Let $R$ be a ring with unity. An element $x \in R$ is called $\textit{nilpotent}$ if $x^m = 0$ for some $m \in \mathbb{N}$. a.) Show that if $n=a^kb$ for some integers $a,b$, and $k$ where $k \neq 0$, then $\overline{ab}$ is a nilpotent element of $\mathbb{Z} / n \mathbb{Z}$.
The above question comes from Dummit & Foote's Abstract Algebra (pg. 231). I do not have a question about how to solve this problem. Rather, I am wondering what the meaning of $\overline{ab}$ is as I have not seen this notation before. For context, a proof I found for this problem gives:
$$(ab)^k=(a^kb)b^{k-1}=nb^{k-1} \equiv 0 \mod n \hspace{1cm} \blacksquare$$
But it is not at all clear to me what exactly $\overline{ab}$ means. Thanks for your time. Note: It's possible that this could be a notation specific to Dummit & Foote if anyone reading this is familiar with their textbook
Not sure what edition you have, but I'm guessing in every edition it has something like this screenshot (which I grabbed from the 3rd edition.)
To find this I just consulted the index of the book...