User:Dfeuer/Archimedean Totally Ordered Group is Abelian
Theorem
Let $G$ be an Archimedean totally ordered group.
Then $G$ is abelian.
Lemma 1
Let $G$ be an Archimedean totally ordered group with a smallest strictly positive element, $\epsilon$.
Then $G$ is the infinite cyclic group generated by $\epsilon$, with the natural ordering.
Lemma 2
Let $G$ be a non-trivial totally ordered group with no smallest strictly positive element.
Let $g$ be a strictly positive element of $G$.
Let $n \in \N_{>0}$.
Then there is a strictly positive $h \in G$ such that $h^n \le g$
Proof
If $G$ has a smallest strictly positive element then by User:Dfeuer/Archimedean Totally Ordered Group is Abelian/Lemma 1, $G$ is a cyclic group and thus abelian.
Suppose that $G$ has no smallest strictly positive element.
Let $e$ be the identity of $G$.
Suppose for the sake of contradiction that $a,b \in G$ and $ab \ne ba$.
Then clearly, neither $a$ nor $b$ is the identity, and $a \ne b$.
Then multiplying on the left and on the right by $a^{-1}$, $ba^{-1} \ne a^{-1}b$.
We can do similarly for $b$. Thus we can assume WLOG that $a > e$ and $b > e$.
By swapping $a$ with $b$ if necessary, we can ensure that $ab < ba$.
Let $c = b^{-1}a^{-1}ba$, so $abc = ba$.
By User:Dfeuer/Archimedean Totally Ordered Group is Abelian/Lemma 2, there is a $d \in G$ such that $e < d^2 < c$.
Since $G$ is Archimedean, there are $n,m\in\N_{>0}$ such that
\((1)\) | $:$ | \(\ds d^n < a \le d^{n+1} \) | |||||||
\((2)\) | $:$ | \(\ds d^m \le b < d^{m+1} \) |
Then multiplying the first inequality by the second yields
- $(3)\quad d^{n+m} < ab$
while multiplying the second by the first yields
- $(4)\quad abc = ba < d^{m+n+2} < d^{m+n}c$
Multiplying $(4)$ on the right by $c^{-1}$ yields $ab < d^{m+n}$.
But we have already shown that $d^{m+n} < ab$, so we have reached a contradiction.
Thus we conclude that $ab = ba$.
As this holds for all $a, b \in G$, $G$ is abelian.
$\blacksquare$