A function satisfying $f \left ( \frac 1 {f(x)} \right ) = x$
(I would put this in a comment but don't have enough reputation.)
The complex function $\frac{z+i}{iz+1}$ seems to satisfy your criterion. There might be a way of making this real?
(I would put this in a comment but don't have enough reputation.)
The complex function $\frac{z+i}{iz+1}$ seems to satisfy your criterion. There might be a way of making this real?