

A054499


Number of pairings on a bracelet; number of chord diagrams that can be turned over and having n chords.


13



1, 1, 2, 5, 17, 79, 554, 5283, 65346, 966156, 16411700, 312700297, 6589356711, 152041845075, 3811786161002, 103171594789775, 2998419746654530, 93127358763431113, 3078376375601255821, 107905191542909828013, 3997887336845307589431
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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

Table of n, a(n) for n=0..20.
W. Y.C. Chen, D. C. Torney, Equivalence classes of matchings and latticesquare designs, Discr. Appl. Math. 145 (3) (2005) 349357.
Étienne Ghys, A Singular Mathematical Promenade, arXiv:1612.06373, 2016. 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. C. Read, Letter to N. J. A. Sloane, Feb 04 1971 (gives initial terms of this sequence)
Index entries for sequences related to bracelets


FORMULA

a(n) = (2*A007769(n) + A047974(n) + A047974(n1))/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*k1)!!, {k, 0, max}];
alpha[p_, q_?OddQ] := q^(p/2)*(p1)!!;
a[0] = 1;
a[n_] := 1/4*(Abs[HermiteH[n1, 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}] (* JeanFrançois Alcover, Sep 05 2013, after R. J. Mathar; corrected by Andrey Zabolotskiy, Jul 27 2016 *)


CROSSREFS

Cf. A007769, A104256, A279207, A279208.
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



