A number of the form n↑...↑_︸_n n, where Knuth up-arrow notation has been used. The first few Ackermann numbers are 1↑1 = 1, 2↑↑2 = 4, and 3↑↑↑3 = 3^(3^(·^(·^(·^3)))) _︸_7625597484987.

Ackermann function | Knuth up-arrow notation | power tower