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 A007769 Number of chord diagrams with n chords; number of pairings on a necklace. 13
 1, 1, 2, 5, 18, 105, 902, 9749, 127072, 1915951, 32743182, 624999093, 13176573910, 304072048265, 7623505722158, 206342800616597, 5996837126024824, 186254702826289089, 6156752656678674792, 215810382466145354405, 7995774669504366055054 (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. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..200 D. Bar-Natan, On the Vassiliev Knot Invariants, Topology 34 (1995) 423-472. D. Bar-Natan, Bibliography of Vassiliev Invariants. W. Y.-C. Chen, D. C. Torney, Equivalence classes of matchings and lattice-square designs, Discr. Appl. Math. 145 (3) (2005) 349-357., table of C_2n. Combinatorial Object Server, Information on Chord Diagrams Étienne Ghys, A Singular Mathematical Promenade, arXiv:1612.06373 [math.GT], 2016. See p. 252. A. Khruzin, Enumeration of chord diagrams, arXiv:math/0008209 [math.CO], 2000. R. J. Mathar, Feynman diagrams of the QED vacuum polarization, vixra:1901.0148 (2019), Section V. Joe Sawada, A fast algorithm for generating nonisomorphic chord diagrams, SIAM J. Discrete Math, Vol. 15, No. 4, 2002, pp. 546-561. Alexander Stoimenow, On the number of chord diagrams, Discr. Math. 218 (2000), 209-233. FORMULA 2n a_n = Sum_{2n=pq} alpha(p, q)phi(q), phi = Euler function, alpha(p, q) = Sum_{k >= 0} binomial(p, 2k) q^k (2k-1)!! if q even, = q^{p/2} (p-1)!! if q odd. MAPLE with(numtheory): alpha:=proc(p, q):if is(q, even) then add(binomial(p, 2*k)*q^k*doublefactorial(2*k-1), k=0..p/2) else q^(p/2)*doublefactorial(p-1) fi end: a:=n->add(alpha(2*n/p, p)*phi(p), p=divisors(2*n))/2/n: a(0):=1:seq(a(k), k=0..20); # Robert FERREOL, Oct 10 2018 MATHEMATICA max = 20; alpha[p_, q_?EvenQ] := Sum[Binomial[p, 2k]*q^k*(2k-1)!!, {k, 0, max}]; alpha[p_, q_?OddQ] := q^(p/2)*(p-1)!!; a[0] = 1; a[n_] := Sum[q = 2n/p; alpha[p, q]*EulerPhi[q], {p, Divisors[2n]}]/(2n); Table[a[n], {n, 0, max}] (* Jean-François Alcover, May 07 2012, after R. J. Mathar *) Stoimenow states that a Mma package is available from his website. - N. J. A. Sloane, Jul 26 2018 PROG (PARI) doublefactorial(n)={ local(resul) ; resul=1 ; forstep(i=n, 2, -2, resul *= i ; ) ; return(resul) ; } alpha(n, q)={ if(q %2, return( q^(p/2)*doublefactorial(p-1)), return( sum(k=0, p/2, binomial(p, 2*k)*q^k*doublefactorial(2*k-1)) ) ; ) ; } A007769(n)={ local(resul, q) ; if(n==0, return(1), resul=0 ; fordiv(2*n, p, q=2*n/p ; resul += alpha(p, q)*eulerphi(q) ; ); return(resul/(2*n)) ; ) ; } { for(n=0, 20, print(n, " ", A007769(n)) ; ) ; } \\ R. J. Mathar, Oct 26 2006 CROSSREFS Cf. A054499, A104255, A279207, A279208. Sequence in context: A005639 A093730 A304918 * A174122 A005805 A058338 Adjacent sequences:  A007766 A007767 A007768 * A007770 A007771 A007772 KEYWORD nonn,easy,nice AUTHOR Jean.Betrema(AT)labri.u-bordeaux.fr EXTENSIONS More terms from Christian G. Bower, Apr 06 2000 Corrected and extended by R. J. Mathar, Oct 26 2006 STATUS approved

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Last modified December 9 16:42 EST 2019. Contains 329879 sequences. (Running on oeis4.)