group theory

# Induced characters

## Definition

Let $\phi\colon H\to G$ be a group homomorphism, $V$ a representation of $H$, and $\chi$ the character of $V$. The induced character $\phi_!(\chi)$ of $f$ is the character of the induced $G$-representation

$\phi_!(V) = Ind^G_H(V) = V\otimes_{k[H]} k[G].$

## Formula

There is a formula for the induced character:

$\phi_!(\chi)(g) = \frac{1}{|H|} \sum_{k^{-1} g k = \phi(h)} \chi(h)$

where the sum is over all pairs $(k\in G, h\in H)$ such that $k^{-1} g k = \phi(h)$.

This formula is usually given only in the case when $\phi$ is injective, when it can be re-expressed as a sum over cosets. The case when $\phi$ is surjective is Exercise 7.1 of (Serre) and the general case is easy to put together from these. It can also be derived abstractly using bicategorical trace.

## References

Revised on March 1, 2012 22:10:25 by Urs Schreiber (82.169.65.155)