In Sym(n) with n ≥ 5 let H be a conjugacy class of elements of order 2 and let Γ be the Cayley graph whose vertex set is the group G generated by H (so G = Sym(n) or Alt(n) ) and whose edge set is determined by H. We are interested in the metric structure of this graph. In particular, for g ∈ G let Br(g) be the metric ball in Γ of radius r and centre g. We show that the intersection numbers Φ(Γ; r, g) := | Br(e) ∩ Br(g) | are generalized Stirling functions in n and r. The results are motivated by the study of error graphs in Levenshtein [15,16,17] and related reconstruction problems.Keywords: Intersection numbers in graphs, the k -transposition Cayley graph on Sym(n) , error graphs, reconstruction AMS Classification: 05C12 Distance in graphs, 05C25 Graphs and abstract algebra, 20B25 Finite automorphism groups of algebraic, geometric, or combinatorial structures
Metric Intersections in Cayley graphsLet G be a finite group and let H be a subset of G which generates it. We assume that H = {h −1 : h ∈ H} and that H does not contain the identity element e of G. In this situation H defines an undirected simple Cayley graph Γ = Γ H G on the vertex set G. The usual graph distance on Γ is denoted d : G × G → N ∪ {0}; thus d = d(u, w) for u, w ∈ G is the least number of h i ∈ H so that w = uh 1 ...h d . This defines a metric on Γ. For an integer r ≥ 0 and g ∈ G the setis the metric ball of radius r and centre g. In this paper we are interested in the metric intersection numbers Φ(Γ; r, g) := | B r (Γ, e) ∩ B r (Γ, g) | considered as a function on G for r ≥ 0. Notice in particular, Φ(Γ; r, e) is the cardinality of B r (Γ, e), and this information also provides the diameter of Γ . We determine these intersection numbers when 1