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A268219
a(n) = (n!/4!)*Sum(1/k!,k=1..n-4).
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
0,6
LINKS
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
CROSSREFS
For others in this series, see A038156, A038158, A268218, A268220.
Sequence in context: A247494 A043025 A190540 * A272494 A185009 A376525
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jan 30 2016
STATUS
approved