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A004231
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Ackermann numbers: n^n^n^...^n (with n n's).
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2
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OFFSET
| 1,2
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COMMENTS
| Using Knuth's arrow notation, this is n^^^2 (n-penta-2) or n^^n (n-tetra-n). - Andrew Robbins, Apr 16 2009
Comment from Trevor Green (green(AT)dlcwest.com): The fourth term in this sequence has about as many digits - 8.07 * 10^153 - as the *square* of the number of protons in the universe.
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REFERENCES
| W. Ackermann, Zum Hilbertschen Aufbau der reellen Zahlen, Math. Ann. 99 (1928), 118-133.
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LINKS
| Eric Weisstein's World of Mathematics, Ackermann Number
Wikipedia, Knuth's up-arrow notation
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CROSSREFS
| Cf. A027747, A008868 (which mentions an older estimate for the same number).
Sequence in context: A161405 A147876 A164796 * A066546 A132653 A115544
Adjacent sequences: A004228 A004229 A004230 * A004232 A004233 A004234
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KEYWORD
| nonn,bref
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AUTHOR
| wild(AT)edumath.u-strasbg.fr (Daniel Wild)
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EXTENSIONS
| Name improved by Arkadiusz Wesolowski, Dec 09 2011
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