%I A054499
%S A054499 1,2,5,17,79,554,5283,65346,966156,16411700,312700297,6589356711,152041845075,
%T A054499 3811786161002,103171594789775,2998419746654530,93127358763431113,3078376375601255821,
%U A054499 107905191542909828013,3997887336845307589431
%N A054499 Number of pairings on a bracelet; number of chord diagrams that can be 
               turned over and having n chords.
%C A054499 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.
%H A054499 A. Khruzin, <a href="http://arXiv.org/abs/math.CO/0008209">Enumeration 
               of chord diagrams</a>
%H A054499 V. A. Liskovets, <a href="http://www.cs.uwaterloo.ca/journals/JIS/index.html">
               Some easily derivable sequences</a>, J. Integer Sequences, 3 (2000), 
               #00.2.2.
%H A054499 <a href="Sindx_Br.html#bracelets">Index entries for sequences related 
               to bracelets</a>
%F A054499 a(n)=(2*A007769(n)+A047974(n)+A047974(n-1))/4.
%Y A054499 Cf. A104256.
%Y A054499 Sequence in context: A099825 A014288 A020096 this_sequence A001186 A125282 
               A020125
%Y A054499 Adjacent sequences: A054496 A054497 A054498 this_sequence A054500 A054501 
               A054502
%K A054499 nonn,easy,nice
%O A054499 0,2
%A A054499 Christian G. Bower (bowerc(AT)usa.net), Apr 06 2000 based on a problem 
               by Wouter Meeussen (wouter.meeussen(AT)pandora.be)
%E A054499 Corrected and extended by N. J. A. Sloane (njas(AT)research.att.com), 
               Oct 29 2006