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Moore graphs and beyond: A survey of the degree/diameter problem
 ELECTRONIC JOURNAL OF COMBINATORICS
, 2013
"... The degree/diameter problem is to determine the largest graphs or digraphs of given maximum degree and given diameter. General upper bounds – called Moore bounds – for the order of such graphs and digraphs are attainable only for certain special graphs and digraphs. Finding better (tighter) upper bo ..."
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The degree/diameter problem is to determine the largest graphs or digraphs of given maximum degree and given diameter. General upper bounds – called Moore bounds – for the order of such graphs and digraphs are attainable only for certain special graphs and digraphs. Finding better (tighter) upper bounds for the maximum possible number of vertices, given the other two parameters, and thus attacking the degree/diameter problem ‘from above’, remains a largely unexplored area. Constructions producing large graphs and digraphs of given degree and diameter represent a way of attacking the degree/diameter problem ‘from below’. This survey aims to give an overview of the current stateoftheart of the degree/diameter problem. We focus mainly on the above two streams of research. However, we could not resist mentioning also results on various related problems. These include considering Moorelike bounds for special types of graphs and digraphs, such as vertextransitive, Cayley, planar, bipartite, and many others, on
A Note on Large Graphs of Diameter Two and Given Maximum Degree
"... Let vt(d; 2) be the largest order of a vertextransitive graph of degree d and diameter two. It is known that vt(d; 2) = d 2 + 1 for d = 1; 2; 3, and 7; for the remaining values of d we have vt(d; 2) d 2 \Gamma 1. The only known general lower bound on vt(d; 2), valid for all d, seems to be vt(d ..."
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Cited by 19 (5 self)
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Let vt(d; 2) be the largest order of a vertextransitive graph of degree d and diameter two. It is known that vt(d; 2) = d 2 + 1 for d = 1; 2; 3, and 7; for the remaining values of d we have vt(d; 2) d 2 \Gamma 1. The only known general lower bound on vt(d; 2), valid for all d, seems to be vt(d; 2) b d+2 2 cd d+2 2 e. Using voltage graphs, we construct a family of vertextransitive nonCayley graphs which shows that vt(d; 2) 8 9 (d + 1 2 ) 2 for all d of the form d = (3q \Gamma 1)=2 where q is a prime power congruent with 1 (mod 4). The construction generalizes to all prime powers and yields large highly symmetric graphs for other degrees as well. In particular, for d = 7 we obtain as a special case the HoffmanSingleton graph, and for d = 11 and d = 13 we have new largest graphs of diameter two and degree d on 98 and 162 vertices, respectively. 1 Introduction The wellknown degree/diameter problem asks for determining the largest possible number n(d; k) of vertic...
Digraphs of degree 3 and order close to the Moore bound
, 1995
"... It is known that Moore digraphs of degree d ? 1 and diameter k ? 1 do not exist (see [20] or [5]). Furthermore, for degree 2, it is shown that for k 3 there are no digraphs of order `close' to, i.e., one less than, Moore bound [18]. In this paper, we shall consider digraphs of diameter k, degr ..."
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Cited by 9 (6 self)
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It is known that Moore digraphs of degree d ? 1 and diameter k ? 1 do not exist (see [20] or [5]). Furthermore, for degree 2, it is shown that for k 3 there are no digraphs of order `close' to, i.e., one less than, Moore bound [18]. In this paper, we shall consider digraphs of diameter k, degree 3 and number of vertices one less than Moore bound. We give a necessary condition for the existence of such digraphs and, using this condition, we deduce that such digraphs do not exist for infinitely many values of the diameter. Keywords  digraphs, Moore bound, diameter, degree. 1. Introduction By a digraph we mean a structure G = (V; A) where V (G) is a nonempty set of distinct elements called vertices; and A(G) is a set of ordered pairs (u; v) of distinct vertices u; v 2 V called arcs. The order of a digraph G is the number of vertices in G, i.e., jV (G)j. An inneighbour of a vertex v in a digraph G is a vertex u such that (u; v) 2 G. Similarly, an outneighbour of a vertex v is a v...
