Semi-Direct Product & Non-Abelian Groups

[OMM!]

Homework Statement

Let p, q be distinct primes s.t. p \equiv 1 (mod q). Prove that there exists a non-Abelian group of order pq and calculate the character table.

Homework Equations

Semi-Direct Product: Let H = < Y | S > and N = < X | R > be groups and let \phi : H \rightarrow Aut (N) be a homomorphism. Then the SDP has the presentation:

N x H = < X, Y | R, S, y^{-1}xyw_{x,y}^{-1} >

With x in X, y in Y, w_{x,y} = (\phi(y(x))) (i.e. \phi of y and then \phi of y of x etc.) in < Y > = N

The Attempt at a Solution

I've done a specific example where p=7 and q=3 and found a group of order 21, using the semi-direct product of 2 cyclic groups C_{7} and C_{3}.

But can't see how to prove that there exists a group more generally. So far I have.

If N = C_{p} = < x > and H = C_{q} = < y >, then \phi : C_{q} \rightarrow Aut (C_{p}) = C_{p-1} = < \alpha >, then we have y \mapsto \phi(y) too.

When p=7 and q=3

I had that \phi(y) = {1, \alpha^{2}, \alpha^{4}}

If \phi(y) = 1, then we'd have that the SDP of C_{7} and C_{3} would be ismorphic to C_{7} x C_{3}.

If \phi(y) = \alpha^{2}, then the SDP of C_{7} and C_{3} would be equal to < x, y >

Then by the relation in the SDP, we'd have: yxy^{-1} = \alpha^{2}(x) = x^{t}

And we have that: t = {1, 2, 3, 4, 5, 6}, with p=7 and q=3, then: t^{3} \equiv 1 (mod 7), and so: t = 1 and t = 2 and t = 4, thus we have groups:

G = < X, Y | x^{7} = y^{3} = 1, yxy^{-1} = x > = C_{7} x C_{3}

G = < X, Y | x^{7} = y^{3} = 1, yxy^{-1} = x^{2} >

G = < X, Y | x^{7} = y^{3} = 1, yxy^{-1} = x^{4} >

But how would I get to this stage more generally?! Thanks for any help!

 

[micromass]

Hi OMM!

I think it's clear that you will want to form the group C_p\rtimes C_q. The first thing you will want to do is check if there exists a suitable homomorphism

\phi:C_q\rightarrow Aut(C_p)

 

[OMM!]

Hi micromass, thanks for your help once again!

So if we say, < y > = C_{q}, then we need to show there is a homomorphism \phi that sends y to \phi(y) which is an element of Aut C_{p} = C_{p-1}.

And since p \equiv 1 (mod q), then C_{p-1} isomorphic to C_{q}?

And clearly there exists a homomorphism from one group to itself (endomorphism?).

Am I on the right track?!

 

[micromass]

You're certainly on the right track, but there is a small mistake. You say that C_q is isomorphic to C_{p-1}, but this is not true.

All you know is that p-1=0 (mod q), but this does not imply that q=p-1. It merely implies that q divides p-1.

 

[OMM!]

So for y in C_{q}, we have y^{q} = 1

And if q divides p-1, then y^{p-1} = 1?

Does the order of the element \phi(y) divide p-1? Or does it divide q?

(Sorry, probably obvious, but having a mental moment!)

Then to find the possible element for \phi(y) for C_{p-1} = < \alpha >

\phi(y) in {1, \alpha, \alpha^{2}, ... , \alpha^{p-1}}

Show which powers of \alpha, when put to the power of q, give a power 1 (mod p)? (i.e. yxy^{-1} = x^{t}, we need t^{q} \equiv 1 (mod p))

Am I missing something out here or is it just a case now of showing that basically t = 1 is one of these values of t, thus getting the non-abelian cyclic group C_{p} x C_{q} of order pq?

 

[micromass]

Hmm, I think you're making it a bit too hard on yourself.

We know that q divides p-1, does that mean that we can always find an element of order q in C_{p-1}?

 

[OMM!]

I'd say yes (With very little conviction! Haha)

As I said in the last post, if we have an element y in C_{q} which is obviously of order q, then y is also in C_{p-1}?

 

[micromass]

I'd say yes (With very little conviction! Haha)

As I said in the last post, if we have an element y in C_{q} which is obviously of order q, then y is also in C_{p-1}?

Aah, yes. I seem to be a bit confused with the notation. You are identifying C_q with elements on the unit circle of \mathbb{C} yes??

 

[OMM!]

I'm not entirely sure!

I am basically saying that C_{q} is the cyclic group of order q, thus is generated by < y >.

In other words: C_{q} = < y | y^{q} = 1 >

The elements of which are: {1, y, y^{2}, ... , y^{q-1}}

 

[micromass]

I'm not entirely sure!

I am basically saying that C_{q} is the cyclic group of order q, thus is generated by < y >.

In other words: C_{q} = < y | y^{q} = 1 >

The elements of which are: {1, y, y^{2}, ... , y^{q-1}}

Yes, that's what I thought.

There are several methods to prove the existence of an element of order q in C_{p-1}, the easiest makes use of the fundamental theorem of abelian groups.

However, you seem to want to do another method (which is also fine): you want to say that if y is an element of C_q, then it is an element of C_{p-1}. Unfortunately, this does not make much sense: the elements in the cyclic group might look quite different from each other!
However, every cyclic group is isomorphic to a part of the unit circle by

\Phi:C^q\rightarrow S^1:y^k\rightarrow e^{2\pi i k/q}

this is an group monomorphism. So now you can indeed say that an element of C_q is also an element of C_{p-1}!

 

[OMM!]

And the order of that element divides q?

It can't be 1, else the group would be Abelian i.e. yxy^{-1} = x ===> yx = xy

So the order must be q, as q is prime?

 

[micromass]

Well, you can choose that element such that the order is q!

 

[OMM!]

Thanks for your help, I'll give it another shot in the morning. Found a bit on Frobenius Groups which seems quite related, which I think may be my bed-time reading!

Thanks again, you're a great help!