TITLE: Proof of the intersection of 2 coprime order cyclic subgroups of G is the set e
QUESTION [5 upvotes]: Let $G$ be an abelian group. Suppose that $a$ is in $G$ and has order $m$ (such that $m$ is finite) and that the positive integer $k$ divides and $b$ has order $n$ with $\gcd(m,n)$=1. Prove that $\langle a\rangle \cap \langle b\rangle = \{e\}$.

since m and n are relatively prime this should be easy we know  $a^{m}  =e=b^{n}$
we know $\langle a\rangle$ is a cyclic subgroup of g and that $\langle b\rangle$ is a cyclic subgroup of g so $\langle a\rangle \cup \langle b\rangle $ is a subset of G. since G is abelian this must a cyclic group or its not a subgroup of G so $\langle b\rangle$ must be a subset of $\langle a\rangle$ or $\langle a\rangle$ must be a subset of $\langle b\rangle$ since the element $a^{m-1}$ cannot equal $b^{n-1}$ and both are generations for a and b respectively we have reached a contradiction.

REPLY [10 votes]: The order of $\langle a\rangle$ is $n$, the order of  $\langle b\rangle$ is $m$. Now if $x$ belongs to the former, its order divides $n$. And if it belongs to the latter, its order divides $m$. So if it belongs to both, its order divides $\gcd(n,m)=1$. So $x=e$.

REPLY [5 votes]: Note that if $x\in H$, then the order of $x$ divides the order of $H$.
if $x\in \langle a\rangle\cap\langle b\rangle$, then, the order of $x$ divides both order of $a$ and order of $b$, which must be $1$.
Therefore $x$ can only be the identity $e$.