Proving that an Abelian group of order pq is isomorphic to Z_pq

[Mr Davis 97]

Homework Statement

Given that G is an abelian group of order pq, I need to show that G is isomorphic to $\mathbb{Z}_{pq}$

Homework Equations

The Attempt at a Solution

I am trying to do this by showing that G is always cyclic, and hence that isomorphism holds. If there is an element of order pq, then we immediately see that G is cyclic.

If there is an element x of order p, I want to show that the there is an element y not in the cyclic subgroup generated by x, such that the order of xy is pq, which would mean that G is cyclic, right. How could I go about doing this?

 

[Dick]

Homework Statement

Given that G is an abelian group of order pq, I need to show that G is isomorphic to $\mathbb{Z}_{pq}$

Homework Equations

The Attempt at a Solution

I am trying to do this by showing that G is always cyclic, and hence that isomorphism holds. If there is an element of order pq, then we immediately see that G is cyclic.

If there is an element x of order p, I want to show that the there is an element y not in the cyclic subgroup generated by x, such that the order of xy is pq, which would mean that G is cyclic, right. How could I go about doing this?

p and q aren't just any old numbers, right? You should state all of the premises explicitly. Can you say whether there are elements of order p and q?

 

[Mr Davis 97]

p and q aren't just any old numbers, right? You should state all of the premises explicitly. Can you say whether there are elements of order p and q?

Sorry. p and q are primes. By Lagrange, all of the element of G must be of order 1, p, q, or pq. THe identity is the only element of order 1. And if there is an element of order pq then G is autmatically cyclic. Now I want to show that if there are only elements of order p and q, that G is still cyclic

 

[Dick]

Sorry. p and q are primes. By Lagrange, all of the element of G must be of order 1, p, q, or pq. THe identity is the only element of order 1. And if there is an element of order pq then G is autmatically cyclic. Now I want to show that if there are only elements of order p and q, that G is still cyclic

Do you know the theorem that tells you that if a prime p divides the order of G, then G has an element of order p? This would be a great help for your other thread as well.

 

[Mr Davis 97]

Do you know the theorem that tells you that if a prime p divides the order of G, then G has an element of order p? This would be a great help for your other thread as well.

Yes, I think that that is Cauchy's theorem

 

[Dick]

Yes, I think that that is Cauchy's theorem

Yes, it is. That means you have an element $a$ of order p and another element $b$ of order q. What's the order of $ab$?