Tetration is binary operation: y=f(a,b), that is defined as iterated exponent
Tetration may be used for calculation of functional root of exponent.
etetr.m - e tetrated to x. Smooth to 5th order, defined for x in (-1,inf).
etetri.m - inverse function to etetr. Quite unoptimized and slow.
expfroot.m - fucntional root of any order exponent. by default order is 2.