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 A004121 M2095 #26 Jan 31 2022 01:27:54
%S A004121 2,16,208,3968,109568,4793344,410662912,82657083392,38274970222592,
%T A004121 37590755515826176,75458309991776124928,305873605165090925969408,
%U A004121 2491832958314452159507202048,40704585435508852018947014262784
%N A004121 Generalized weak orders on n points.
%D A004121 I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.
%D A004121 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%D A004121 C. G. Wagner, Enumeration of generalized weak orders. Arch. Math. (Basel) 39 (1982), no. 2, 147-152.
%H A004121 Vincenzo Librandi, <a href="/A004121/b004121.txt">Table of n, a(n) for n = 1..80</a>
%H A004121 C. G. Wagner, <a href="/A004121/a004121_1.pdf">Enumeration of generalized weak orders</a>, Preprint, 1980. [Annotated scanned copy]
%H A004121 C. G. Wagner and N. J. A. Sloane, <a href="/A004121/a004121.pdf">Correspondence, 1980</a>
%F A004121 E.g.f.: 1/(1 - Sum_{i >= 1} 2^binomial(i+1, 2)*x^i/i!).
%t A004121 max = 14; f[x_] := 1/(1 - Sum[(2^(i*(i+1)/2)*x^i)/i!, {i, 1, max}]); Drop[ CoefficientList[ Series[f[x], {x, 0, max}], x]*Range[0, max]!, 1] (* _Jean-François Alcover_, Oct 21 2011, after g.f. *)
%Y A004121 Cf. A004122, A004123, A000670 (asymmetric generalized weak orders on n points).
%K A004121 nonn,nice,easy
%O A004121 1,1
%A A004121 _N. J. A. Sloane_
%E A004121 Formula and more terms from _Vladeta Jovovic_, Mar 27 2001