What is $\operatorname{syt}$? I came across the following definition of the set on this web page

But what is $\operatorname{syt}$?
 A: After a bit of poking around, I found the same result as @SiongthyeGoh, but not from a mathematical P.O.V:
From The Census of Mathematical Notations:

On the Helsinki University of Technology, Department of Mathematics website, we find that they use syt instead of $\gcd$, and it is called 'Suurin yhteinen tekijä' in a Finnish context.
Also find syt in Wikipedia. Usually at university level notation (a,b) is used for greatest common divisor of numbers a and b instead of letter combination syt. For example see Lecture notes by Pentti Haukkanen at University of Tampere.

For further reference, here is the link to the webpage quoted. As you might expect, it supposedly (I don't speak Finnish) translates somewhat literally to..."greatest common divisor"
A: I believe it is just a typo. syt is just gcd.
\begin{align*}
A_d &= \left\{ x: 1 \leq x \leq n\text{ and } gcd(x,n)=d \right\}\\
&= \left\{ dx': 1 \leq x' \leq n/d \text{ and } gcd(dx',n)=d \right\}\\
&= \left\{ dx': 1 \leq x' \leq n/d \text{ and } gcd(x',n/d)=1 \right\}.\\
\end{align*}
We can see that the set above have the same cardinality with the set 
$$A_d'=\left\{ x: 1 \leq x \leq n/d \text{ and } gcd(x,n/d)=1 \right\}.$$
