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 A054499 Number of pairings on a bracelet; number of chord diagrams that can be turned over and having n chords. 16
 1, 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; text; internal format)
 OFFSET 0,3 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. REFERENCES R. C. Read, Some Enumeration Problems in Graph Theory. Ph.D. Dissertation, Department of Mathematics, Univ. London, 1958. LINKS W. Y.-C. Chen, D. C. Torney, Equivalence classes of matchings and lattice-square designs, Discr. Appl. Math. 145 (3) (2005) 349-357. Étienne Ghys, A Singular Mathematical Promenade, arXiv:1612.06373 [math.GT], 2016-2017. See p. 252. A. Khruzin, Enumeration of chord diagrams, arXiv:math/0008209 [math.CO], 2000. V. A. Liskovets, Some easily derivable sequences, J. Integer Sequences, 3 (2000), #00.2.2. R. J. Mathar, Chord Diagrams A054499 (2018) R. J. Mathar, Feynman diagrams of the QED vacuum polarization, vixra:1901.0148 (2019) R. C. Read, Letter to N. J. A. Sloane, Feb 04 1971 (gives initial terms of this sequence) Alexander Stoimenow, On the number of chord diagrams, Discr. Math. 218 (2000), 209-233. FORMULA a(n) = (2*A007769(n) + A047974(n) + A047974(n-1))/4 for n > 0. EXAMPLE For n=3, there are 5 bracelets with 3 pairs of beads. They are represented by the words aabbcc, aabcbc, aabccb, abacbc, and abcabc. All of the 6!/(2*2*2) = 90 combinations can be derived from these by some combination of relabeling the pairs, rotation, and reflection. So a(3) = 5. - Michael B. Porter, Jul 27 2016 MATHEMATICA max = 19; alpha[p_, q_?EvenQ] := Sum[Binomial[p, 2*k]*q^k*(2*k-1)!!, {k, 0, max}]; alpha[p_, q_?OddQ] := q^(p/2)*(p-1)!!; a[0] = 1; a[n_] := 1/4*(Abs[HermiteH[n-1, I/2]] + Abs[HermiteH[n, I/2]] + (2*Sum[Block[{q = (2*n)/p}, alpha[p, q]*EulerPhi[q]], {p, Divisors[ 2*n]}])/(2*n)); Table[a[n], {n, 0, max}] (* Jean-François Alcover, Sep 05 2013, after R. J. Mathar; corrected by Andrey Zabolotskiy, Jul 27 2016 *) CROSSREFS Cf. A007769, A104256, A279207, A279208, A003437 (loopless chord diagrams), A322176 (marked chords). Sequence in context: A289739 A243337 A259622 * A001186 A125282 A020125 Adjacent sequences:  A054496 A054497 A054498 * A054500 A054501 A054502 KEYWORD nonn,easy,nice AUTHOR Christian G. Bower, Apr 06 2000 based on a problem by Wouter Meeussen EXTENSIONS Corrected and extended by N. J. A. Sloane, Oct 29 2006 a(0)=1 prepended back again by Andrey Zabolotskiy, Jul 27 2016 STATUS approved

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Last modified December 10 23:29 EST 2019. Contains 329910 sequences. (Running on oeis4.)