I usually denote a set whose elements are distinct by $\{a\_p\}\_{p \in P}$. And I have a function $f$ which takes a set as argument, so we could write $f(\{a\_p\}\_{p \in P})$. My question is how to write it when $f$ takes a set which contains only 1 element (a singleton). If we write it $f(\{a\_p\})$, it is still not obvious that the set is a singleton; if we write $f((a\_p))$ or $f(a\_p)$, it contradicts to the fact that f accepts only set as argument.

Does anyone have any idea?