Regular Digraphs of Diameter 2 and Maximum Order
, 1994
"... It is known that Moore digraphs of degree d ? 1 and diameter k ? 1 do not exist (see [16] or [4]). For degree 2, it has been shown that for diameter k 3 there are no digraphs of order `close' to, i.e., one less than, the Moore bound [14]. For diameter 2, it is known that digraphs close to Moor ..."
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Cited by 8 (6 self)
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It is known that Moore digraphs of degree d ? 1 and diameter k ? 1 do not exist (see [16] or [4]). For degree 2, it has been shown that for diameter k 3 there are no digraphs of order `close' to, i.e., one less than, the Moore bound [14]. For diameter 2, it is known that digraphs close to Moore bound exist for any degree because the line digraphs of complete digraphs are an example of such digraphs. However, it is not known whether these are the only digraphs close to Moore digraphs. In this paper, we shall consider the general case of directed graphs of diameter 2, degree d 3 and with the number of vertices n = d + d², that is, one less than the Moore bound. Using the eigenvalues of the corresponding adjacency matrices we give a number of necessary conditions for the existence of such digraphs. Furthermore, for degree 3 we prove that there are no digraphs close to Moore bound other than the line digraph of K 4 .
On the structure of digraphs with order close to the Moore bound
, 1996
"... The Moore bound for a diregular digraph of degree d and diameter k is M d;k = 1 + d + : : : + d k . It is known that digraphs of order M d;k do not exist for d ? 1 and k ? 1 ([24] or [6]). In this paper we study digraphs of degree d, diameter k and order M d;k \Gamma 1, denoted by (d; k)digraphs ..."
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Cited by 8 (5 self)
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The Moore bound for a diregular digraph of degree d and diameter k is M d;k = 1 + d + : : : + d k . It is known that digraphs of order M d;k do not exist for d ? 1 and k ? 1 ([24] or [6]). In this paper we study digraphs of degree d, diameter k and order M d;k \Gamma 1, denoted by (d; k)digraphs. Miller and Fris showed that (2; k) digraphs do not exist for k 3 [22]. Subsequently, we gave a necessary condition of the existence of (3; k)digraphs, namely, (3; k)digraphs do not exist if k is odd or if k + 1 does not divide 9 2 (3 k \Gamma 1) [3]. The (d; 2)digraphs were considered in [4]. In this paper, we present further necessary conditions for the existence of (d; k)digraphs. In particular, for d; k 3, we show that a (d; k)digraph contains either no cycle of length k or exactly one cycle of length k. 1 Introduction By a digraph we mean a structure G = (V; A) where V (G) is a nonempty set of elements called vertices; and A(G) is a set of ordered pairs (u; v) of disti...
A note on constructing large Cayley graphs of given degree and diameter by voltage assignments
, 1997
"... Voltage graphs are a powerful tool for constructing large graphs (called lifts) with prescribed properties as covering spaces of small base graphs. This makes them suitable for application to the degree/diameter problem, which is to determine the largest order of a graph with given degree and diame ..."
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Voltage graphs are a powerful tool for constructing large graphs (called lifts) with prescribed properties as covering spaces of small base graphs. This makes them suitable for application to the degree/diameter problem, which is to determine the largest order of a graph with given degree and diameter. Many currently known largest graphs of degree # 15 and diameter # 10 have been found by computer search among Cayley graphs of semidirect products of cyclic groups. We show that all of them can in fact be described as lifts of smaller Cayley graphs of cyclic groups, with voltages in (other) cyclic groups. # This research started when J. Plesnk and J. Siran were visiting the Department of Computer Science and Software Engineering of the University of Newcastle NSW Australia in 1995, supported by small ARC grant. the electronic journal of combinatorics 5 (1998), #R9 2 This opens up a new possible direction in the search for large vertextransitive graphs of given degree and diameter....
On the existence of graphs of diameter two and defect two
 Discrete Mathematics 309 (2009
"... In the context of the degree/diameter problem, the ‘defect ’ of a graph represents the difference between the corresponding Moore bound and its order. Thus, a graph with maximum degree d and diameter two has defect two if its order is n = d2 − 1. Only four extremal graphs of this type, referred to a ..."
