Groups whose elements have order 2

In summary: I'll be honest in return. What does (ab)(ba)=(-1) mean? Are you sure you understand multiplicative group notation? There's no such thing as (-1).
  • #1
halvizo1031
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0

Homework Statement



Suppose that G is a group in which every non-identity element has order two. show that G is commutative.

Homework Equations





The Attempt at a Solution


DOES THIS ANSWER THE QUESTION?:

Notice first that x2 = 1 is equivalent to x = x−1. Since every element of G
has an inverse, we can distribute the elements of G into subsets {x, x−1}. Since
the inverse of x−1 is x, these sets are all disjoint. All of them have either two
elements (if x = x−1) or one element (if x = x−1). Let k be the number of
two-element sets, and let j be the number of one-element sets. Then the total
number of elements of G is 2k + j.
Now notice that j > 0, since there is at least one element such that x = x−1,
namely x = 1. So we know that 2k + j is even and that j > 0. It follows that
j ≥ 2, so there must exist at least one more element x such that x = x−1. That’s
the element we were looking for.
 
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  • #2
No, that doesn't show anything. You assumption is that every nonidentity element has order two. That mean EVERY element is its own inverse. Try this. What you want to show is that ab=ba for every two nonidentity elements of G. If ab is not the identity then (ab)(ab)=1, that's your assumption. What's (ab)(ba)?
 
  • #3
Dick said:
No, that doesn't show anything. You assumption is that every nonidentity element has order two. That mean EVERY element is its own inverse. Try this. What you want to show is that ab=ba for every two nonidentity elements of G. If ab is not the identity then (ab)(ab)=1, that's your assumption. What's (ab)(ba)?


to be honest, I am lost now. I'm not sure I understand your response. Does (ab)(ba)=-1?
 
  • #4
halvizo1031 said:
to be honest, I am lost now. I'm not sure I understand your response. Does (ab)(ba)=-1?

I'll be honest in return. What does (ab)(ba)=(-1) mean? Are you sure you understand multiplicative group notation? There's no such thing as (-1).
 

FAQ: Groups whose elements have order 2

What is the definition of a group whose elements have order 2?

A group whose elements have order 2 is a mathematical structure consisting of a set of elements and a binary operation that satisfies the group axioms, where every element in the group has an order of 2.

What are some examples of groups whose elements have order 2?

One example is the group of symmetries of a rectangle, where the elements are rotations of 180 degrees and reflections across the diagonals. Another example is the group of complex numbers with absolute value 1, where the elements are 1 and -1.

What is the identity element in a group whose elements have order 2?

The identity element in a group whose elements have order 2 is the element that, when combined with any other element in the group, results in that same element. In other words, it is the element that has no effect on the operation.

Can a group whose elements have order 2 have more than one identity element?

No, a group whose elements have order 2 can only have one identity element. This is because the identity element must be unique and cannot be the product of any other elements in the group.

Are groups whose elements have order 2 commutative?

Not necessarily. While some groups whose elements have order 2 are commutative, meaning the order of the elements does not change when the operation is switched, this is not a defining characteristic of such groups. It depends on the specific group and its operation.

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