# Frobenius reciprocity

In mathematics, and in particular representation theory, Frobenius reciprocity is a theorem expressing a duality between the process of restricting and inducting. It can be used to leverage knowledge about representations of a subgroup to find and classify representations of "large" groups that contain them. It is named for Ferdinand Georg Frobenius, the inventor of the representation theory of finite groups.

## Statement

### Character theory

The theorem was originally stated in terms of character theory. Let G be a finite group with a subgroup H, let ${\displaystyle \operatorname {Res} _{H}^{G}}$ denote the restriction of a character, or more generally, class function of G to H, and let ${\displaystyle \operatorname {Ind} _{H}^{G}}$ denote the induced class function of a given class function on H. For any finite group A, there is an inner product ${\displaystyle \langle -,-\rangle _{A}}$ on the vector space of class functions ${\displaystyle A\to \mathbb {C} }$ (described in detail in the article Schur orthogonality relations). Now, for any class functions ${\displaystyle \psi :H\to \mathbb {C} }$ and ${\displaystyle \varphi :G\to \mathbb {C} }$, the following equality holds:

${\displaystyle \langle \operatorname {Ind} _{H}^{G}\psi ,\varphi \rangle _{G}=\langle \psi ,\operatorname {Res} _{H}^{G}\varphi \rangle _{H}}$
.[1][2]

In other words, ${\displaystyle \operatorname {Ind} _{H}^{G}}$ and ${\displaystyle \operatorname {Res} _{H}^{G}}$ are Hermitian adjoint.

Proof of Frobenius reciprocity for class functions

Let ${\displaystyle \psi :H\to \mathbb {C} }$ and ${\displaystyle \varphi :G\to \mathbb {C} }$ be class functions.

Proof. Every class function can be written as a linear combination of irreducible characters. As ${\displaystyle \langle \cdot ,\cdot \rangle }$ is a bilinear form, we can, without loss of generality, assume ${\displaystyle \psi }$ and ${\displaystyle \varphi }$ to be characters of irreducible representations of ${\displaystyle H}$ in ${\displaystyle W}$ and of ${\displaystyle G}$ in ${\displaystyle V,}$ respectively. We define ${\displaystyle \psi (s)=0}$ for all ${\displaystyle s\in G\setminus H.}$ Then we have

{\displaystyle {\begin{aligned}\langle {\text{Ind}}(\psi ),\varphi \rangle _{G}&={\frac {1}{|G|}}\sum _{t\in G}{\text{Ind}}(\psi )(t)\varphi (t^{-1})\\&={\frac {1}{|G|}}\sum _{t\in G}{\frac {1}{|H|}}\sum _{s\in G \atop s^{-1}ts\in H}\psi (s^{-1}ts)\varphi (t^{-1})\\&={\frac {1}{|G|}}{\frac {1}{|H|}}\sum _{t\in G}\sum _{s\in G}\psi (s^{-1}ts)\varphi ((s^{-1}ts)^{-1})\\&={\frac {1}{|G|}}{\frac {1}{|H|}}\sum _{t\in G}\sum _{s\in G}\psi (t)\varphi (t^{-1})\\&={\frac {1}{|H|}}\sum _{t\in G}\psi (t)\varphi (t^{-1})\\&={\frac {1}{|H|}}\sum _{t\in H}\psi (t)\varphi (t^{-1})\\&={\frac {1}{|H|}}\sum _{t\in H}\psi (t){\text{Res}}(\varphi )(t^{-1})\\&=\langle \psi ,{\text{Res}}(\varphi )\rangle _{H}\end{aligned}}}

In the course of this sequence of equations we used only the definition of induction on class functions and the properties of characters. ${\displaystyle \Box }$

Alternative proof. In terms of the group algebra, i.e. by the alternative description of the induced representation, the Frobenius reciprocity is a special case of a general equation for a change of rings:

