Linear representation theory of projective general linear group of degree two over a finite field

This article gives specific information, namely, linear representation theory, about a family of groups, namely: projective general linear group of degree two.
View linear representation theory of group families | View other specific information about projective general linear group of degree two

This article describes the linear representation theory of the projective general linear group of degree two over a finite field. The order (size) of the field is $q$, and the characteristic prime is $p$. $q$ is a power of $p$. The group is denoted $PGL(2,q)$ or $PG{L}_2(q)$.

See also the linear representation theory for: special linear group, projective special linear group, and general linear group.

Summary

Item	Value
degrees of irreducible representations over a splitting field	Case $q$ odd: 1 (2 times), $q-1$ ($(q-1)/2$ times), $q$ (2 times), $q+1$ ($(q-3)/2$ times) Case $q$ even: 1 (1 time), $q-1$ ($q/2$ times), $q$ (1 time), $q+1$ ($(q-2)/2$ times)
number of irreducible representations	Case $q$ odd: $q+2$, case $q$ even: $q+1$ See number of irreducible representations equals number of conjugacy classes, element structure of projective general linear group of degree two over a finite field#Conjugacy class structure
quasirandom degree (minimum degree of nontrivial ireducible representation)	1
maximum degree of irreducible representation	$q+1$
lcm of degrees of irreducible representations	Case $q$ odd: $q(q+1)(q-1)/2=(q^3-q)/2$; Case $q$ even: $q(q+1)(q-1)=q^3-q$
sum of squares of degrees of irreducible representations	$q(q+1)(q-1)=q^3-q$, equal to the group order; see sum of squares of degrees of irreducible representations equals group order

Particular cases

$q$ (field size)	$p$ (underlying prime, field characteristic)	Case for $q$	Group $PGL(2,q)$	Order of the group ($=q^3-q$)	Degrees of irreducible representations (ascending order)	Number of irreducible representations ($=q+1$ if $q$ even, $q+2$ if $q$ odd)	Linear representation theory page
2	2	even	symmetric group:S3	6	1,1,2	3	linear representation theory of symmetric group:S3
3	3	odd	symmetric group:S4	24	1,1,2,3,3	5	linear representation theory of symmetric group:S4
4	2	even	alternating group:A5	60	1,3,3,4,5	5	linear representation theory of alternating group:A5
5	5	odd	symmetric group:S5	120	1,1,4,4,5,5,6	7	linear representation theory of symmetric group:S5
7	7	odd	projective general linear group:PGL(2,7)	336	1,1,6,6,6,7,7,8,8	9	linear representation theory of projective general linear group:PGL(2,7)
8	2	even	projective special linear group:PSL(2,8)	504	1,7,7,7,7,8,9,9,9	9	linear representation theory of projective special linear group:PSL(2,8)
9	3	odd	projective general linear group:PGL(2,9)	720	1,1,8,8,8,8,9,9,10,10,10	11	linear representation theory of projective general linear group:PGL(2,9)

Irreducible representations

Case $p\ne 2$, $q$ odd

Description of collection of representations	Parameter for describing each representation	How the representation is described	Degree of each representation	Number of representations	Sum of squares of degrees
Trivial	--	$x\mapsto 1$	1	1	1
Sign representation	--	Kernel is projective special linear group of degree two, image is $\{\pm 1\}$	1	1	1
Unclear	a nontrivial homomorphism $\phi :{\mathbb{F}}_{q^2}^{\ast }\to {\mathbb{C}}^{\ast }$, with the property that $\phi (x{)}^{q+1}=1$ for all $x$, and $\phi$ takes values other than $\pm 1$. Identify $\phi$ and ${\phi }^q$.	unclear	$q-1$	$(q-1)/2$	$(q-1{)}^3/2=(q^3-3q^2+3q-1)/2$
Nontrivial component of permutation representation of $PG{L}_2$ on the projective line over ${\mathbb{F}}_q$	--	--	$q$	1	$q^2$
Tensor product of sign representation and nontrivial component of permutation representation on projective line	--	--	$q$	1	$q^2$
Induced from one-dimensional representation of Borel subgroup	$\alpha$ homomorphism ${\mathbb{F}}_q^{\ast }\to {\mathbb{C}}^{\ast }$, with $\alpha$ taking values other than $\pm 1$, up to inverses.	Induced from the following representation of the image of the Borel subgroup: $\left(\begin{array}{cc}a& b\\ 0& d\\ \end{array}\right)\mapsto \alpha (a)\alpha (d{)}^{-1}$	$q+1$	$(q-3)/2$	$(q+1{)}^2(q-3)/2=(q^3-q^2-5q-3)/2$
Total	NA	NA	NA	$q+2$	$q^3-q$