This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A038158 #16 Jul 08 2025 22:08:17 %S A038158 0,0,0,3,18,100,615,4326,34636,311760,3117645,34294150,411529866, %T A038158 5349888336,74898436795,1123476552030,17975624832600,305585622154336, %U A038158 5500541198778201,104510282776785990,2090205655535719990,43894318766250120000 %N A038158 a(n) = (n!/2)*Sum(1/k!, k=1..n-2). %H A038158 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 A038158 a(n) = A038157(n) / 2. - _Sean A. Irvine_, Jan 09 2021 %o A038158 (PARI) a(n) = (n!/2)*sum(k=1, n-2, 1/k!); \\ _Michel Marcus_, Jan 30 2016 %Y A038158 Row sums of A268217. %Y A038158 Cf. A038157. %K A038158 nonn %O A038158 0,4 %A A038158 _N. J. A. Sloane_