OFFSET
0,5
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 680
FORMULA
E.g.f.: (1/2)/x^2*(1-x-(1-2*x+x^2-4*x^3)^(1/2))-(1/2)/x*(1-x-(1-2*x+x^2-4*x^3)^(1/2))-x
Recurrence: {a(1)=0, a(2)=0, a(3)=0, a(4)=24, a(6)=2160, a(7)=30240, (38*n^4+120*n^3-48-4*n+130*n^2+4*n^5)*a(n) +(-193*n^2-52*n^3-302*n-5*n^4-168)*a(n+1) +(96+29*n^2+92*n+3*n^3)*a(n+2) +(-52-3*n^2-25*n)*a(n+3) +(n+6)*a(n+4)=0, a(5)=240}
a(n) = n!*A052702(n). - R. J. Mathar, Oct 18 2013
MAPLE
spec := [S, {B=Prod(Z, C), C=Union(B, S, Z), S=Prod(B, B)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
seq(n!*add(binomial(n-k-2, 2*k-1)*binomial(2*k, k)/(k+1), k=0..n-2), n=0..20); # Mark van Hoeij, May 12 2013
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved