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Search: id:A007147
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| A007147 |
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Number of self-dual 2-colored necklaces with 2n beads.
(Formerly M0319)
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+0
3
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| 1, 1, 2, 2, 4, 5, 9, 12, 23, 34, 63, 102, 190, 325, 612, 1088, 2056, 3771, 7155, 13364, 25482, 48175, 92205, 175792, 337594, 647326, 1246863, 2400842, 4636390, 8956060, 17334801, 33570816, 65108062, 126355336, 245492244, 477284182
(list; graph; listen)
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OFFSET
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1,3
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
E. M. Palmer and R. W. Robinson, Enumeration of self-dual configurations, Pacific J. Math., 110 (1984), 203-221.
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LINKS
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Index entries for sequences related to necklaces
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PROGRAM
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(PARI) a(n)= (1/2) *(2^((n-1)\2)+sumdiv(n, k, (k%2)*eulerphi(k)*2^(n/k))/(2*n))
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CROSSREFS
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Cf. A000016, A016116. a(n)= (1/2) *(A016116(n-1)+A000016(n)).
Sequence in context: A001224 A102526 A050192 this_sequence A127968 A037026 A116651
Adjacent sequences: A007144 A007145 A007146 this_sequence A007148 A007149 A007150
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KEYWORD
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nonn,nice,easy
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com).
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EXTENSIONS
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More terms from Michael Somos
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