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A002845
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Number of distinct values taken by 2^2^...^2 (with n 2's and parentheses inserted in all possible ways).
(Formerly M1139 N0435)
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24
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1, 1, 1, 2, 4, 8, 17, 36, 78, 171, 379, 851, 1928, 4396, 10087, 23273, 53948, 125608, 293543, 688366, 1619087, 3818818, 9029719, 21400706, 50828664, 120963298, 288405081, 688821573, 1647853491, 3948189131, 9473431479
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OFFSET
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1,4
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COMMENTS
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REFERENCES
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J. Q. Longyear, personal communication.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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R. K. Guy, Letter to N. J. A. Sloane, June 24 1971: front, back [Annotated scanned copy, with permission]
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EXAMPLE
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When n=3, 2^(2^2) = (2^2)^2 = 16, so a(3) = 1.
When n=4, ((2^2)^2)^2 = (2^(2^2))^2 = 16^2 = 2^8 = 4^4 = (2^2)^(2^2), 2^(2^(2^2)) = 2^((2^2)^2) = 2^16, so a(4) = 2.
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CROSSREFS
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KEYWORD
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nonn,nice,more,changed
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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