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From Groupprops

This article gives the statement and possibly, proof, of an implication relation between two automorphism properties. That is, it states that every automorphism satisfying the first automorphism property (i.e., inner automorphism) must also satisfy the second automorphism property (i.e., extensible automorphism)
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Contents 1 Statement 1.1 Verbal statement 1.2 Symbolic statement 2 Proof

Statement

Verbal statement

Any inner automorphism of a group is extensible.

Symbolic statement

Let be groups and σ an inner automorphism of G, there is an automorphism σ' of H such that the restriction of σ' < math > to < math > G is σ.

Proof

The proof in fact follows from the result:

Inner is extensibility-stable

and the fact that every inner automorphism is an automorphism.