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A007148 Number of self-complementary 2-colored bracelets (turn over necklaces) with 2n beads.
(Formerly M0774)
+0
5
1, 2, 3, 6, 10, 20, 37, 74, 143, 284, 559, 1114, 2206, 4394, 8740, 17418, 34696, 69194, 137971, 275280, 549258, 1096286, 2188333, 4369162, 8724154, 17422652, 34797199, 69505908, 138845926, 277383872, 554189329, 1107297290, 2212558942 (list; graph; listen)
OFFSET

1,2

REFERENCES

E. M. Palmer and R. W. Robinson, Enumeration of self-dual configurations, Pacific J. Math., 110 (1984), 203-221.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

F. Ruskey, Necklaces, Lyndon words, De Bruijn sequences, etc.

Index entries for sequences related to bracelets

PROGRAM

(PARI) a(n)= (1/2) *(2^(n-1)+sumdiv(n, k, eulerphi(2*k)*2^(n/k))/(2*n))

CROSSREFS

Cf. A000013, A000079, A007147. a(n)= (1/2) *(A000079(n-1)+A000013(n)).

Sequence in context: A164047 A158291 A045690 this_sequence A093371 A003214 A123423

Adjacent sequences: A007145 A007146 A007147 this_sequence A007149 A007150 A007151

KEYWORD

nonn,nice,easy

AUTHOR

N. J. A. Sloane (njas(AT)research.att.com).

EXTENSIONS

Description corrected by Christian G. Bower (bowerc(AT)usa.net). More terms from Michael Somos

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Last modified November 25 20:09 EST 2009. Contains 167514 sequences.


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