# 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 $\operatorname {Res} _{H}^{G}$ denote the restriction of a character, or more generally, class function of G to H, and let $\operatorname {Ind} _{H}^{G}$ denote the induced class function of a given class function on H. For any finite group A, there is an inner product $\langle -,-\rangle _{A}$ on the vector space of class functions $A\to \mathbb {C}$ (described in detail in the article Schur orthogonality relations). Now, for any class functions $\psi :H\to \mathbb {C}$ and $\varphi :G\to \mathbb {C}$, the following equality holds:
    $\langle \operatorname {Ind} _{H}^{G}\psi ,\varphi \rangle _{G}=\langle \psi ,\operatorname {Res} _{H}^{G}\varphi \rangle _{H}.$
In other words, $\operatorname {Ind} _{H}^{G}$ and $\operatorname {Res} _{H}^{G}$ are Hermitian adjoint.

### 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]. 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 $K[G]\otimes _{K[H]}M$ corresponds to the induced representation $\operatorname {Ind} _{H}^{G}$, whereas the restriction of scalars ${_{K[H]}}N$ corresponds to the restriction $\operatorname {Res} _{H}^{G}$. Accordingly, the statement is as follows: The following sets of module homomorphisms are in bijective correspondence:
    $\operatorname {Hom} _{K[G]}(K[G]\otimes _{K[H]}M,N)\cong \operatorname {Hom} _{K[H]}(M,{_{K[H]}}N)$.
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 $\operatorname {Res} _{H}^{G},\operatorname {Ind} _{H}^{G}$ be defined as above. For any group A and field K let ${\textbf {Rep}}_{A}^{K}$ denote the category of linear representations of A over K. There is a forgetful functor
    ${\begin{aligned}\operatorname {Res} _{H}^{G}:{\textbf {Rep}}_{G}&\longrightarrow {\textbf {Rep}}_{H}\\(V,\rho )&\longmapsto \operatorname {Res} _{H}^{G}(V,\rho )\end{aligned}}$
This functor acts as the identity on morphisms. There is a functor going in the opposite direction:
    ${\begin{aligned}\operatorname {Ind} _{H}^{G}:{\textbf {Rep}}_{H}&\longrightarrow {\textbf {Rep}}_{G}\\(W,\tau )&\longmapsto \operatorname {Ind} _{H}^{G}(W,\tau )\end{aligned}}$
These functors form an adjoint pair $\operatorname {Ind} _{H}^{G}\dashv \operatorname {Res} _{H}^{G}$. 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.