OFFSET
0,1
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 593
FORMULA
E.g.f.: -(-2+x^2+2*x)/(-1+2*x)/(-1+x^2)
Recurrence: {a(1)=2, a(2)=10, a(0)=2, (12+2*n^3+12*n^2+22*n)*a(n)+(-n^2-5*n-6)*a(n+1)+(-2*n-6)*a(n+2)+a(n+3)=0}
(2^n+Sum(1/2*_alpha^(-n), _alpha=RootOf(-1+_Z^2)))*n!
n!*[2^n+(n mod 2)].
a(n) = n!*A052531(n). - R. J. Mathar, Nov 27 2011
MAPLE
spec := [S, {S=Union(Sequence(Prod(Z, Z)), Sequence(Union(Z, Z)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved