Perhaps it's time to back-up a step. Why are you looking for a function that does square-roots without using the sqrt operation? Is this some sort of performance optimization?
I don't need square root function though at all, only the condition of mine you have exactly appointed to hold true about that function.
I too think there is only squreroot, or more exactly, only: f(x)=xm/n where m/n are whole numbers such that m/n=1/2 .
This is so far what I think, but I'm in no position to hold that as definitive, or even proven, I just think that by the intiuiton.
Any ohther furthering of this discussion, as to what f(x) possible formula is, is welcomed.