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A052729
A simple context-free grammar in a labeled universe.
0
0, 0, 2, 12, 168, 3120, 75600, 2237760, 78382080, 3169393920, 145297152000, 7446479040000, 421880659200000, 26181558769766400, 1766275594501017600, 128700516232544256000, 10073126652104355840000
OFFSET
0,3
FORMULA
E.g.f.: RootOf(-_Z+_Z^2+_Z^3+x)^2
Recurrence: {a(1)=0, a(2)=2, a(3)=12, (21*n-27*n^3-6)*a(n) +(-49*n^2-6-65*n)*a(n+1) +(-33-17*n)*a(n+2) +5*a(n+3) =0.
a(n) = n!*A052706(n). - R. J. Mathar, Oct 18 2013
MAPLE
spec := [S, {B=Prod(S, C), S=Prod(C, C), C=Union(B, S, Z)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
seq(2*add((n+k-1)!/(n-k-2)!/(2*k+2-n)!, k= ceil(n/2)-1..n-2), n=0..20); # Mark van Hoeij, May 12 2013
CROSSREFS
Sequence in context: A368981 A255163 A052728 * A098431 A231085 A059522
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved