A268220 a(n) = (n!/5!)*Sum(1/k!,k=1..n-5).

0, 0, 0, 0, 0, 0, 6, 63, 560, 5166, 51912, 571494, 6858720, 89164647, 1248307060, 18724608903, 299593746816, 5093093702060, 91675686645648, 1741838046278940, 34836760925594304, 731571979437500733, 16094583547625042460, 370175421595376010229, 8884210118289024288000

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, A268218, A268219.

Programs

Maple

g:=(r,n)->(n!/r!)*add(1/k!,k=1..n-r);
g2:=r->[seq(g(r,n),n=0..30)];
g2(5);

PARI

a(n) = (n!/5!)*sum(k=1, n-5, 1/k!); \\ Michel Marcus, Jan 30 2016