# Modular representation theory of projective special linear group of degree two over a finite field in its defining characteristic

This article gives specific information, namely, modular representation theory, about a family of groups, namely: projective special linear group of degree two.

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This article discusses the modular representation theory in characteristic (where is a prime number) for the projective special linear group of degree two over a finite field of size and characteristic . Note in particular that the characteristic of the field over which we are taking representations is the same as the characteristic of the field over which we are considering the projective special linear group of degree two.

See also:

- Linear representation theory of projective special linear group of degree two over a finite field (for the non-modular case)
- Modular representation theory of special linear group of degree two over a finite field in its defining characteristic.

## Particular cases

## Case

See modular representation theory of symmetric group:S3 at 2.

## Case is an odd prime

In this case, the degrees of irreducible representations are (a total of representations) as follows:

- Consider the natural action of on a two-dimensional vector space over with basis labeled .
- Use this to induce an action of on the polynomial ring .
- For a nonnegative integer, the space of homogeneous polynomials of degree is an invariant subspace of degree with basis . thus acquires a representation on this space of degree . For the case , the induced action on this space gives an
*irreducible*representation of degree . When is even (so the degree is odd) the center acts trivially, and we thus get a representation of the quotient group .

Note that for , we still get a representation but this is not irreducible.

## General case

First, we recall the general case for the special linear group of degree two:

## General case

In this case, there are irreducible representations, given as follows. Suppose where is the characteristic.

Each irreducible representation can be described as a tensor product of representations:

where each is a representation of degree obtained by looking at homogeneous degree polynomials in and where the action is given by composing the usual -action with the power of Frobenius, i.e., with the field automorphism .

As before, we have , so we get the following degree for the whole representation:

This number is bounded from below by and from above by . However, for , it is not the case that each degree appears once. Rather, each degree appears based on the number of distinct ways it can be written as an ordered product of numbers between 1 and .

For , all these descend to the projective special linear group because the special linear group is the same as the projective special linear group.

Of these representations, not all descend to the projective special linear group in the case of odd . For odd , the criterion for a representation to descend is the following: The number of s that are even (so that is odd) should be even.

A short combinatorial argument shows that there are exactly possible ways of choosing these s when is odd.