(n)⩽m̄. Moreover we show that maxF(n)=m̄ for infinite values of n

Non-existence of (3,2)-Equicolourings in C k -Designs

Abstract A block colouring of a C k -design Σ = (X, B) of order v (odd) is a mapping φ : B → C, where blocks and d(x) is called, using graph theoretic terminology, the degree of the vertex x. A partition of degree D into s parts defines a colouring of type s in which the blocks containing a vertex x are coloured exactly with s colours. For a vertex x and for i = 1, 2, . . . , s, indicate by B x,i the set of all the blocks incident in x and coloured with the colour C i . A colouring of type s is equitable if, for every vertex x, it is |B x,i −B x,j | ≤ 1, for all i, j = 1, . . . , s. If |C| = r, such a colouring will said an (r, s)-equiblock-colouring. In this paper we prove the non-existence of (r, s)-equiblock-colourings, having s = 2 and r = 3, for some classes of C 4 -designs. Mathematics Subject Classification: 05B0

Balanced and strongly balanced Pk-designs

AbstractGiven a graph G, a G-decomposition of the complete graph Kv is a set of graphs, all isomorphic to G, whose edge sets partition the edge set of Kv. A G-decomposition of Kv is also called a G-design and the graphs of the partition are said to be the blocks. A G-design is said to be balanced if the number of blocks containing any given vertex of Kv is a constant.In this paper the concept of strongly balanced G-design is introduced and strongly balanced path-designs are studied. Furthermore, we determine the spectrum of those path-designs which are balanced, but not strongly balanced

An edge colouring of multigraphs

We consider a strict k-colouring of a multigraph G as a surjection f from the vertex set of G into a set of colours {1,2,…,k} such that, for every non-pendant vertex χ of G, there exist at least two edges incident to χ and coloured by the same colour. The maximum number of colours in a strict edge colouring of G is called the upper chromatic index of G and is denoted by χ(G). In this paper we prove some results about it

α-Resolvable λ-fold G-designs

A λ-fold G-design is said to be α-resolvable if its blocks can be partitioned into classes such that every class contains each vertex exactly α times. In this paper we study the existence problem of an α-resolvable λ-fold G-design oforder v in the case when G is any connected subgraph of K_4 and prove that the necessary conditions for its existence are also sufficient

Perfect Octagon Quadrangle Systems with an upper C4-system and a large spectrum

An octagon quadrangle is the graph consisting of an 8-cycle (x1, x2,..., x8) with two additional chords: the edges {x1, x4} and {x5, x8}. An octagon quadrangle system of order ν and index λ [OQS] is a pair (X,H), where X is a finite set of ν vertices and H is a collection of edge disjoint octagon quadrangles (called blocks) which partition the edge set of λKν defined on X. An octagon quadrangle system Σ=(X,H) of order ν and index λ is said to be upper C4-perfect if the collection of all of the upper 4-cycles contained in the octagon quadrangles form a μ-fold 4-cycle system of order ν; it is said to be upper strongly perfect, if the collection of all of the upper 4-cycles contained in the octagon quadrangles form a μ-fold 4-cycle system of order ν and also the collection of all of the outside 8-cycles contained in the octagon quadrangles form a ρ-fold 8-cycle system of order ν. In this paper, the authors determine the spectrum for these systems, in the case that it is the largest possible

The Spectrum of Balanced P^(3)(1, 5)-Designs

Given a 3-uniform hypergraph H(3), an H(3)-decomposition of the complete hypergraph K(3)_v is a collection of hypergraphs, all isomorphic to H(3), whose edge sets partition the edge set of K(3)_v. An H(3)-decomposition of K(3)_v is also called an H(3)-design and the hypergraphs of the partition are said to be the blocks. An H(3)-design is said to be balanced if the number of blocks containing any given vertex of K(3)_v is a constant. In this paper, we determine completely, without exceptions, the spectrum of balanced P(3)(1 5)-designs