OFFSET
0,3
COMMENTS
Previous name was: A simple grammar.
LINKS
INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 826
FORMULA
E.g.f.: log(-1/(-1+x*exp(x)-x))
a(n) ~ (n-1)! * r^n, where r = 1.23997788765655... is the root of the equation log(1+r)=1/r. - Vaclav Kotesovec, Sep 29 2013
a(n) = n!*Sum_{k=1..n/2}((k-1)!*stirling2(n-k,k)/(n-k)!). - Vladimir Kruchinin, Mar 22 2016
MAPLE
spec := [S, {B=Set(Z, 1 <= card), C=Prod(Z, B), S=Cycle(C)}, labeled]: seq(combstruct[count](spec, size=n), n=0..20);
MATHEMATICA
CoefficientList[Series[Log[-1/(-1+x*E^x-x)], {x, 0, 20}], x]* Range[0, 20]! (* Vaclav Kotesovec, Sep 29 2013 *)
PROG
(Maxima)
a(n):=(n)!*sum((k-1)!*stirling2(n-k, k)/(n-k)!, k, 1, n/2); /* Vladimir Kruchinin, Mar 22 2016 */
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
encyclopedia(AT)pommard.inria.fr, Jan 25 2000
EXTENSIONS
New name using e.g.f. by Joerg Arndt, Sep 30 2013
STATUS
approved