OFFSET
1,2
COMMENTS
An Ackermann-like function. The underlying puzzle was invented by Hans Zantema. The derivation and proof of the general formula involving a palindromic sequence of up-arrows is by Richard Stong.
The next term is too large to include (2^16385, it has 4933 digits).
LINKS
Zuming Feng, Po-Shen Loh, and Yi Sun, 51st International Mathematical Olympiad, Math. Mag. 83 (2010), pp. 320-323.
Terence Tao, Minipolymath2 project: IMO 2010 Q5 (2010)
A. van den Brandhof, J. Guichelaar, and A. Jaspers, Half a Century of Pythagoras Magazine, MAA, 2015, 225
Stan Wagon, The Generous Automated Teller Machine
Stan Wagon, Richard Stong's proof of the uparrow formula
Wikipedia, Knuth's up-arrow notation
FORMULA
Let f_n(x) = 2↑↑...↑x, with n Knuth up-arrows, so f_0(x) = 2x, f_1(x) = 2^x, f_2(x) = 2↑↑x = 2^2^...^2 with x copies of 2, etc.
Let F_n be the composition of f_0, f_1,...,f_(n-4).
Let G_n be the same composition but in the opposite order.
Then a(n) = G_n(F_n(7)), a formula due to Richard Stong.
EXAMPLE
a(5) = f_0(f_1(f_1(f_0(7)))) = 2*2^(2^(2*7)) = 2*2^(2^14) = 2^16385.
CROSSREFS
KEYWORD
nonn,nice
AUTHOR
Stan Wagon, Jan 27 2017
STATUS
approved