A self inverse function is a function $f$, such that $y=f(x)$, with the special property that $ff(x)=x$, or written another way, $f(x) = f^-1(x)$
Example:
Imagine there's a function $f$, such that $y = 1/x$
$f^-1(x) = 1/x$, which means that $f(x) = f^-1(x)$, therefore this specific function is said to be a self-inverse function.
Another example:
Let the function $f$ be such that $y = 3-x$
$ff(x) = 3-x$, { so, $ff(x) = x$ } therefore this specific function is a self-inverse function.
What is/are the domain(s) of these types of functions?