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 A007474 Number of circular chord diagrams with n chords, up to rotational symmetry. (Formerly M1800) 2
 1, 0, 1, 2, 7, 36, 300, 3218, 42335, 644808, 11119515, 213865382, 4537496680, 105270612952, 2651295555949, 72042968876506, 2100886276796969, 65446290562491916, 2169090198219290966, 76211647261082309466, 2829612806029873399561 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Gheorghe Coserea, Table of n, a(n) for n = 0..300 Dror Bar-Natan, On the Vassiliev Knot Invariants, Topology 34 (1995) 423-472. D. Bar-Natan, Bibliography of Vassiliev Invariants E. Krasko, A. Omelchenko, Enumeration of Chord Diagrams without Loops and Parallel Chords, arXiv preprint arXiv:1601.05073 [math.CO], 2016. E. Krasko, A. Omelchenko, Enumeration of Chord Diagrams without Loops and Parallel Chords, The Electronic Journal of Combinatorics, 24(3) (2017), #P3.43. MATHEMATICA m = 20; Clear[M]; M[_, _] = 0; Mget[n_, k_] := Which[n < 0, 0, n == 0, 1, n == 1, 1 - Mod[k, 2], n == 2, k - Mod[k, 2], True, M[n, k]]; Mset[n_, k_, v_] := (M[n, k] = v); Minit[] = (tmp = 0; For[n = 3, n <= 2*m, n++, For[k = 1, k <= 2*m, k++, tmp = If[Mod[k, 2] == 1, k*(n-1)*Mget[n-2, k] + Mget[n-4, k], Mget[n-1, k] + k*(n-1) * Mget[n-2, k] - Mget[n-3, k] + Mget[n-4, k]]; Mset[n, k, tmp]]]; ); a[n_] := DivisorSum[2*n, EulerPhi[#] * (Mget[2*n/#, #] - Mget[2*n/# - 2, #])&] / (2*n); Minit[]; Prepend[ Array[a, m], 1] (* Jean-François Alcover, Apr 24 2017, after Gheorghe Coserea *) PROG (PARI) N = 20; M = matrix(2*N, 2*N); Mget(n, k) = { if (n<0, 0, n==0, 1, n==1, 1-(k%2), n==2, k-(k%2), M[n, k]) }; Mset(n, k, v) = { M[n, k] = v; }; Minit() = {   my(tmp = 0);   for (n=3, 2*N, for(k=1, 2*N,     tmp = if (k%2, k*(n-1) * Mget(n-2, k) + Mget(n-4, k),     Mget(n-1, k) + k*(n-1) * Mget(n-2, k) - Mget(n-3, k) + Mget(n-4, k));     Mset(n, k, tmp))); }; a(n) = sumdiv(2*n, d, eulerphi(d) * (Mget(2*n/d, d) - Mget(2*n/d-2, d))) / (2*n); Minit(); concat(1, vector(N, n, a(n)))  \\ Gheorghe Coserea, Dec 10 2016 CROSSREFS Sequence in context: A012363 A012717 A072236 * A002724 A292206 A203900 Adjacent sequences:  A007471 A007472 A007473 * A007475 A007476 A007477 KEYWORD nonn,nice AUTHOR STATUS approved

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Last modified December 6 04:14 EST 2019. Contains 329784 sequences. (Running on oeis4.)