nLab induced character

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

• Jean-Pierre Serre: Linear Representations of Finite Groups, Graduate Texts in Mathematics 42, Springer (1977) [doi:10.1007/978-1-4684-9458-7, pdf]

• MO question about the non-injective case.