Brauer's induction theorem

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This article states an induction theorem: a result relating the linear characters and linear representations of a group with the characters/representations induced from the linear characters/representations of subgroups
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This fact is related to: linear representation theory
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Name

This result is also termed the characterization of characters lemma or characterization of linear characters lemma.

Statement

Version involving elementary subgroups and arbitrary characters

Let G be a finite group. Brauer's induction theorem states that every character of G is a combination, with integer coefficients, of characters induced from Elementary subgroup (?)s of G. In other words, the character ring of G (over integers, for representations over complex numbers) is generated as a \Z-module by characters induced from elementary subgroups.

Here, the term induced is used in the sense of Induced class function (?). For a character of a representation on H, the induced class function on G by the character is the same as the character of the Induced representation (?) from H to G.

Version involving elementary subgroups and linear characters

Let G be a finite group. This strong form of Brauer's induction theorem states that every character of G is a combination, with (possibly negative) integer coefficients, of characters induced from Linear character (?)s (i.e., characters of one-dimensional representations) on elementary subgroups.

Version involving linear characters

Let G be a finite group. Every character of G is a combination, with (possibly negative) integer coefficients, of characters induced from linear characters (i.e., characters of one-dimensional representations) on subgroups.

Related facts

Similar facts

For a complete list of induction theorems, refer Category:Induction theorems.

Applications

Facts used

  1. Elementary implies nilpotent, Nilpotent implies supersolvable
  2. Finite supersolvable implies monomial
  3. Elementariness is subgroup-closed

Proof

The first formulation implies the other two

By facts (1) and (2), every linear representation of an elementary subgroup is monomial. In particular, every irreducible linear representation is induced from a degree-one representation, i.e., from a linear character. Further, by fact (3), any subgroup of an elementary group is elementary. Thus, using the fact that induction of representations is transitive, we obtain that the \Z-span of representations induced from linear characters of elementary subgroups is the same as the \Z-span of representations induced from all characters of elementary subgroups.

This shows that the first formulation implies the second. The third formulation follows naturally from the second.

Proof of the first formulation

The idea is to show that for each p dividing the order of the group, the constant function sending every element to the index of the p-Sylow subgroup, is a linear combination of characters induced from p-elementary subgroups. Then, we can combine these constant functions to obtain the constant function that sends everything to 1. After that, suitable tensoring gives the result that every character occurs as a linear combination of characters induced from elementary subgroups.

References

Textbook references

  • Linear representations of finite groups by Jean-Pierre Serre, 10-digit ISBN 0287901906 (English), ISBN 3540901906 (French), Page 75, Theorem 18, Section 10.2, More info