A038158 a(n) = (n!/2)*Sum(1/k!, k=1..n-2).

0, 0, 0, 3, 18, 100, 615, 4326, 34636, 311760, 3117645, 34294150, 411529866, 5349888336, 74898436795, 1123476552030, 17975624832600, 305585622154336, 5500541198778201, 104510282776785990, 2090205655535719990, 43894318766250120000

Offset: 0

Author

N. J. A. Sloane

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

Row sums of A268217.

Cf. A038157.

Programs

PARI

a(n) = (n!/2)*sum(k=1, n-2, 1/k!); \\ Michel Marcus, Jan 30 2016

Formula

a(n) = A038157(n) / 2. - Sean A. Irvine, Jan 09 2021