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%I #14 Jan 09 2021 17:53:50
%S 0,0,0,3,18,100,615,4326,34636,311760,3117645,34294150,411529866,
%T 5349888336,74898436795,1123476552030,17975624832600,305585622154336,
%U 5500541198778201,104510282776785990,2090205655535719990,43894318766250120000
%N a(n) = (n!/2)*Sum(1/k!, k=1..n-2).
%H G. A. Kamel, <a href="http://www.aascit.org/journal/archive2?journalId=928&paperId=2310">Partial Chain Topologies on Finite Sets</a>, Computational and Applied Mathematics Journal. Vol. 1, No. 4, 2015, pp. 174-179.
%F a(n) = A038157(n) / 2. - _Sean A. Irvine_, Jan 09 2021
%o (PARI) a(n) = (n!/2)*sum(k=1, n-2, 1/k!); \\ _Michel Marcus_, Jan 30 2016
%Y Row sums of A268217.
%Y Cf. A038157.
%K nonn
%O 0,4
%A _N. J. A. Sloane_.