A268218 a(n) = (n!/3!)*Sum(1/k!,k=1..n-3).

0, 0, 0, 0, 4, 30, 200, 1435, 11536, 103908, 1039200, 11431365, 137176600, 1783296086, 24966145568, 374492183975, 5991874944160, 101861874051400, 1833513732926016, 34836760925595273, 696735218511906600, 14631439588750039930, 321891670952500880000, 7403508431907520241771

Offset: 0

Author

N. J. A. Sloane, Jan 30 2016

Keywords

• nonn

Links

• G. A. Kamel, Partial Chain Topologies on Finite Sets, Computational and Applied Mathematics Journal. Vol. 1, No. 4, 2015, pp. 174-179.

Crossrefs

For others in this series, see A038156, A038158, A268219, A268220.

Programs

Maple

g:=(r,n)->(n!/r!)*add(1/k!,k=1..n-r);
g2:=r->[seq(g(r,n),n=0..30)];
g2(3);

PARI

a(n) = (n!/3!)*sum(k=1, n-3, 1/k!); \\ Michel Marcus, Jan 30 2016