Some properties of n-capable and n-perfect groups (original) (raw)
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Let (G, *) be a semigroup, D ⊆ G, and n ≥ 2 be an integer. We say that (D, *) is an n-closed subset of G if a 1 * • • • * an ∈ D for every a 1 , ..., an ∈ D. Hence every closed set is a 2-closed set. The concept of n-closed sets arise in so many natural examples. For example, let D be the set of all odd integers, then (D, +) is a 3-closed subset of (Z, +) that is not a 2-closed subset of (Z, +). If K = {1, 4, 7, 10, ...} , then (K, +) is a 4-closed subset of (Z, +) that is not an n-closed subset of (Z, +) for n = 2, 3. In this paper, we show that if (H, *) is a subgroup of a group (G, *) such that [H : G] = n < ∞, then H is a normal subgroup of G if and only if every left coset of H is an n + 1-closed subset of G.