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A003437
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Number of unlabeled Hamiltonian circuits on n-octahedron (cross polytope); also number of circular chord diagrams with n chords, modulo symmetries.
(Formerly M1781)
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12
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0, 1, 2, 7, 29, 196, 1788, 21994, 326115, 5578431, 107026037, 2269254616, 52638064494, 1325663757897, 36021577975918, 1050443713185782, 32723148860301935, 1084545122297249077, 38105823782987999742, 1414806404051118314077
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OFFSET
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1,3
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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FORMULA
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MATHEMATICA
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nn = 20; M = Array[0&, {2nn, 2nn}];
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 = Module[{tmp = 0}, For[n = 3, n <= 2nn, n++, For[k = 1, k <= 2nn, k++, tmp = If[OddQ[k], 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]]]];
A007474[n_] := Sum[EulerPhi[d] (Mget[2n/d, d] - Mget[2n/d - 2, d]), {d, Divisors[2n]}]/(2n);
a[n_] := A007474[n]/2 + (Mget[n, 2] - Mget[n-1, 2] + Mget[n-2, 2])/4;
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PROG
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(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)));
};
Minit();
A007474(n) = sumdiv(2*n, d, eulerphi(d) * (Mget(2*n/d, d) - Mget(2*n/d-2, d)))/(2*n);
a(n) = A007474(n)/2 + (Mget(n, 2) - Mget(n-1, 2) + Mget(n-2, 2))/4;
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CROSSREFS
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KEYWORD
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nonn,nice
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AUTHOR
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STATUS
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approved
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