# Abstract algebra question concerning center of a group

1. Nov 30, 2011

### xcr

1. The problem statement, all variables and given/known data

If a is the only element of order 2 in a group G, prove that a is an element of Z(G).
[Z(G) is the notation used by the book for center of group G]

2. Relevant equations

Z(G)={a is an element of G: ag=ga for every g that is an element of G}

3. The attempt at a solution

I know that if a has order 2 (|a|=2) then a ≠ the identity of the group, say e, and a=a^-1.
I just don't see where I would go from here in showing the center of a group.

2. Nov 30, 2011

### micromass

Staff Emeritus
What is the order of $gag^{-1}$??

3. Nov 30, 2011

### xcr

I would say two but I don't really have any reasoning for saying that...

4. Nov 30, 2011

### xcr

Actually, after looking at it, I would say that the order of gag^-1 is 2 because if the order of a is 2, then (a^2)=e. So (gag^-1)^2=(g^2)(a^2)(g^-2)=(g^2)(e)(g^-2)=(g^2)(g^-2)=e

5. Dec 1, 2011

### micromass

Staff Emeritus
Indeed. Now use that there is only one element of order 2...

6. Dec 1, 2011

### xcr

Still don't see where you are going with it

7. Dec 1, 2011

### micromass

Staff Emeritus
There is only one element of order 2. What can you conclude??

8. Dec 1, 2011

### xcr

That the element is not the identity and it is also its inverse.

9. Dec 1, 2011

### micromass

Staff Emeritus
You have found that both a and $g^{-1}ag$ are elements of order 2.

But the question states that there is ONLY ONE element of order 2. So what can you conclude??

10. Dec 1, 2011

### xcr

Then a=gag^-1. So multiplying on the right by g would give me ag=ga, ta-da