${\displaystyle {\text{Hom}}_{\mathbb {C} [H]}(W,U)={\text{Hom}}_{\mathbb {C} [G]}(\mathbb {C} [G]\otimes _{\mathbb {C} [H]}W,U).}$

This equation is by definition equivalent to

${\displaystyle \langle W,{\text{Res}}(U)\rangle _{H}=\langle W,U\rangle _{H}=\langle {\text{Ind}}(W),U\rangle _{G}.}$

As this bilinear form tallies the bilinear form on the corresponding characters, the theorem follows without calculation. ${\displaystyle \Box }$

### Module theory

As explained in the section Representation theory of finite groups#Representations, modules and the convolution algebra, the theory of the representations of a group G over a field K is, in a certain sense, equivalent to the theory of modules over the group algebra K[G].[3] Therefore, there is a corresponding Frobenius reciprocity theorem for K[G]-modules.

Let G be a group with subgroup H, let M be an H-module, and let N be a G-module. In the language of module theory, the induced module ${\displaystyle K[G]\otimes _{K[H]}M}$ corresponds to the induced representation ${\displaystyle \operatorname {Ind} _{H}^{G}}$, whereas the restriction of scalars ${\displaystyle {_{K[H]}}N}$ corresponds to the restriction ${\displaystyle \operatorname {Res} _{H}^{G}}$. Accordingly, the statement is as follows: The following sets of module homomorphisms are in bijective correspondence:

${\displaystyle \operatorname {Hom} _{K[G]}(K[G]\otimes _{K[H]}M,N)\cong \operatorname {Hom} _{K[H]}(M,{_{K[H]}}N)}$
.[4][5]

As noted below in the section on category theory, this result applies to modules over all rings, not just modules over group algebras.

### Category theory

Let G be a group with a subgroup H, and let ${\displaystyle \operatorname {Res} _{H}^{G},\operatorname {Ind} _{H}^{G}}$ be defined as above. For any group A and field K let ${\displaystyle {\textbf {Rep}}_{A}^{K}}$ denote the category of linear representations of A over K. There is a forgetful functor

{\displaystyle {\begin{aligned}\operatorname {ResF} _{H}^{G}:{\textbf {Rep}}_{G}&\longrightarrow {\textbf {Rep}}_{H}\\(V,\rho )&\longmapsto \operatorname {Res} _{G}^{H}(V,\rho )\end{aligned}}}

This functor acts as the identity on morphisms. There is a functor going in the opposite direction:

{\displaystyle {\begin{aligned}\operatorname {IndF} _{H}^{G}:{\textbf {Rep}}_{H}&\longrightarrow {\textbf {Rep}}_{G}\\(W,\tau )&\longmapsto \operatorname {Ind} _{H}^{G}(W,\tau )\end{aligned}}}

These functors form an adjoint pair ${\displaystyle \operatorname {IndF} _{H}^{G}\dashv \operatorname {ResF} _{H}^{G}}$.[6][7] In the case of finite groups, they are actually both left- and right-adjoint to one another. This adjunction gives rise to a universal property for the induced representation (for details, see Induced representation#Properties).

In the language of module theory, the corresponding adjunction is an instance of the more general relationship between restriction and extension of scalars.

## Notes

1. Serre 1977, p. 56.
2. Sengupta 2012, p. 246.
3. Specifically, there is an isomorphism of categories between K[G]-Mod and RepGK, as described on the pages Isomorphism of categories#Category of representations and Representation theory of finite groups#Representations, modules and the convolution algebra.
4. James, Gordon Douglas (1945–2001). Representations and characters of groups. Liebeck, M. W. (Martin W.) (2nd ed.). Cambridge, UK: Cambridge University Press. ISBN 9780521003926. OCLC 52220683.
5. Sengupta 2012, p. 245.
6. "Frobenius reciprocity on planetmath.org". planetmath.org. Retrieved 2017-11-02.
7. "Frobenius reciprocity in nLab". ncatlab.org. Retrieved 2017-11-02.