A268219 - OEIS

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

0,6

LINKS

Table of n, a(n) for n=0..24.

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

MAPLE

g:=(r, n)->(n!/r!)*add(1/k!, k=1..n-r);

g2:=r->[seq(g(r, n), n=0..30)];

g2(4);

PROG

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