Strongly regular graphs

In this paper we have tried to summarize the known results on strongly regular graphs. Both groupal and combinatorial aspects of the theory have been included. We give the list of all known strongly regular graphs and a large bibliography of this subject.     

Factorization of regular multigraphs into regular graphs

A regular multigraph with maximum multiplicity r and degree rs cannot always be factored into r s-regular simple graphs. It is shown, however, that under general conditions a similar factorization can be achieved if we first allow the addition or deletion of a relatively small number of hamilton cycles. Based on this result, we give extensions of some known factorization results on simple graphs to new results on multigraphs. © 1995 John Wiley & Sons, Inc.     

Retarded regular graphs are regular or semiregular

A graph G is said to be retarded regular if there is a positive integral number s such that the number of walks of length s starting at vertices of G is a constant function. Regular and semiregular graphs are retarded regular with s?=?1 and s\!≤ \!2, respectively. We prove that any retarded regular connected graph is either regular or semiregular.     

Locally regular coloured graphs

In this work we introduce the concept of locally regular coloured graph as a generalization to any dimension of the concept of regularity for maps on surfaces of W. Threlfall. We prove that locally regular coloured graphs can be obtained from the classical spherical, euclidean and hyperbollic tessellations. Finally we describe locally regular coloured graphs on spherical 3-manifolds.Partially supported by British-Spanish Join Research Program and DGICYT.     

On Hamiltonian-connected regular graphs

In this paper it is shown that any m-regular graph of order 2m (m ≧ 3), not isomorphic to K_m,m, or of order 2m + 1 (m even, m ≧ 4), is Hamiltonian connected, which extends a previous result of Nash-Williams. As a corollary, it is derived that any such graph contains atleast m Hamiltonian cycles for odd m and atleast 1/2m Hamiltonian cycles for even m.     

Two-ended regular median graphs

We show that regular median graphs of linear growth are the Cartesian product of finite hypercubes with the two-way infinite path. Such graphs are Cayley graphs and have only two ends.For cubic median graphs G the condition of linear growth can be weakened to the condition that G has two ends. For higher degree the relaxation to two-ended graphs is not possible, which we demonstrate by an example of a median graph of degree four that has two ends, but nonlinear growth.     

On regular self-complementary graphs

A regular self-complementary graph is presented which has no complementing permutation consisting solely of cycles of length four. This answers one of Kotzig's questions.     

Matchings in regular graphs

We consider k-regular graphs with specified edge connectivity and show how some classical theorems and some new results concerning the existence of matchings in such graphs can be proved by using the polyhedral characterization of Edmonds. In addition, we show that lower bounds of Lovász and Plummer on the number of perfect matchings in bicritical graphs can be improved for cubic bicritical graphs.     

Completely regular clique graphs

Let Γ=(X,R) be a connected graph. Then Γ is said to be a completely regular clique graph of parameters (s,c) with s≥1 and c≥1, if there is a collection \(\mathcal{C}\) of completely regular cliques of size s+1 such that every edge is contained in exactly c members of  \(\mathcal{C}\) . In this paper, we show that the parameters of \(C\in\mathcal{C}\) as a completely regular code do not depend on \(C\in\mathcal{C}\) . As a by-product we have that all completely regular clique graphs are distance-regular whenever \(\mathcal {C}\) consists of edges. We investigate the case when Γ is distance-regular, and show that Γ is a completely regular clique graph if and only if it is a bipartite half of a distance-semiregular graph.     

Generating random regular graphs

An algorithm is described which generates a random labeled cubic graph on n vertices. Also described is a procedure which, if successful, generates a random (0,1)-matrix with prescribed row and column sums. The latter yields procedures which, if successful, generate random labeled graphs with specified degree sequence and random labeled bipartite graphs with specified degree sequences. These procedures can be implemented so that each trial requires time which is linear in the number of vertices plus edges, but in generating a random r-regular graph, the probability of success of a given trial is about $\text{exp}\text{(}\text{(1 ? r}{}^2\text{)}\text{4}\text{)}$, which is prohibitively small for large r. Comparisons are made between the complexities of the two methods of generating random cubic graphs. The two general schemes presented derive from methods which have been used to enumerate regular graphs, both asymptotically and exactly.     

