# Classifying the irreducible representations of $\mathbb{Z}/p\mathbb{Z}\rtimes \mathbb{Z}/n \mathbb{Z}$

I need help with the following problem:

Suppose $n$ is some positive integer, and $n|p-1$. Classify all irreducible representations of $$\mathbb{Z}/p\mathbb{Z}\rtimes \mathbb{Z}/n \mathbb{Z}.$$

I am not sure where to start,

#### Solutions Collecting From Web of "Classifying the irreducible representations of $\mathbb{Z}/p\mathbb{Z}\rtimes \mathbb{Z}/n \mathbb{Z}$"

I assume that you are talking about a faithful action of $\mathbb{Z}/n\mathbb{Z}$ on $\mathbb{Z}/p\mathbb{Z}$. First, there are the obvious lifts of the irreducible representations of $\mathbb{Z}/n\mathbb{Z}$ to $G$. For the rest:

Exercise 1: The induction of a non-trivial character of $\mathbb{Z}/p\mathbb{Z}$ to $G$ is irreducible.

Exercise 2: When are two such inductions isomorphic?

Exercise 3: Now count the sums of squares of the degrees of the characters that you get this way.

I have to say that this is rather tough homework if you were not given any of these hints. But with them, you should be able to do the rest yourself.

Edit: More generally, suppose that $G=A\rtimes H$ where $A$ is abelian. Then all irreducible characters of $G$ are obtained as follows. $H$ acts on the irreducible characters of $A$ by $(h\cdot\chi)(a) = \chi(h^{-1}ah)$. Let $\chi$ be a linear character of $A$. Extend it to $S_\chi=A\rtimes \text{Stab}_H(\chi)$, where $\text{Stab}_H(\chi)\leq H$ is the stabiliser of $\chi$ in $H$ under the above action, by setting $\chi(as) = \chi(a)$ for $a\in A,s\in \text{Stab}_H(\chi)$. Let $\rho$ be an irreducible character of $\text{Stab}_H(\chi)$, lift it to an irreducible character of $S_\chi$. Then $\text{Ind}_{G/S_\chi}(\chi\otimes \rho)$ is an irreducible character of $G$ and they all arise in this way. I will leave it to you to determine when two such inductions are isomorphic, so as not to spoil the homework exercise.

Note that your question is a special case of this, since $H=\mathbb{Z}/n\mathbb{Z}$ acts faithfully on $\mathbb{Z}/p\mathbb{Z}$, and therefore also on its irreducible characters. Thus, $\text{Stab}_H(\chi)$ is trivial in your case, whenever $\chi$ is non-trivial.

Have a look at Weintraub – “Representation theory of finite group” in the section Mackey Machine. They give the right tool for exactly studying the irreducible representation of a semidirect product $H \rtimes G$, where $H$ is abelian.

In fact, the Mackey Machine holds in greater generality for an exact sequence of locally compact groups
$1 \rightarrow H \rightarrow K \rightarrow G \rightarrow 1,$
with some mild conditions on $H$ (being type 1).

Here is a place to start, although not a complete solution by any means. Let $G=\mathbb{Z}/p\mathbb{Z}\rtimes \mathbb{Z}/n \mathbb{Z}$. Then $\mathbb{Z}/n\subset G$ and $\mathbb{Z}\subset G$.

If we restrict a representation to one of these two subgroups, it will split as a direct sum of irreducibles, and because the subgroups are abelian, this means that we can find a basis of eigenvectors for the action of $\mathbb{Z}/p$ (or a basis of eigenvectors for the action of $\mathbb{Z}/n$).

Suppose that $\omega$ is a $p$th root of unity, and that $v$ is a vector such that $k.v=\omega^k v$ for $k\in \mathbb{Z}/p$. If $j\in \mathbb{Z}/n$, then how does $k$ act on $j.v$? Use the commutation relations that you know you have in $G$ between elements of $\mathbb{Z}/n$ and $\mathbb{Z}/p$