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A002831
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Number of 3-edge-colored connected trivalent graphs with 2n nodes.
(Formerly M3424 N1388)
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20
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1, 4, 11, 60, 318, 2806, 29359, 396196, 6231794, 112137138, 2249479114, 49691965745, 1197158348160, 31230408793660, 876971159096883, 26374570956403684, 845812191249484022, 28812214090645864661, 1038982259432805270094, 39540452134474760212909
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OFFSET
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1,2
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COMMENTS
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In a letter to N. J. A. Sloane dated Feb 04 1971 (see link), R. C. Read enclosed a table listing 14 sequences, all of which, he says, appeared in his 1958 Ph.D. thesis. The values he gave for terms a(5) and a(6) in the present sequence are apparently incorrect (the terms given here are correct; the incorrect terms are shown in A246598). - N. J. A. Sloane, Sep 08 2014
Comment from Max Alekseyev, Sep 09 2014: the relationship between "all graphs" and "connected graphs" is of course a version of the Euler transform - see for example the third formula in the Euler Transform link.
Number of oriented unrooted pavings (after Arques & Koch, Spehner, Lienhardt) with 2n darts.
Also the number of conjugacy classes of free index 2n subgroups in the free product Z_2*Z_2*Z_2. (End)
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REFERENCES
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R. C. Read, Some Enumeration Problems in Graph Theory. Ph.D. Dissertation, Department of Mathematics, Univ. London, 1958.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
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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G.f.: sum(mobius(k) * log(G(x^k)) / k, k >= 1) where G(x) is the g.f. for A002830. - Sean A. Irvine, Sep 09 2014
Asymptotics: a(n) ~ (2/Pi)^(1/2)*(2/e)^n*n^{n - 1/2}; cf. Ciobanu and Kolpakov in Links. - Sasha Kolpakov, Dec 17 2017
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MATHEMATICA
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terms = 20;
permcount[v_] := Module[{m = 1, s = 0, k = 0, t}, For[i = 1, i <= Length[v], i++, t = v[[i]]; k = If[i > 1 && t == v[[i - 1]], k + 1, 1]; m *= t k; s += t]; s!/m];
b[k_, q_] := If[OddQ[q], If[OddQ[k], 0, j = k/2; q^j (2 j)!/(j! 2^j)], Sum[ Binomial[k, 2 j] q^j (2 j)!/(j! 2^j), {j, 0, Quotient[k, 2]}]];
pm[v_] := Module[{p = Total[x^v]}, Product[ b[Coefficient[p, x, i], i], {i, 1, Exponent[p, x]}]];
a2830[n_] := Module[{s = 0}, Do[ s += permcount[p] pm[p]^3, {p, IntegerPartitions[2 n]}]; s/(2 n)!];
G[x_] = 1 + Sum[a2830[n] x^n, {n, 1, terms+1}];
gf = Sum[MoebiusMu[k] Log[G[x^k]]/k, {k, 1, terms+1}] + O[x]^(terms+1);
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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a(5) and a(6) corrected and new terms a(7) and a(8) computed by Sean A. Irvine, Sep 09 2014
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STATUS
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approved
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