On quasi-strongly regular graphs

We study the quasi-strongly regular graphs, which are a combinatorial generalization of the strongly regular and the distance regular graphs. Our main focus is on quasi-strongly regular graphs of grade 2. We prove a “spectral gap”-type result for them which generalizes Seidel's well-known formula for the eigenvalues of a strongly regular graph. We also obtain a number of necessary conditions for the feasibility of parameter sets and some structural results. We propose the heuristic principle that the quasi-strongly regular graphs can be viewed as a “lower-order approximation” to the distance regular graphs. This idea is illustrated by extending a known result from the distance-regular case to the quasi-strongly regular case. Along these lines, we propose a number of conjectures and open problems. Finally, we list the all the proper connected quasi-strongly graphs of grade 2 with up to 12 vertices.     

On quasi-strongly regular graphs

We study the quasi-strongly regular graphs, which are a combinatorial generalization of the strongly regular and the distance regular graphs. Our main focus is on quasi-strongly regular graphs of grade 2. We prove a “spectral gap”-type result for them which generalizes Seidel's well-known formula for the eigenvalues of a strongly regular graph. We also obtain a number of necessary conditions for the feasibility of parameter sets and some structural results. We propose the heuristic principle that the quasi-strongly regular graphs can be viewed as a “lower-order approximation” to the distance regular graphs. This idea is illustrated by extending a known result from the distance-regular case to the quasi-strongly regular case. Along these lines, we propose a number of conjectures and open problems. Finally, we list the all the proper connected quasi-strongly graphs of grade 2 with up to 12 vertices.     

Disconnecting strongly regular graphs

In this paper, we show that the minimum number of vertices whose removal disconnects a connected strongly regular graph into non-singleton components equals the size of the neighborhood of an edge for many graphs. These include block graphs of Steiner 2-designs, many Latin square graphs and strongly regular graphs whose intersection parameters are at most a quarter of their valency.     

Distance degree regular graphs

In this note, inequalities between the distance degrees of distance degree regular graphs and to characterize the graphs when one of the equalities holds are proved.     

Connected domination of regular graphs

A dominating setD of a graph G is a subset of V(G) such that for every vertex v∈V(G), either v∈D or there exists a vertex u∈D that is adjacent to v in G. Dominating sets of small cardinality are of interest. A connected dominating setC of a graph G is a dominating set of G such that the subgraph induced by the vertices of C in G is connected. A weakly-connected dominating setW of a graph G is a dominating set of G such that the subgraph consisting of V(G) and all edges incident with vertices in W is connected. In this paper we present several algorithms for finding small connected dominating sets and small weakly-connected dominating sets of regular graphs. We analyse the average-case performance of these heuristics on random regular graphs using differential equations, thus giving upper bounds on the size of a smallest connected dominating set and the size of a smallest weakly-connected dominating set of random regular graphs.     

Regular factors of regular graphs

Given r ? 3 and 1 ? λ ? r, we determine all values of k for which every r-regular graph with edge-connectivity λ has a k-factor.     

Maximum genus of regular graphs

This paper provides tight lower bounds on the maximum genus of a regular graph in terms of its cycle rank. The main tool is a relatively simple theorem that relates lower bounds with the existence (or non-existence) of induced subgraphs with odd cycle rank that are separated from the rest of the graph by cuts of size at most three. Lower bounds on the maximum genus are obtained by bounding from below the size of these odd subgraphs. As a special case, upper-embeddability of a class of graphs is caused by an absence of such subgraphs. A well-known theorem stating that every 4-edge-connected graph is upper-embeddable is a straightforward corollary of the employed method.     

Vertex-antimagic labelings of regular graphs

Let G = (V, E) be a finite, simple and undirected graph with p vertices and q edges. An (a, d)-vertex-antimagic total labeling of G is a bijection f from V (G) ∪ E(G) onto the set of consecutive integers 1, 2, . . . , p + q, such that the vertex-weights form an arithmetic progression with the initial term a and difference d, where the vertex-weight of x is the sum of the value f (x) assigned to the vertex x together with all values f (xy) assigned to edges xy incident to x. Such labeling is called super if the smallest possible labels appear on the vertices. In this paper, we study the properties of such labelings and examine their existence for 2r-regular graphs when the difference d is 0, 1, . . . , r + 1.