A268219 a(n) = (n!/4!)*Sum(1/k!,k=1..n-4).

0, 0, 0, 0, 0, 5, 45, 350, 2870, 25956, 259770, 2857800, 34294095, 445823950, 6241536301, 93623045880, 1497968735900, 25465468512680, 458378433231300, 8709190231398576, 174183804627976365, 3657859897187509650, 80472917738125219615, 1850877107976880060000, 44421050591445121450626

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, 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(4);

PARI

a(n) = (n!/4!)*sum(k=1, n-4, 1/k!); \\ Michel Marcus, Jan 30 2016