Groupprops, The Group Properties Wiki (pre-alpha)

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., linearly pushforwardable automorphism) must also satisfy the second automorphism property (i.e., class-preserving automorphism)
View all automorphism property implications | View all automorphism property non-implications |Get help on looking up automorphism property implications/non-implications
Get more facts about linearly pushforwardable automorphism| Get more facts about class-preserving automorphism

Contents 1 Statement 2 Definitions used 2.1 Class-separating field 2.2 Class-preserving automorphism 2.3 Linearly pushforwardable automorphism 3 Related facts 4 Related survey articles 5 Proof

Statement

In a class-separating field, any linearly pushforwardable automorphism is class-preserving.

Definitions used

Class-separating field

Further information: Class-separating field

A field k is termed class-separating for a group G if, given two elements such that:

For every finite-dimensional linear representation , ρ(g) and ρ(h) are conjugate in GL(V)

Then, g and h are conjugate in G.

Class-preserving automorphism

Further information: class-preserving automorphism

An automorphism of a group is termed class-preserving if it sends every element of the group to an element in its conjugacy class.

Linearly pushforwardable automorphism

Further information: Linearly pushforwardable automorphism

An automorphism σ of a group G is termed linearly pushforwardable over a field k if, for any finite-dimensional linear representation , there exists an element such that for every , we have:

ρ(σ(g)) = aρ(g)a ^{− 1}

Related facts

• Class-preserving implies linearly pushforwardable in a class-determining field.

Related survey articles

• Conjugacy class-representation duality

Proof

Given: A group G, a class-separating field k for G. A linearly extensible automorphism σ for G.

To prove: For any , g and σ(g) are conjugate.

Proof: Let be any finite-dimensional linear representation of G over k. Then, since σ is linearly pushforwardable, the elements ρ(g) and ρ(σ(g)) are conjugate inside GL(V).

Since this is true for every finite-dimensional linear representation ρ, the definition of class-separating field forces us to conclude that g and σ(g) are conjugate.