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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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25
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OFFSET
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0,1
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COMMENTS
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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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FORMULA
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Product_{n>=0} (1 + 1/a(n)) = 3/2. - Amiram Eldar, Jan 29 2021
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MATHEMATICA
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PROG
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CROSSREFS
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Subsequence of A000244 (powers of 3).
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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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