General semilinear group

Definition

Suppose $R$ is a commutative unital ring and $n$ is a natural number. The general semilinear group of degree $n$ over $R$, denoted $\Gamma L(n,R)$, is defined as the group of all invertible $R$-semilinear transformations from a $n$-dimensional free module over $R$ to itself.

It can also be described explicitly as an external semidirect product of the general linear group $GL(n,R)$ by the automorphism group of $R$ as a commutative unital ring (acting entry-wise on the matrices), i.e.:

$\Gamma L(n,R)=GL(n,R)\rtimes \operatorname {Aut} (R)$

Special case of fields

Suppose $k$ is the prime subfield of $K$ and suppose that $K$ is a Galois extension over $k$ (this is always true for $K$ a finite field). Then, $\operatorname {Aut} (K)=\operatorname {Gal} (K/k)$ and we can rewrite the group as:

$\Gamma L(n,K)=GL(n,K)\rtimes \operatorname {Gal} (K/k)$

where the action of $\operatorname {Gal} (K/k)$ on $GL(n,K)$ is obtained by inducing the corresponding Galois automorphism on each matrix entry.

If $q$ is a prime power, we denote by $\Gamma L(n,q)$ the group $\Gamma L(n,\mathbb {F} _{q})$ where $\mathbb {F} _{q}$ is the (unique up to isomorphism) field of size $q$.

Arithmetic functions

For a finite field

We consider here a field $K=\mathbb {F} _{q}$ of size $q=p^{r}$ where $p$ is the field characteristic, so $r$ is a natural number.

The prime subfield is $k=\mathbb {F} _{p}$, and the extension $K/k$ has degree $r$. The Galois group of the extension thus has size $r$. Note that the Galois group of the extension is always a cyclic group of order $r$ and is generated by the Frobenius automorphism $x\mapsto x^{p}$.

We are interested in the group $\Gamma L(n,q)$.

Function	Value	Explanation
order	$rq^{\binom {n}{2}}\prod _{i=1}^{n}(q^{i}-1)$	order of semidirect product is product of orders: we multiply the order $r$ of the Galois group with the order of the general linear group.

Particular cases

Finite cases

We consider a field of size $q=p^{r}$ where $p$ is the underlying prime and field characteristic, and therefore $r$ is the degree of the extension over the prime subfield and also the order of the Galois group.

Note that in the case $r=1$, the general semilinear group coincides with the general linear group.

$q$ (field size)	$p$ (underlying prime, field characteristic)	$r$ (degree of extension over prime subfield)	$n$	$\Gamma L(n,q)$	order of $\Gamma L(n,q)$
2	2	1	1	trivial group	1
3	3	1	1	cyclic group:Z2	1
4	2	2	1	symmetric group:S3	6
5	5	1	1	cyclic group:Z4	4
7	7	1	1	cyclic group:Z6	6
8	2	3	1	semidirect product of Z7 and Z3	21
2	2	1	2	symmetric group:S3	6
3	3	1	2	general linear group:GL(2,3)	48
4	2	2	2	general semilinear group:GammaL(2,4)	360
5	5	1	2	general linear group:GL(2,5)	480