I'm looking for a method to solve:
$$f(f(x))=x$$
Where $f$ is defined for $x \in R$
So far by inverting both sides I have:
$f(x)=f^{-1}(x)$
Which means that my function should be symmetrical over $y=x$. I may "guess" the functions:
$y=x$
$y=c-x$
However I'm wondering is there a way to solve this without "guessing".