Group Theory: Element of Order 2 in Groups of Even Order

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Homework Help Overview

The discussion centers around a group theory problem involving groups of even order and the existence of an element of order 2. The original poster attempts to demonstrate that in a group G of even order, there exists an element g, distinct from the identity, such that g squared equals the identity.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants explore the implications of the group's even order and the nature of inverses within the group. Questions arise regarding the validity of the original poster's reasoning and the consequences of pairing elements with their inverses.

Discussion Status

The discussion is ongoing, with participants examining the original argument and questioning its assumptions. Some guidance is provided regarding the properties of inverses in groups, but no consensus has been reached on the correctness of the original claim.

Contextual Notes

Participants note that the removal of the identity from a group of even order results in an odd number of elements, which raises questions about the pairing of elements with their inverses. The implications of this observation are under consideration.

PsychonautQQ
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Homework Statement


If G is a group of even order, show that it has an element g not equal to the identity such that g^2 = 1.

Homework Equations


None

The Attempt at a Solution


What I wrote:

If |G| = n, then g^n = 1 for some g in G. Thus, (g^(n/2))(g^(n/2)) = 1, so g^(n/2) is the element of order 2.

Is this a flawed argument? there guaranteed to be an element such that g^n = 1?
 
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The identity is, of course, its own inverse. Since the group is "of even order", i.e. it has an even number of elements, removing the identity leaves an odd number of elements. Pairing each number with its inverse, what happens?
 
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HallsofIvy said:
The identity is, of course, its own inverse. Since the group is "of even order", i.e. it has an even number of elements, removing the identity leaves an odd number of elements. Pairing each number with its inverse, what happens?
I see your argument. Is this true though? Can't ab = 1, bc = 1, cd = 1 etc etc but ba not equal 1?
 
PsychonautQQ said:
I see your argument. Is this true though? Can't ab = 1, bc = 1, cd = 1 etc etc but ba not equal 1?

No, it's not possible. If ab=1 then ba=1. Prove it!
 
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