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A052730
A simple context-free grammar in a labeled universe.
0
0, 0, 0, 6, 72, 1440, 35280, 1058400, 37376640, 1520830080, 70053984000, 3603888288000, 204806714112000, 12742502111539200, 861483843034214400, 62887137880799232000, 4929809613351727104000, 413044721987184967680000
OFFSET
0,4
FORMULA
E.g.f.: RootOf(-_Z+_Z^2+_Z^3+x)-RootOf(-_Z+_Z^2+_Z^3+x)^2-x
D-finite Recurrence: {a(1)=0, a(2)=0, a(3)=6, a(4)=72, (-105*n+135*n^3+30)*a(n) +(-130-107*n+29*n^2)*a(n+1) +(-99-91*n)*a(n+2) +15*a(n+3)=0.
a(n) = n!*A052703(n). - R. J. Mathar, Oct 18 2013
MAPLE
spec := [S, {C=Prod(B, B), B=Union(S, Z, C), S=Prod(B, C)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
CROSSREFS
Sequence in context: A113133 A302355 A089252 * A010796 A038095 A012242
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved