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 A336012 #9 Jul 04 2020 22:15:16
%S A336012 1,1,1,3,7,35,121,847,3907,35163,202741,2230151,15430207,200592691,
%T A336012 1619195761,24287936415,224061282907,3809041809419,39531606447181,
%U A336012 751100522496439,8661323866026007,181887801186546147,2307185279184885001,53065261421252355023,734307168916191403507,18357679222904785087675
%N A336012 a(n) is the number of chains from {} to a top element in the poset of even sized subsets of {1,2,...,n} ordered by inclusion.
%C A336012 Note that when n is even there is one top element, namely {1,2,...,n}. When n is odd there are C(n,n-1) = n top elements in the poset.
%H A336012 Philippe Flajolet and Robert Sedgewick, <a href="http://algo.inria.fr/flajolet/Publications/AnaCombi/anacombi.html">Analytic Combinatorics</a>, Cambridge Univ. Press, 2009, page 179.
%F A336012 E.g.f.: 1/(2-cosh(x))*(1+x).
%t A336012 nn = 25; Range[0, nn]! CoefficientList[Series[1/(2 - Cosh[z])*(1 + z), {z, 0, nn}], z]
%Y A336012 Cf. A260464.
%K A336012 nonn
%O A336012 0,4
%A A336012 _Geoffrey Critzer_, Jul 04 2020