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In the context of the degree/diameter problem, the ‘defect ’ of a graph represents the difference between the corresponding Moore bound and its order. Thus, a graph with maximum degree d and diameter two has defect two if its order is n = d2 − 1. Only four extremal graphs of this type, referred to as (d, 2, 2)graphs, are known at present: two of degree d = 3 and one of degree d = 4 and 5, respectively. In this paper we prove, by using algebraic and spectral techniques, that for all values of the degree d within a certain range, (d, 2, 2)graphs do not exist. The enumeration of (d, 2, 2)graphs is equivalent to the search of binary symmetric matrices A fulfilling that AJn = dJn and A2 + A + (1 − d)In = Jn + B, where Jn denotes the allone matrix and B is the adjacency matrix of a union of graph cycles. In order to get the factorization of the characteristic polynomial of A in Q[x], we consider the polynomials Fi,d(x) = fi(x2 + x + 1 − d), where fi(x) denotes the minimal polynomial of the Gauss period ζi + ζi, being ζi a primitive ith root of unity. We formulate a conjecture on the irreducibility of Fi,d(x) in Q[x] and we show that its proof would imply the nonexistence of (d, 2, 2)graphs for any degree d> 5.
The Complete Catalog of 3Regular, Diameter3 Planar Graphs
, 1996
"... The largest known 3regular planar graph with diameter 3 has 12 vertices. We consider the problem of determining whether there is a larger graph with these properties. We find all nonisomorphic 3regular, diameter3 planar graphs, thus solving the problem completely. There are none with more than 12 ..."
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The largest known 3regular planar graph with diameter 3 has 12 vertices. We consider the problem of determining whether there is a larger graph with these properties. We find all nonisomorphic 3regular, diameter3 planar graphs, thus solving the problem completely. There are none with more than 12 vertices. An Upper Bound A graph with maximum degree \Delta and diameter D is called a (\Delta; D)graph. It is easily seen ([9], p. 171) that the order of a (\Delta,D)graph is bounded above by the Moore bound, which is given by 1+ \Delta + \Delta (\Delta \Gamma 1) + \Delta \Delta \Delta + \Delta(\Delta \Gamma 1) D\Gamma1 = 8 ? ! ? : \Delta(\Delta \Gamma 1) D \Gamma 2 \Delta \Gamma 2 if \Delta 6= 2; 2D + 1 if \Delta = 2: Figure 1: The regular (3,3)graph on 20 vertices (it is unique up to isomorphism) . For D 2 and \Delta 3, this bound is attained only if D = 2 and \Delta = 3; 7, and (perhaps) 57 [3, 14, 23]. Now, except for the case of C 4 (the cycle on four vertices), the num...
Listcoloring the square . . .
, 2007
"... The square G 2 of a graph G is the graph with the same vertex set as G and with two vertices adjacent if their distance in G is at most 2. Thomassen showed that for a planar graph G with maximum degree ∆(G) = 3 we have χ(G 2) ≤ 7. Kostochka and Woodall conjectured that for every graph, the listch ..."
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The square G 2 of a graph G is the graph with the same vertex set as G and with two vertices adjacent if their distance in G is at most 2. Thomassen showed that for a planar graph G with maximum degree ∆(G) = 3 we have χ(G 2) ≤ 7. Kostochka and Woodall conjectured that for every graph, the listchromatic number of G 2 equals the chromatic number of G 2, that is χl(G 2) = χ(G 2) for all G. If true, this conjecture (together with Thomassen’s result) implies that every planar graph G with ∆(G) = 3 satisfies χl(G 2) ≤ 7. We prove that every graph (not necessarily planar) with ∆(G) = 3 other than the Petersen graph satisfies χl(G 2) ≤ 8 (and this is best possible). In addition, we show that if G is a planar graph with ∆(G) = 3 and girth g(G) ≥ 7, then χl(G 2) ≤ 7. Dvo˘rák, ˘ Skrekovski, and Tancer showed that if G is a planar graph with ∆(G) = 3 and girth g(G) ≥ 10, then χl(G 2) ≤ 6. We improve the girth bound to show that if G is a planar graph with ∆(G) = 3 and g(G) ≥ 9, then χl(G 2) ≤ 6. All of our proofs can be easily translated into lineartime coloring algorithms.