Hey no fair taking my name in vain! Talk about living in an "unreal universe"! No one would ever really want to calculate A(5,5) -- so it is quite appropriate for VU. The result would be of no real value. The whole point of A(i,j) is that it is a counterexample, a "recursive function which is not primitive recursive". (And, no, I never took Computer Science, so I don't know what that means.)
Actually, it is NOT my name: >From <http://www.nist.gov/dads/HTML/ackermann.html>: Ackermann's function (algorithm) Definition: A function of two parameters whose value grows very fast. Formal Definition: A(0, j)=j+1 for j ? 0 A(i, 0)=A(i-1, 1) for i > 0 A(i, j)=A(i-1, A(i, j-1)) for i, j > 0 See also inverse Ackermann function. Note: In 1928, Wilhelm Ackermann observed that A(x,y,z), the z-fold iterated exponentiation of x with y, is an example of a recursive function which is not primitive recursive. A(x,y,z) was simplified to a function of 2 variables by Rózsa Péter in 1935. Raphael M. Robinson simplified the initial condition in 1948. Many people have given other versions of Ackermann's function, some of which are not simply a restating of this one. Author: PEB
