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A054499
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Number of pairings on a bracelet; number of chord diagrams that can be turned over and having n chords.
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10
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1, 2, 5, 17, 79, 554, 5283, 65346, 966156, 16411700, 312700297, 6589356711, 152041845075, 3811786161002, 103171594789775, 2998419746654530, 93127358763431113, 3078376375601255821, 107905191542909828013, 3997887336845307589431
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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COMMENTS
| Place 2n points equally spaced on a circle. Draw lines to pair up all the points so that each point has exactly one partner. Allow turning over.
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LINKS
| W. Y.-C. Chen, D. C. Torney,Equivalence classes of matchings and lattice-square designs, Discr. Appl. Math. 145 (3) (2005) 349-357.
A. Khruzin, Enumeration of chord diagrams
V. A. Liskovets, Some easily derivable sequences, J. Integer Sequences, 3 (2000), #00.2.2.
Index entries for sequences related to bracelets
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FORMULA
| a(n)=(2*A007769(n)+A047974(n)+A047974(n-1))/4.
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CROSSREFS
| Cf. A104256.
Sequence in context: A184509 A020096 A187245 * A001186 A125282 A020125
Adjacent sequences: A054496 A054497 A054498 * A054500 A054501 A054502
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KEYWORD
| nonn,easy,nice
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AUTHOR
| Christian G. Bower (bowerc(AT)usa.net), Apr 06 2000 based on a problem by Wouter Meeussen (wouter.meeussen(AT)pandora.be)
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EXTENSIONS
| Corrected and extended by N. J. A. Sloane (njas(AT)research.att.com), Oct 29 2006
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