OFFSET
0,2
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 540
FORMULA
E.g.f.: -1/(-1+3*x+x^2)
Recurrence: {a(0)=1, a(1)=3, (-2-n^2-3*n)*a(n)+(-6-3*n)*a(n+1)+a(n+2)=0}
Sum(1/13*(2*_alpha+3)*_alpha^(-1-n), _alpha=RootOf(-1+3*_Z+_Z^2))*n!
a(n)= n!*A006190(n+1). - R. J. Mathar, Nov 27 2011
MAPLE
spec := [S, {S=Sequence(Union(Z, Z, Z, Prod(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