Induced representation

Let G be a finite group and H any subgroup of G. Furthermore let (π, V) be a representation of H. Let n = [G : H] be the index of H in G and let g₁, ..., g_n be a full set of representatives in G of the left cosets in G/H. The induced representation Ind^G
_H π can be thought of as acting on the following space:

${\displaystyle W=\bigoplus _{i=1}^{n}g_{i}V.}$

Here each g_i V is an isomorphic copy of the vector space V whose elements are written as g_i v with v∈V. For each g in G and each g_i there is an h_i in H and j(i) in {1, ..., n} such that g g_i = g_j(i) h_i . (This is just another way of saying that g₁, ..., g_n is a full set of representatives.) Via the induced representation G acts on W as follows:

${\displaystyle g\cdot \sum _{i=1}^{n}g_{i}v_{i}=\sum _{i=1}^{n}g_{j(i)}\pi (h_{i})v_{i}}$

where ${\displaystyle v_{i}\in V}$ for each i.

Alternatively, one can construct induced representations using the tensor product: any K-linear representation ${\displaystyle \pi }$ of the group H can be viewed as a module V over the group ring K[H]. We can then define

${\displaystyle \operatorname {Ind} _{H}^{G}\pi =K[G]\otimes _{K[H]}V.}$

This latter formula can also be used to define Ind^G
_H π for any group G and subgroup H, without requiring any finiteness.[1]

For any group, the induced representation of the trivial representation of the trivial subgroup is the right regular representation. More generally the induced representation of the trivial representation of any subgroup is the permutation representation on the cosets of that subgroup.

An induced representation of a one dimensional representation is called a monomial representation, because it can be represented as monomial matrices. Some groups have the property that all of their irreducible representations are monomial, the so-called monomial groups.

If H is a subgroup of the group G, then every K-linear representation ρ of G can be viewed as a K-linear representation of H; this is known as the restriction of ρ to H and denoted by Res(ρ). In the case of finite groups and finite-dimensional representations, the Frobenius reciprocity theorem states that, given representations σ of H and ρ of G, the space of H-equivariant linear maps from σ to Res(ρ) has the same dimension over K as that of G-equivariant linear maps from Ind(σ) to ρ.[2]

The universal property of the induced representation, which is also valid for infinite groups, is equivalent to the adjunction asserted in the reciprocity theorem. If ${\displaystyle (\sigma ,V)}$ is a representation of H and ${\displaystyle (\operatorname {Ind} (\sigma ),{\hat {V}})}$ is the representation of G induced by ${\displaystyle \sigma }$, then there exists a H-equivariant linear map ${\displaystyle j:V\to {\hat {V}}}$ with the following property: given any representation (ρ,W) of G and H-equivariant linear map ${\displaystyle f:V\to W}$, there is a unique G-equivariant linear map ${\displaystyle {\hat {f}}:{\hat {V}}\to W}$ with ${\displaystyle {\hat {f}}j=f}$. In other words, ${\displaystyle {\hat {f}}}$ is the unique map making the following diagram commute:[3]

The Frobenius formula states that if χ is the character of the representation σ, given by χ(h) = Tr σ(h), then the character ψ of the induced representation is given by

${\displaystyle \psi (g)=\sum _{x\in G/H}{\widehat {\chi }}\left(x^{-1}gx\right),}$

where the sum is taken over a system of representatives of the left cosets of H in G and

${\displaystyle {\widehat {\chi }}(k)={\begin{cases}\chi (k)&{\text{if }}k\in H\\0&{\text{otherwise}}\end{cases}}}$

If G is a locally compact topological group (possibly infinite) and H is a closed subgroup then there is a common analytic construction of the induced representation. Let (π, V) be a continuous unitary representation of H into a Hilbert space V. We can then let:

${\displaystyle \operatorname {Ind} _{H}^{G}\pi =\left\{\phi \colon G\to V\$ :\ \phi (gh^{-1})=\pi (h)\phi (g){\text{ for all }}h\in H,\;g\in G{\text{ and }}\ \phi \in L^{2}(G/H)\right\}.}

Here φ∈L²(G/H) means: the space G/H carries a suitable invariant measure, and since the norm of φ(g) is constant on each left coset of H, we can integrate the square of these norms over G/H and obtain a finite result. The group G acts on the induced representation space by translation, that is, (g.φ)(x)=φ(g⁻¹x) for g,x∈G and φ∈Ind^G
_H π.

This construction is often modified in various ways to fit the applications needed. A common version is called normalized induction and usually uses the same notation. The definition of the representation space is as follows:

${\displaystyle \operatorname {Ind} _{H}^{G}\pi =\left\{\phi \colon G\to V\$ :\ \phi (gh^{-1})=\Delta _{G}^{-{\frac {1}{2}}}(h)\Delta _{H}^{\frac {1}{2}}(h)\pi (h)\phi (g){\text{ and }}\phi \in L^{2}(G/H)\right\}.}

Here Δ_G, Δ_H are the modular functions of G and H respectively. With the addition of the normalizing factors this induction functor takes unitary representations to unitary representations.

One other variation on induction is called compact induction. This is just standard induction restricted to functions with compact support. Formally it is denoted by ind and defined as:

${\displaystyle \operatorname {ind} _{H}^{G}\pi =\left\{\phi \colon G\to V\$ :\ \phi (gh^{-1})=\pi (h)\phi (g){\text{ and }}\phi {\text{ has compact support mod }}H\right\}.}

Note that if G/H is compact then Ind and ind are the same functor.

Suppose G is a topological group and H is a closed subgroup of G. Also, suppose π is a representation of H over the vector space V. Then G acts on the product G × V as follows:

${\displaystyle g.(g',x)=(gg',x)}$

where g and g′ are elements of G and x is an element of V.

Define on G × V the equivalence relation

${\displaystyle (g,x)\sim (gh,\pi (h^{-1})(x)){\text{ for all }}h\in H.}$

Denote the equivalence class of ${\displaystyle (g,x)}$ by ${\displaystyle [g,x]}$. Note that this equivalence relation is invariant under the action of G; consequently, G acts on (G × V)/~ . The latter is a vector bundle over the quotient space G/H with H as the structure group and V as the fiber. Let W be the space of sections ${\displaystyle \phi :G/H\to (G\times V)/\!\sim }$ of this vector bundle. This is the vector space underlying the induced representation Ind^G
_H π. The group G acts on a section ${\displaystyle \phi :G/H\to {\mathcal {L}}_{W}}$ given by ${\displaystyle gH\mapsto [g,\phi _{g}]}$ as follows:

${\displaystyle (g\cdot \phi )(g'H)=[g',\phi _{g^{-1}g'}]\ {\text{ for }}g,g'\in G.}$

In the case of unitary representations of locally compact groups, the induction construction can be formulated in terms of systems of imprimitivity.