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A108411
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3^floor(n/2). Powers of 3 repeated.
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27
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1, 1, 3, 3, 9, 9, 27, 27, 81, 81, 243, 243, 729, 729, 2187, 2187, 6561, 6561, 19683, 19683, 59049, 59049, 177147, 177147, 531441, 531441, 1594323, 1594323, 4782969, 4782969, 14348907, 14348907, 43046721, 43046721, 129140163
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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COMMENTS
| a(n) is the Parker sequence for the automorphism group of the limit of the class of oriented graphs; a(n) counts the finite circulant structures in that class. - Nour-Eddine Fahssi (fahssin(AT)yahoo.fr), Feb 18 2008
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LINKS
| Vincenzo Librandi, Table of n, a(n) for n = 0..3000
D. A. Gewurz and F. Merola, Sequences realized as Parker vectors of oligomorphic permutation groups, J. Integer Seq., 6 (2003), 03.1.6.
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FORMULA
| O.g.f.: (1+x)/(1-3*x^2). [R. J. Mathar, Apr 01 2008]
a(n)=3^(n/2)*((1+(-1)^n)/2+(1-(-1)^n)/(2*sqrt(3))). [From Paul Barry, Nov 12 2009]
a(n+3) = a(n+2)*a(n+1)/a(n). [Reinhard Zumkeller, Mar 04 2011]
a(n) = (-1)^n*Sum_{k, 0<=0<=n} A158020(n,k)*2^k. - DELEHAM Philippe, Dec 01 2011
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PROG
| (PARI) a(n)=3^floor(n/2).
(MAGMA) [3^Floor(n/2): n in [0..50]]; // Vincenzo Librandi, Aug 17 2011
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CROSSREFS
| Essentially the same as A056449.
Cf. A000244, A016116.
Sequence in context: A146474 A145957 A128019 * A056449 A162436 A146788
Adjacent sequences: A108408 A108409 A108410 * A108412 A108413 A108414
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KEYWORD
| nonn
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AUTHOR
| Ralf Stephan, Jun 05 2005
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