6.3.1 Permutation groups Definition 9: a bijection from a set s to itself is called a permutation of s Lemma 6.1: let s be a set (1) Let fand g be two permutations of s. Then the composition of f and g is a permutation of S (2) Letf be a permutation of s. Then the inverse of f is a permutation of s. 6.3.1 Permutation groups  Definition 9: A bijection from a set S to itself is called a permutation of S  Lemma 6.1：Let S be a set.  (1) Let f and g be two permutations of S. Then the composition of f and g is a permutation of S.  (2) Let f be a permutation of S. Then the inverse of f is a permutation of S