OFFSET
0,3
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 577
FORMULA
E.g.f.: -x/(-1 + 2*x + x^2).
Recurrence: {a(1)=1, a(0)=0, (-2-n^2-3*n)*a(n)+(-4-2*n)*a(n+1)+a(n+2)}.
Sum((-1/4)*(-1+_alpha)*_alpha^(-1-n), _alpha=RootOf(-1+2*_Z+_Z^2))*n!.
MAPLE
spec := [S, {S=Prod(Z, Sequence(Union(Z, Z, Prod(Z, Z))))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
with(combstruct):ZL:=[T, {T=Union(Z, Prod(Epsilon, Z, T), Prod(T, Z, Epsilon), Prod(T, Z, Z))}, labeled]:seq(count(ZL, size=i), i=0..19); # Zerinvary Lajos, Dec 16 2007
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
STATUS
approved