A085119 a(n) = number at which the standard Ackermann function mod n stabilizes, or -1 if it does not stabilize.

1, 1, 1, 1, 1, 1, 5, 2, 1, 1, 1, 9, 13, 13, 13, 13, 13, 2, 13, 13, 17, 13, 13, 5, 13, 13, 13, 22, 13, 5, 29, 1, 13, 13, 13, 29, 13, 13, 13, 7, 13, 1, 17, 13, 29, 1, 13, 1, 33, 49, 13, 20, 31, 11, 13, 11, 3, 19, 13, 19, 61, 13, 61, 13, 61, 27, 49, 10, 13, 40, 13, 34, 37

Offset: 2

Author

Jerrold Grossman, Apr 25 2004

Keywords

• nonn

Comments

a(1969) = -1, but otherwise a(n) is positive for all n >= 2 and < 8000000.

[Needs program(s). - N. J. A. Sloane, May 25 2025]

Links

• J. Froemke and J. W. Grossman, A mod-n Ackermann function, or what's so special about 1969?, Amer. Math. Monthly, 100 (1993), 180-183.
• Mark Rickert, The first 8 million terms a(n) [a gzipped file], March 2022.
• Stan Wagon, Problem of the Week POW #1340: Modular Ackermann, March 2022.
• Stan Wagon, Problem of the Week POW #1340: Solution, March 2022.

Crossrefs

Cf. A352196, A383460.

Extensions

Revised by N. J. A. Sloane, May 25 2025