# Math Help - A question about Group

1. ## A question about Group

Hi I am new to Group Theory and I am confused by a problem :

Let $G$ be any group of order $n$ and $k$ be an integer prime to $n$ .

Is it true that $x^k = y^k ~ \implies x = y ~~~ x,y \in G$ ?

I know there exists $A,B ~\in \mathbb{N}$ such that $Ak - Bn = 1$

Then $(x^k)^A = (y^k)^A$

$x^{Ak} = y^{Ak}$

$x^{1 + Bn} = y^{1 + Bn}$

$x (x^n)^B = y (y^n)^B$

I think if $a^n = e$ , then i can prove it but is it really true ? Or my proof has some mistakes ? Actually , it is what i guess and i don't know if it is correct .

Thanks !

2. Originally Posted by simplependulum
Hi I am new to Group Theory and I am confused by a problem :

Let $G$ be any group of order $n$ and $k$ be an integer prime to $n$ .

Is it true that $x^k = y^k ~ \implies x = y ~~~ x,y \in G$ ?

I know there exists $A,B ~\in \mathbb{N}$ such that $Ak - Bn = 1$

Then $(x^k)^A = (y^k)^A$

$x^{Ak} = y^{Ak}$

$x^{1 + Bn} = y^{1 + Bn}$

$x (x^n)^B = y (y^n)^B$

I think if $a^n = e$ , then i can prove it but is it really true ? Or my proof has some mistakes ? Actually , it is what i guess and i don't know if it is correct .

Thanks !
Looks fine to me!

3. Originally Posted by chiph588@
Looks fine to me!

But how to prove it true ? $a^n =e$

$e$ is the neutral element of $G$

4. Originally Posted by simplependulum
But how to prove it true ? $a^n =e$

$e$ is the neutral element of $G$
See here.

5. Originally Posted by chiph588@
See here.

Oh i see . If the group is finite , then we would find that

$\{\ e,a,a^2,... \}\$ contains some repeated elements and write $a^s = a^t ~ \implies a^{s-t} = e ~~ s>t$

$\{\ e , a , a^2 ,..., a^{s-t-1} \}\$ is a subgroup of $G$ then use Lagrange's Theorem to show that $a^n = e$ .

Thank you very much

6. Originally Posted by simplependulum
Hi I am new to Group Theory and I am confused by a problem :

Let $G$ be any group of order $n$ and $k$ be an integer prime to $n$ .

Is it true that $x^k = y^k ~ \implies x = y ~~~ x,y \in G$ ?

I know there exists $A,B ~\in \mathbb{N}$ such that $Ak - Bn = 1$

Then $(x^k)^A = (y^k)^A$

$x^{Ak} = y^{Ak}$

$x^{1 + Bn} = y^{1 + Bn}$

$x (x^n)^B = y (y^n)^B$

I think if $a^n = e$ , then i can prove it but is it really true ? Or my proof has some mistakes ? Actually , it is what i guess and i don't know if it is correct .

Thanks !
The property $1 \neq x^n = y^n \Rightarrow x=y$ is called the `unique roots property', and it does not always hold. It does hold in, for example, free groups (infinite groups that can be viewed as the sort of top-down building blocks of group theory).

Such groups have the property that $g^mh^n = h^ng^m \Rightarrow gh=hg$.

However, this property does not always hold. A finite example would be the elements $\alpha = (123)(45)$ and $\beta = (123)$. Then $\alpha^2 = \beta^2$ but $\alpha \neq \beta$.

What I believe you proved in your post is that if $|G| = n$ and $gcd(n, k) = 1$ then if $a^k=b^k$ we have that $a=b$.

(An infinite example of a group without the unique roots property is the group given by the presentation (rules) $\langle a, b; a^2=b^2\rangle$. This is non-trivial and infinite as it has a homomorphism onto the infinite dihedral group, $C_2 \ast C_2$)