Geometric Group Theory Notes
نویسنده
چکیده
Definition. Let Γ be a group. Take A ⊂ Γ to be any subset. The subgroup generated by A, denoted 〈A〉, is the smallest subgroup of Γ containing A. Equivalently, 〈A〉 is the intersection of all subgroups of Γ containing A. Equivalently, 〈A〉 = {a1 1 · · · an n | ai ∈ A, ǫi = ±1}, the set of finite products of elements of A ∪A−1. Notation. We will frequently use Γ to denote a group. If A is a set, A = {a−1 | a ∈ A}. Remark. 〈A〉 is characterized by the following properties: (1) A ⊂ 〈A〉 (2) 〈A〉 ≤ Γ (3) If G ≤ Γ and A ⊂ G, then 〈A〉 ⊂ G. Definition. If A ⊂ Γ and 〈A〉 = Γ, we say that A generates Γ. If A is a finite set which generates Γ, then we say that Γ is finitely generated.
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تاریخ انتشار 2010