OFFSET
1,2
COMMENTS
Number of all unrooted directed trees on n nodes.
Ditrees are well-colored directed trees. Well-colored means, each green vertex has at least a red child, each red vertex has no red child.
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..352
Federico Ardila, Matthias Beck, Jodi McWhirter, The Arithmetic of Coxeter Permutahedra, arXiv:2004.02952 [math.CO], 2020.
C. Banderier, J.-M. Le Bars and V. Ravelomanana, Generating functions for kernels of digraphs, arXiv:math/0411138 [math.CO], 2004.
Vsevolod Gubarev, Rota-Baxter operators on a sum of fields, arXiv:1811.08219 [math.RA], 2018.
Jean-Christophe Novelli and Jean-Yves Thibon, Free quasi-symmetric functions and descent algebras for wreath products, and noncommutative multi-symmetric functions (2008); arXiv:0806.3682 [math.CO], 2008. Discrete Math. 310 (2010), no. 24, 3584-3606.
Jean-Christophe Novelli and Jean-Yves Thibon, Duplicial algebras and Lagrange inversion, arXiv preprint arXiv:1209.5959 [math.CO], 2012.
J.-B. Priez, A. Virmaux, Non-commutative Frobenius characteristic of generalized parking functions: Application to enumeration, arXiv:1411.4161 [math.CO], 2014-2015.
FORMULA
E.g.f. satisfies: A(x) = 1 + 2*Sum_{n>=1} x^(2*n-1)/(2*n-1)! * A(x)^((4*n-1)/2) when offset=0: A(x) = Sum_{n>=0} a(n)*x^n/n!. - Paul D. Hanna, Sep 07 2012
E.g.f. satisfies: A(x) = 1/A(-x*A(x)^2) when offset=0. - Paul D. Hanna, Sep 07 2012
a(n) = sum(k=0..n-1, k!*stirling2(n-1,k)*binomial(2*n,k))/n. - Vladimir Kruchinin, Nov 19 2014
E.g.f.: -LambertW(-2*x)*(1+LambertW(-2*x)/2)/2. - Vaclav Kotesovec, Dec 08 2014
MATHEMATICA
Table[2*(2*n)^(n - 2), {n, 1, 50}] (* or *) With[{nmax = 40}, CoefficientList[Series[-LambertW[-2*x]*(1+LambertW[-2*x]/2)/2, {x, 0, nmax}], x]*Range[0, nmax]!] (* G. C. Greubel, Nov 15 2017 *)
PROG
(PARI) /* E.g.f. when offset=0 satisfies: */
{a(n)=local(A=1+2*x); for(i=1, 21, A=1+2*sum(n=1, 21, x^(2*n-1)/(2*n-1)!*A^((4*n-1)/2))+x*O(x^n)); n!*polcoeff(A, n)} \\ Paul D. Hanna, Sep 07 2012
for(n=0, 20, print1(a(n), ", "))
(Maxima) a(n):=sum(k!*stirling2(n-1, k)*binomial(2*n, k), k, 0, n-1)/(n); /* Vladimir Kruchinin, Nov 19 2014 */
(Magma) [1] cat [2*(2*n)^(n-2): n in [2..20]]; // Vincenzo Librandi, Nov 19 2014
(PARI) x='x+O('x^50); Vec(serlaplace(-lambertw(-2*x)*(1 + lambertw(-2*x)/2)/2)) \\ G. C. Greubel, Nov 15 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ralf Stephan, Aug 17 2004
STATUS
approved