# Trouble with Group Theory Proofs: Help Needed

• margaret23
In summary: But even if you do that, you won't be able to say that S_3 is cyclic because it could have a subgroup of order 2 or 3. So you can't really prove or disprove that it is cyclic.
margaret23
I m having trouble with a couple group theory proofs. I just have no clue how to start. If u could put me on the right path that would be great.

first

prove of disprove that if every subgroup of a group G is cyclic, then G is cyclic.

and second

prove or disprove that every group X of order 6 is communtative.

For the first, I'm assuming G doesn't count as a subgroup of itself. What are your thoughts on whether these are true or not?

the question doesn't state that its not including the whole group as a subgroup, however i think that its safe to assume that they want it proved without.

I' m thinking from what i know about groups that the first one is true.. and that the second one is false.

I think that the second one probably has a counter example somewhat like the quaternions, Q8.

but i don't really know what else to look at.

For the first, remember that every group of prime order is cyclic. So if you had a group of order pq, where p and q are prime, all its proper subgroups must have order p, q, or 1, and so must be cyclic. Are all such groups cyclic?

For the second, can you list all the groups of order 6? There are actually very few of them.

There are actually 2 of them, and since it is trivial to write down two non-isomorphic groups of order 6... (and any proper subgroup of them is cyclic, by the way, by Lagrange's theorem)

Last edited:
Thanks for the help on the second one .. it makes sense now.. for the first question about proving that if all subgroups are cyclic then the group is cyclic. I would think that its true however i do not know how to start the proof. Could you put me on the right track again?? thanks

margaret23 said:
first

prove of disprove that if every subgroup of a group G is cyclic, then G is cyclic.

and second

prove or disprove that every group X of order 6 is communtative.

i think you can actually use the second part to prove the first

If you have the answer to the second question, then you have a counterexample to the first question.

ha i beat you to it :tongue:

my counter example to the second one is the diherdral group containing the permentation of 3 elements. i m having trouble rapping my mind around how this is cyclic.. i ve delt a lot with simpler groups like the intergers mod n , i don't quite understand what makes my counter example cyclic if infact it is.. ?

ps.. thanks

You are trying to prove or disprove that if every subgroup of a group G is cyclic, then G is cyclic.

Take the group that you found. What do you know about its subgroups?

BTW - This group is not cyclic, as you have said.

Thanks soo much

By definition a group of order n is cyclic if and only if it has an element of order n. The group you have (S_3, or D_3, it's called) has only 6 elements. You can write them all down and work out their orders.

## What is group theory?

Group theory is a branch of mathematics that deals with the study of symmetry and the properties of objects that remain unchanged under certain transformations. It is used in various fields such as physics, chemistry, and computer science to describe and understand the relationships between different objects or systems.

## What are the common challenges in understanding group theory proofs?

Some common challenges in understanding group theory proofs include abstract concepts, complex mathematical notation, and the need for a strong foundation in algebra and set theory. Additionally, group theory proofs often require a deep understanding of the definitions and properties of groups, which can be difficult to grasp for beginners.

## How can I improve my understanding of group theory proofs?

One way to improve your understanding of group theory proofs is to practice solving problems and working through proofs on your own. It is also helpful to seek out resources such as textbooks, online courses, or other educational materials that provide step-by-step explanations and examples.

## What are some common mistakes to avoid when working on group theory proofs?

Some common mistakes to avoid when working on group theory proofs include not fully understanding the definitions and properties of groups, misinterpreting notation, and making incorrect assumptions or skipping steps in the proof. It is important to carefully read and follow each step of the proof and to take your time to fully understand each concept.

## Where can I find help with understanding group theory proofs?

There are many resources available for help with understanding group theory proofs, including online forums and communities, tutoring services, and study groups. It can also be helpful to seek guidance from a professor or mentor who has a strong understanding of group theory and can provide personalized assistance.

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