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A038158 a(n) = (n!/2)*Sum(1/k!, k=1..n-2). 4
0, 0, 0, 3, 18, 100, 615, 4326, 34636, 311760, 3117645, 34294150, 411529866, 5349888336, 74898436795, 1123476552030, 17975624832600, 305585622154336, 5500541198778201, 104510282776785990, 2090205655535719990, 43894318766250120000 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,4

LINKS

Table of n, a(n) for n=0..21.

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

FORMULA

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

PROG

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

CROSSREFS

Row sums of A268217.

Cf. A038157.

Sequence in context: A132848 A321032 A180036 * A327828 A009021 A303519

Adjacent sequences:  A038155 A038156 A038157 * A038159 A038160 A038161

KEYWORD

nonn

AUTHOR

N. J. A. Sloane.

STATUS

approved

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Last modified July 28 13:49 EDT 2021. Contains 346334 sequences. (Running on oeis4.)