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A011764
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a(n) = 3^(2^n) (or: write in base 3, read in base 9).
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22
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OFFSET
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0,1
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COMMENTS
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a(n) is the second-highest value k such that A173419(k) = n+2. - Charles R Greathouse IV, Oct 03 2012
Let b(0) = 6; b(n+1) = smallest number such that b(n+1) + Product_{i=0..n} b(i) divides b(n+1)*Product_{i=0..n} b(i). Then b(n+1) = a(n) for n >= 0. - Derek Orr, Dec 13 2014
Changing "+" to "-": Let b(0) = 6; b(n+1) = smallest number such that b(n+1) - Product_{i=0..n} b(i) divides b(n+1)*Product_{i=0..n} b(i). Then b(n+2) = a(n) for n >= 0. - Derek Orr, Jan 04 2015
With offset = 1, a(n) is the number of collections C of subsets of {1,2,...,n} such that if S is in C then the complement of S is not in C. - Geoffrey Critzer, Feb 06 2017
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LINKS
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Vincenzo Librandi, Table of n, a(n) for n = 0..11
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FORMULA
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a(0)=3 and a(n+1) = a(n)^2. - Benoit Jubin, Jun 27 2009
Sum_{n>=0} 1/a(n) = A078885. - Amiram Eldar, Nov 09 2020
Product_{n>=0} (1 + 1/a(n)) = 3/2. - Amiram Eldar, Jan 29 2021
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MATHEMATICA
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3^(2^Range[0, 10]) (* Harvey P. Dale, Oct 14 2012 *)
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PROG
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(MAGMA) [3^(2^n): n in [0..8]]; // Vincenzo Librandi, Sep 15 2011
(PARI) a(n)=3^2^n \\ Charles R Greathouse IV, Oct 03 2012
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CROSSREFS
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Cf. A001146, A078885, A176594.
Sequence in context: A216206 A038062 A218149 * A018624 A274032 A032078
Adjacent sequences: A011761 A011762 A011763 * A011765 A011766 A011767
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KEYWORD
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nonn,easy
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AUTHOR
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Stephan Y Solomon (ilans(AT)way.com)
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STATUS
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approved
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