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Let G = (V,A) be a directed graph. With any subset X of V is associated the directed subgraph G[X] = (X,A ∩ (X×X)) of G induced by X. A subset X of V is an interval of G provided that for a,b ∈ X and x ∈ V∖X, (a,x) ∈ A if and only if (b,x) ∈ A, and similarly for (x,a) and (x,b). For example ∅, V, and {x}, where x ∈ V, are intervals of G which are the trivial intervals. A directed graph is indecomposable if all its intervals are trivial. Given an integer k > 0, a directed graph G = (V,A) is called...
Let V be a finite vertex set and let (, +) be a finite abelian group. An -labeled and reversible 2-structure defined on V is a function g : (V × V) (v, v) : v ∈ V → such that for distinct u, v ∈ V, g(u, v) = −g(v, u). The set of -labeled and reversible 2-structures defined on V is denoted by ℒ(V, ). Given g ∈ ℒ(V, ), a subset X of V is a clan of g if for any x, y ∈ X and v ∈ V X, g(x, v) = g(y, v). For example, ∅, V and v (for v ∈ V) are clans of g, called trivial. An element g of ℒ(V, ) is primitive...
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