is there a name for these sets?

I have a question related to number theory. For positive integers n
and k, define S(n, k) by S(n, k)={x: gcd(n,x)=k}. For example, S(12,
3) is {3, 9}.

The quesion is, is there a name for these S(n, k) sets? and where could
I find more info about them? I didn't see anything in my number theory
book.

Thanks,
jq

.