Proving Isomorphism of Groups with Elements of Same Order

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Discussion Overview

The discussion revolves around proving a statement related to group theory, specifically concerning isomorphic groups and the existence of elements of the same order in both groups. The participants explore the implications of group isomorphism on the orders of elements.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks how to prove that if group G has an element of order n, then group H, which is isomorphic to G, must also have an element of order n.
  • Another participant suggests that to show this, one can use the properties of isomorphisms, specifically that they preserve the order of elements.
  • A participant clarifies that the proof involves showing that if an element x in G has order n, then its image under the isomorphism f, denoted f(x), must also have order n in H.
  • There is a discussion about the definition of the order of an element and the nature of group isomorphisms, emphasizing that f(x^r) = f(x)^r and its implications for the orders of elements.
  • One participant expresses confusion regarding the proof and seeks further clarification on the concepts involved in group theory.
  • Another participant reassures that understanding the proof takes time and emphasizes the importance of grappling with the definitions and properties involved.

Areas of Agreement / Disagreement

Participants generally agree on the approach to proving the statement regarding isomorphic groups, but there is some disagreement about the clarity and understanding of the proof itself, particularly from those less familiar with group theory concepts.

Contextual Notes

Some participants express uncertainty about the definitions and concepts of group theory, indicating that there may be gaps in understanding the foundational aspects necessary for the proof.

SquareCircle
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How would I go about proving the following:

If G has an element of order n, then H has an element of order n.

I am not sure how to start, if I should some how go about proving one to one and onto.

Help
 
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Who knows, since you've not explained what G and H are.

But, guessing you mean let G and H be isomorphic groups, show that G has an element of order n iff H does.

Suppose f is an iso from G to H. Let x be in G, then, f(x^r)=f(x)^r, hence ord(f(x))<=ord(x). by symmetry ord(x)=ord(f(x)).
 
Last edited:
Isomorphism

Sorry, I left that part out.

The whole problem states

Assume that G and H are groups and that G and H are isomorphic
Then prove the statement
If G has an element of order n, then H has an element of order n.
 
Last edited:
Which is what I showed, albeit in a very quick fashion. Do you understand the proof?
 
Isomorphism

No, I do not understand the proof. I am taking group theory and I do not understand the concepts. Do you know what I can do to help me understand some of the concepts?
 
The concept is simply a definition.

the order of an element is the smallest positive r such that x composed with itself r times is the identity

a group isomorphism is a structure preserving map f(xy)=f(x)f(y)

so it follows f(x^r)=f(x)^r

if x^r=e, the identity, then f(x)^r = e, so if r is minimal and positive such that x^r = e then f(x) has order at most r. So by symmetry, with g the inverse iso to f, it follows they must be equal.

you need to think about it. it shouldn't be instantly obvious, it'll take time to understand, but it's supposed to
 
Isomorphism

Thank you, your explanation of the proof helped.
 

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