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 A079144 Number of labeled interval orders on n elements: (2+2)-free posets. 13
 1, 1, 3, 19, 207, 3451, 81663, 2602699, 107477247, 5581680571, 356046745023, 27365431508779, 2494237642655487, 266005087863259291, 32815976815540917183, 4636895313201764853259, 743988605732990946684927 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Alois P. Heinz, Table of n, a(n) for n = 0..260 Graham Brightwell and Mitchel T. Keller, Asymptotic Enumeration of Labelled Interval Orders, arXiv:1111.6766 [math.CO], 2011. Anders Claesson, Mark Dukes and Martina Kubitzke, Partition and composition matrices, arXiv:1006.1312 [math.CO], 2010-2011. Don Zagier, Vassiliev invariants and a strange identity related to the Dedekind eta-function, Topology, vol.40, pp.945-960 (2001); see p. 952. Yan X Zhang, Four Variations on Graded Posets, arXiv preprint arXiv:1508.00318 [math.CO], 2015. FORMULA a(n) = (1/(24^n))*sum(binomial(n, k)*A002439(k), k=0..n). Zagier 2001, p. 954. G.f.: Sum(Product(1-exp(-k*x),k = 1 .. n),n = 0 .. infinity). a(n) = Sum_{k=0..n} k!*Stirling2(n,k)*A138265(k). - Vladeta Jovovic, Mar 11 2008 From Peter Bala, Mar 19 2009: (Start) Conjectural form for the o.g.f. as a continued fraction: 1/(1-x/(1-2*x/(1-5*x/(1-7*x/(1-12*x/(1-15*x/(1- ...))))))) = 1 + x + 3*x^2 + 19*x^3 + 207*x^4 + ..., where the sequence [1,2,5,7,12,15,..] is the sequence of generalized pentagonal numbers A001318. Compare with the continued fraction form of the o.g.f. of A002105. (End) E.g.f.: 1+(exp(x)-1)/(G(0)+1-exp(x)), where G(k)= 2*exp(x*(k+1))-1-exp(x*(k+1))*(exp(x*(k+2))-1)/G(k+1); (continued fraction, Euler's kind, 1-step). - Sergei N. Gladkovskii, Jan 06 2012 Asymptotics (Brightwell and Keller, 2011): a(n) ~ 12*sqrt(3)/Pi^(5/2) * (n!)^2 * sqrt(n) * (6/Pi^2)^n. - Vaclav Kotesovec, May 03 2014 From Peter Bala, May 11 2017: (Start) For a proof of above conjectural continued fraction representation of the o.g.f. see the Bala link. G.f. 1/(1 + x - 2*x/(1 - 1*x/(1 + x - 7*x/(1 - 5*x/(1 + x - 15*x/(1 - 12*x/(1 + x - 26*x/(1 - 22*x/(1 + x - ...))))))))), where the sequence of unsigned partial numerators [2, 1, 7, 5, 15, 12, ...] is obtained from A001318 by swapping adjacent terms. E.g.f. is F(q) = Sum_{n >= 0} q^(n+1)*Product_{i = 1..n} (1 - q^i)^2 at q = exp(t). Note that F(q) at q = 1/(1 - t) is a g.f. for unlabeled interval orders A022493, and at q = 1 - t gives a g.f. for A138265. (End) EXAMPLE 1 + x + 3*x^2 + 19*x^3 + 207*x^4 + 3451*x^5 + 81663*x^6 + 2602699*x^7 + ... MATHEMATICA nmax=20; rk=Rest[CoefficientList[Series[Sum[Product[1-1/(1+x)^j, {j, 1, n}], {n, 0, nmax}], {x, 0, nmax}], x]]; Flatten[{1, Table[Sum[rk[[k]] * k! * StirlingS2[n, k], {k, 1, n}], {n, 1, nmax}]}] (* Vaclav Kotesovec, May 03 2014 *) PROG (PARI) {a(n) = if( n<0, 0, n! * polcoeff( subst( sum( i=0, n, prod( j=1, i, 1 - (1 - x + O(x^(n - i + 2)))^j )), x, 1 - exp( -x + x * O(x^n))), n))} /* Michael Somos, Apr 01 2012 */ CROSSREFS Cf. A022493 (unlabeled interval orders), A002439 (Glaisher's T numbers). Cf. A001318, A138265. Sequence in context: A052886 A180563 A294330 * A049056 A204262 A165356 Adjacent sequences:  A079141 A079142 A079143 * A079145 A079146 A079147 KEYWORD nonn,easy AUTHOR Detlef Pauly (dettodet(AT)yahoo.de), Dec 27 2002 STATUS approved

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Last modified October 22 23:18 EDT 2018. Contains 316518 sequences. (Running on oeis4.)