OFFSET
0,2
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 519
FORMULA
E.g.f.: (-x^2+2*x^3-1)/(-1+2*x)
Recurrence: {a(0)=1, a(1)=2, (-2*n-2)*a(n)+a(n+1)=0, a(2)=10, a(3)=48}
2^n*n!, n>2.
MAPLE
spec := [S, {S=Union(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