# Math Help - usng a conjugate

1. ## usng a conjugate

If G is a group and $a,b \in G$, prove that ab and ba have the same order. I think i need to use the conjugate to prove this, because conjugate elements have the same order.

2. Originally Posted by ux0
If G is a group and $a,b \in G$, prove that ab and ba have the same order. I think i need to use the conjugate to prove this, because conjugate elements have the same order.

$ba=a^{-1}(ab)a .$

Tonio

3. its just that simple?

$ba=ba$

$ba = a^{-1}a(ba)$ ---------Multiplication of the identity

$ba=a^{-1}(ab)a$ ---------- Associativity

$=$ --------------- Conjugate elements have the same order

4. Originally Posted by ux0
its just that simple?

$ba=ba$

$ba = a^{-1}a(ba)$ ---------Multiplication of the identity

$ba=a^{-1}(ab)a$ ---------- Associativity

$=$ --------------- Conjugate elements have the same order

As painfully and embarrasingly simple as that. Why, did you expect something very difficult?
And what did you mean in your last line $=$? This is not true, not even close, in general. What you sure meant is that either $|ab|=|ba|\,\,\, or \,\,\,ord (ab)=ord (ba)$

Tonio

5. Ya I kind of did, and thanks for the notation correction!