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
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.: A(x) = C(2x)/2+C(-C(2x)/2), C(x) = e.g.f. of A000169.
MATHEMATICA
terms = 16;
c[x_] = -LambertW[-x];
A[x_] = c[2x]/2 + c[-c[2x]/2];
CoefficientList[A[x]/x + O[x]^terms, x] Range[terms]! (* Jean-François Alcover, Jan 15 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Ralf Stephan, Aug 17 2004
STATUS
approved