OFFSET
0,3
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 698
FORMULA
E.g.f.: (1/2-1/2*x-1/2*(1-6*x+x^2)^(1/2))*x
Recurrence: {a(1)=0, a(2)=2, a(3)=12, (-4*n+n^2-4+n^3)*a(n) +(-6*n^2+6-9*n)*a(n+1) +(n+1)*a(n+2) =0.
a(n) ~ sqrt(3/sqrt(2)-2)*n^(n-1)*(3+2*sqrt(2))^(n-1)/exp(n). - Vaclav Kotesovec, Oct 05 2013
a(n) = n!*A006318(n-2). - R. J. Mathar, Oct 18 2013
MAPLE
spec := [S, {B=Prod(C, C), C=Union(B, S, Z), S=Prod(Z, C)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20); # end of program
0, 0, seq(simplify(2*n!*hypergeom([ 3-n, n], [2], -1)), n=2..20); # Mark van Hoeij, May 29 2013
MATHEMATICA
CoefficientList[Series[(1/2-1/2*x-1/2*(1-6*x+x^2)^(1/2))*x, {x, 0, 20}], x]* Range[0, 20]! (* Vaclav Kotesovec, Oct 05 2013 *)
PROG
(PARI) x='x+O('x^66); concat([0, 0], Vec( serlaplace( (1/2-1/2*x-1/2*(1-6*x+x^2)^(1/2))*x))) \\ Joerg Arndt, May 29 2013
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved