OFFSET
1,2
COMMENTS
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
Vincenzo Librandi, Table of n, a(n) for n = 1..100
C. Banderier, J.-M. Le Bars and V. Ravelomanana, Generating functions for kernels of digraphs, arXiv:math/0411138 [math.CO], 2004.
FORMULA
E.g.f.: -C(-C(2*x)/2), where C(x) is the e.g.f. of A000169.
a(n) ~ LambertW(1/2) * n^(n-1) * 2^n / (1+LambertW(1/2)). - Vaclav Kotesovec, Oct 08 2013
MATHEMATICA
Rest[CoefficientList[Series[LambertW[-LambertW[-2*x]/2], {x, 0, 20}], x]* Range[0, 20]!] (* Vaclav Kotesovec, Oct 08 2013 *)
PROG
(PARI) x='x+O('x^30); Vec(serlaplace(lambertw(-lambertw(-2*x)/2))) \\ G. C. Greubel, Nov 15 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Ralf Stephan, Aug 17 2004
STATUS
approved