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 A011764 a(n) = 3^(2^n) (or: write in base 3, read in base 9). 23
 3, 9, 81, 6561, 43046721, 1853020188851841, 3433683820292512484657849089281, 11790184577738583171520872861412518665678211592275841109096961 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS 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 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..11 FORMULA 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 MATHEMATICA 3^(2^Range[0, 10]) (* Harvey P. Dale, Oct 14 2012 *) PROG (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 CROSSREFS Cf. A001146, A078885, A176594. Sequence in context: A216206 A038062 A218149 * A018624 A274032 A032078 Adjacent sequences:  A011761 A011762 A011763 * A011765 A011766 A011767 KEYWORD nonn,easy AUTHOR Stephan Y Solomon (ilans(AT)way.com) STATUS approved

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Last modified September 26 20:37 EDT 2022. Contains 357044 sequences. (Running on oeis4.)