OFFSET
2,2
COMMENTS
Also number of labeled acyclic digraphs with n nodes and binomial(n,2)-1 arcs, cf. A081064. - Vladeta Jovovic, Jul 14 2008
REFERENCES
L. Comtet, Advanced Combinatorics, Reidel, 1974, p. 60.
FORMULA
E.g.f.: (x^2/2)(1+x)/(1-x)^3. a(n)=n!(n-1)^2/2, if n>1. - Vladeta Jovovic, Dec 01 2002
D-finite with recurrence (n-2)^2*a(n) -n*(n-1)^2*a(n-1)=0. - R. J. Mathar, Mar 08 2021
PROG
(PARI) a(n)=if(n<2, 0, n!/2*(n-1)^2) /* Michael Somos, Mar 06 2004 */
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jul 10 2000
EXTENSIONS
More terms from Vladeta Jovovic, Dec 01 2002
STATUS
approved