Let $G=BS(m,n)$ denote the [Baumslag–Solitar groups][1] defined by 
the presentation $\langle a,b: b^m a=a b^n\rangle$.

We assume that G is non-abelian.

>Question: Find $m,n$ such that $G$ is an [ordered group][2], i.e. $G$ is a group  on which a partial order relation $\le $ is given such that for any elements $x,y,z \in G$, from $x \le y$ it follows that $xz \le  yz$ and $zx \le  zy$.


  [1]: http://www.encyclopediaofmath.org/index.php/Baumslag%E2%80%93Solitar_group
  [2]: http://www.encyclopediaofmath.org/index.php/Partially_ordered_group