Ackermann's function


Definition: A function of two parameters whose value grows very fast.

Formal Definition:

See also inverse Ackermann function.

Note: Many people have defined other similar functions which are not simply a restating of this one.

In 1928, Wilhelm Ackermann observed that A(x,y,z), the z-fold iterated exponentiation of x with y, is a recursive function that 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.

Author: PEB


History of the function and (Modula-2) code.

More information

Robert Munafo's Versions of Ackermann's Function and analysis. Cowles and Bailey Several Versions of Ackermann's function.

Wilhelm Ackermann, Zum Hilbertschen Aufbau der reellen Zahlen, Mathematische Annalen 99(1):118-133, December 1928.

The formal definition given here is Gnx in the first page of
Raphael M. Robinson, Recursion and Double Recursion, Bulletin of the American Mathematical Society 54:987-993, October 1948.

Go to the Dictionary of Algorithms and Data Structures home page.

If you have suggestions, corrections, or comments, please get in touch with Paul Black.

Entry modified 9 November 2020.
HTML page formatted Mon Dec 21 09:49:05 2020.

Cite this as:
Paul E. Black, "Ackermann's function", in Dictionary of Algorithms and Data Structures [online], Paul E. Black, ed. 9 November 2020. (accessed TODAY) Available from: