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 A201684 #14 Nov 26 2018 04:06:53 %S A201684 1,-1,0,0,4,34,274,2312,21034,207790,2228892,25890642,324477994, %T A201684 4370180744,63007469346,968847653702,15834053988732,274170226919434, %U A201684 5015004366420178,96645631069563928,1957433140982004026 %N A201684 a(n) = 2*A052186(n) - n!. %H A201684 Vaclav Kotesovec, <a href="/A201684/b201684.txt">Table of n, a(n) for n = 0..446</a> %H A201684 J.-L. Baril, <a href="http://www.combinatorics.org/ojs/index.php/eljc/article/view/v18i1p178">Classical sequences revisited with permutations avoiding dotted pattern</a>, Electronic Journal of Combinatorics, 18 (2011), #P178. See Table 1. %F A201684 a(n) ~ n! * (1 - 4/n + 2/n^2 - 2/n^3 - 18/n^4 - 118/n^5 - 948/n^6 - 9120/n^7 - 100728/n^8 - 1250770/n^9 - 17245316/n^10), for coefficients see 2*A256168. - _Vaclav Kotesovec_, Mar 17 2015 %Y A201684 Cf. A052186, A256168. %K A201684 sign %O A201684 0,5 %A A201684 _N. J. A. Sloane_, Dec 03 2011