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 A100728 #15 Oct 10 2019 04:24:22 %S A100728 1,31,337,2570,16865,104858,650761,4145956,27483392,190522216, %T A100728 1382087111,10478149999,82860356456,682066659044,5832719543338, %U A100728 51724107920729,474869705028520,4506715494154371,44152005320340946 %N A100728 Number of rank-(n-2) simple matroids on S_n. %H A100728 W. M. B. Dukes, <a href="http://www.stp.dias.ie/~dukes/matroid.html">Tables of matroids</a>. %H A100728 W. M. B. Dukes, <a href="https://web.archive.org/web/20030208144026/http://www.stp.dias.ie/~dukes/phd.html">Counting and Probability in Matroid Theory</a>, Ph.D. Thesis, Trinity College, Dublin, 2000. %H A100728 W. M. B. Dukes, <a href="https://arxiv.org/abs/math/0411557">The number of matroids on a finite set</a>, arXiv:math/0411557 [math.CO], 2004. [See Lemma 2.2(iii).] %H A100728 W. M. B. Dukes, <a href="http://emis.impa.br/EMIS/journals/SLC/wpapers/s51dukes.html">On the number of matroids on a finite set</a>, Séminaire Lotharingien de Combinatoire 51 (2004), Article B51g. [See Lemma 2.2(iii).] %F A100728 a(n) = Bell(n+1) - (n^2+n+4)*2^(n-2) + n*(n+1)*(3*n^2-n+10)/24. %Y A100728 Cf. A000110 (Bell numbers). Diagonal of A058720. %K A100728 nonn %O A100728 4,2 %A A100728 _Ralf Stephan_, Nov 29 2004