
Orders of Groups
Hi i have completed the answer to this question. Just need your verification on whether it's completely correct or not:
Question:
If G is a group and xEG we define the order ord(x) by:
ord(x) = min{$\displaystyle r \geq 1: x^r = 1$}
If $\displaystyle \theta$: G > H is an injective group homomorphism show that, for each xEG, ord($\displaystyle \theta(x)$) = ord(x)
My answer: Please verify
If $\displaystyle \theta(x)$ = {$\displaystyle x^r: r \epsilon Z$} then ord($\displaystyle \theta(x)$) = ord(x).
For any integer r, we have x^r = e (or 1) if and only if ord(x) divides r.
In general the order of any subgroup of G divides the order of G. If H is a subgroup of G then "ord (G) / ord(H) = [G:H]" where [G:H] is an index of H in G, an integer.
So order for any xEG divides order of the group. So ord($\displaystyle \theta(x)$) = ord(x)
any suggestions or changes please? thnx :)

If ord(x)=a then $\displaystyle \left[ {\phi (x)} \right]^a = \left[ {\phi (x^a )} \right] = \phi (e) = e'$.
Now suppose that $\displaystyle ord\left[ {\phi (x)} \right] = b < a$.
Then
$\displaystyle \left[ {\phi (x)} \right]^b = e' = \left[ {\phi (x)} \right]^a $
$\displaystyle \phi (x^b ) = \phi (x^a )$
$\displaystyle x^b = x^a$ (injective)
$\displaystyle x^{a  b} = e$
Do you see the contradiction?

yes i think i see the contradiction.
i believe to do this question we have to show 2 things:
i) $\displaystyle (\theta(x))^a = e'$
ii) $\displaystyle 0 < b < a \implies (\theta(x))^b \neq e'$.
therefore the contradiction seems to arise from the fact that if $\displaystyle x^{a} = x^{b}$, then $\displaystyle \left[ {\phi (x)} \right]^b = e'$ which is not what statement (ii) says.
am i correct in this assumption? any ideas on how to deal with this?