Symmetric groups on infinite sets are complete

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Let S be an infinite set. The symmetric group on S, denoted \operatorname{Sym}(S), is a complete group: it is centerless and every automorphism of it is inner.

Facts used

  1. Finitary symmetric group is characteristic in symmetric group
  2. Automorphism group of finitary symmetric group equals symmetric group
  3. Finitary symmetric group is automorphism-faithful in symmetric group


Given: S is an infinite set, K = \operatorname{Sym}(S), \sigma is an automorphism of K.

To prove: \sigma is inner.

Proof: Let G = \operatorname{FSym}(S) be the subgroup of K comprising the finitary permutations.

  1. By fact (1), \sigma restricts to an automorphism, say \tau of G.
  2. By fact (2), the automorphism \tau of G arises from some inner automorphism, say \sigma', of K.
  3. Consider the ratio \sigma'\sigma^{-1}. The restriction of this automorphism to G is \tau\tau^{-1} which is the identity map. By fact (3), \sigma'\sigma^{-1} is the identity map on K, so \sigma = \sigma'. Thus, \sigma is inner.