# Thread: Automorphisms over a quotient group of GLn\SLn over Z7

1. ## Automorphisms over a quotient group of GLn\SLn over Z7

I have a problem that I have no idea how to begin solving. I need to describe the following group using an 'action table' (when the action is composition of homomorphisms). Moreover, I need to find out what is isomorphic to this group.

The group is : $\displaystyle Aut(GL_n(Z_7) / SL_n(Z_7))$ , which is a quotient group, with a given n>0, and knowing that $\displaystyle Z_7$ is a field.

Now, I believe I need to find $\displaystyle GL_n(Z_7) , SL_n(Z_7)$ , and find the order of each group. Then, I'll be able to know how many cosets the quotient group contains. Then, I'll have to actually FIND and describe the different automorphisms on this quotient group... Afterward, I'll need to check what happens whenever I composite each and every automorphism to another, and put it in a table, as I was asked to.

At last, I'll need to find what this group of automorphisms is isomorphic to...

Any help ??

Thanks a million!

I have a problem that I have no idea how to begin solving. I need to describe the following group using an 'action table' (when the action is composition of homomorphisms). Moreover, I need to find out what is isomorphic to this group.

The group is : $\displaystyle Aut(GL_n(Z_7) / SL_n(Z_7))$ , which is a quotient group, with a given n>0, and knowing that $\displaystyle Z_7$ is a field.

Now, I believe I need to find $\displaystyle GL_n(Z_7) , SL_n(Z_7)$ , and find the order of each group. Then, I'll be able to know how many cosets the quotient group contains. Then, I'll have to actually FIND and describe the different automorphisms on this quotient group... Afterward, I'll need to check what happens whenever I composite each and every automorphism to another, and put it in a table, as I was asked to.

At last, I'll need to find what this group of automorphisms is isomorphic to...

Any help ??

Thanks a million!

I'd rather write $\displaystyle \mathbb{F}_7:=\mathbb{Z}_7=\mathbb{Z}/7\mathbb{Z}$

Hints:

(1) For $\displaystyle q=p^r\,,\,\,p$ a prime, we have that $\displaystyle |GL_n(\mathbb{F}_q)|=(q^n-1)(q^n-q)\cdot\ldots\cdot(q^n-q^{n-1})=q^{n(n-1)/2}\prod\limits_{i=1}^n(q^i-1)$ . Hint for proof: count how many inversible matrices are there by counting in how many ways can you choose the first column of such a matrix, then in how many ways can its 2nd column be chosen, etc.

2) Since $\displaystyle \Delta:GL_n(\mathbb{F}_q)\rightarrow \mathbb{F}_q^{*}\,,\,\,\Delta(A):=\det(A)$ is an epimorphism of groups, we get that $\displaystyle |SL_n(\mathbb{F}_q)|=\frac{|GL_n(\mathbb{F}_q)|}{q-1}$ (Hint: what is $\displaystyle \ker(\Delta)$ ?)

3) From the above it follows at once that $\displaystyle Aut\left(GL_n(\mathbb{F}_q)/SL_n(\mathbb{F}_q)\right)$ is the automorphism group of a cyclic group, so...

Tonio