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 A005155 Number of degree sequences of n-node graphs. (Formerly M1886) 2
 1, 1, 2, 8, 54, 533, 6944, 111850, 2135740, 47003045, 1168832808, 32363244260, 986532609608, 32810811179569, 1181865951824800, 45823912079507918, 1902469319507438352, 84195282530581058825, 3956365033583165905568, 196716723188140236180160 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS Given a simple graph, the degree sequence maps each vertex to the valence or degree of that vertex. REFERENCES N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). R. P. Stanley, Enumerative Combinatorics, Cambridge, Vol. 2, 1999; see Problem 5.16. LINKS James Spahlinger and Alois P. Heinz, Table of n, a(n) for n = 0..386 (first 101 terms from James Spahlinger) R. Simion, Convex Polytopes and Enumeration, Adv. in Appl. Math. 18 (1997) pp. 149-180. R. P. Stanley, A zonotope associated with graphical degree sequences, in Applied Geometry and Discrete Combinatorics. DIMACS Series in Discrete Math., Amer. Math. Soc., Vol. 4, pp. 555-570, 1991. Kai Wang, Efficient Counting of Degree Sequences, arXiv:1604.04148 [math.CO], 2016, p. 2 and p. 13. FORMULA There is an explicit formula and e.g.f. E.g.f.: (sqrt((1-LambertW(-x))/(1+LambertW(-x)))-LambertW(-x)/x)*exp(-LambertW(-x)^2/2)/2. - Vladeta Jovovic, Jun 21 2007 a(n) ~ Gamma(3/4) * n^(n-1/4) / (2^(3/4) * exp(1/2) * sqrt(Pi)) * (1 - 11*Pi/(24*Gamma(3/4)^2*sqrt(n))). - Vaclav Kotesovec, Jul 09 2013 EXAMPLE 1 + x + 2*x^2 + 8*x^3 + 54*x^4 + 533*x^5 + 6944*x^6 + 111850*x^7 + 2135740*x^8 + ... a(3)=8 because we have: {0, 0, 0}, {0, 1, 1}, {1, 0, 1}, {1, 1, 0}, {1, 1, 2}, {1, 2, 1}, {2, 1, 1}, {2, 2, 2}. - Geoffrey Critzer, Aug 24 2016 MATHEMATICA max = 18; w = ProductLog; f[x_] := (Sqrt[(1 - w[-x])/(1 + w[-x])] - w[-x]/x)*(Exp[-w[-x]^2/2]/ 2); CoefficientList[ Series[f[x], {x, 0, max}], x]*Range[0, max]! (* Jean-François Alcover, Dec 12 2011, after Vladeta Jovovic *) PROG (PARI) {a(n) = local(A, B, C); if( n<0, 0, A = sum( k=1, n, k^k * x^k / k!, x * O(x^n)); B = intformal( 1 + A); C = intformal( 1 / (1 - B)); n! * polcoeff( (1 + (1 - B) * sqrt(1 + 2*A)) / 2 * exp(C), n))} /* Michael Somos, Aug 19 2005 */ CROSSREFS Cf. A004251 for graphs up to isomorphism. Sequence in context: A052662 A073564 A199576 * A133316 A234301 A005440 Adjacent sequences:  A005152 A005153 A005154 * A005156 A005157 A005158 KEYWORD nonn,nice,easy AUTHOR EXTENSIONS Minor edits by Vaclav Kotesovec, Mar 31 2014 STATUS approved

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Last modified January 24 04:10 EST 2019. Contains 319412 sequences. (Running on oeis4.)