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 A075756 #13 Feb 24 2019 11:08:06 %S A075756 1,1,6,52,588,8174,134537,2554647,54909468,1316675221,34820961457, %T A075756 1006230148609,31529224324159,1064355502971193,38497326001639439, %U A075756 1484865225798412485,60822449267067095601,2636248249383130776940,120520100503562054999860,5794815395039941996204424 %N A075756 Number of hierarchies of hierarchies of hierarchies on n points. %H A075756 N. J. A. Sloane and Thomas Wieder, <a href="https://arxiv.org/abs/math/0307064">The Number of Hierarchical Orderings</a>, arXiv:math/0307064 [math.CO], 2003; Order 21 (2004), 83-89. %F A075756 E.g.f.: exp(f(x)-1) where f(x) = e.g.f. for A075744. %t A075756 m = 20; %t A075756 f[x_] = E^(-1 + E^(-1 + 1/(2 - E^x))); %t A075756 CoefficientList[Exp[f[x] - 1] + O[x]^m, x]*Range[0, m - 1]! (* _Jean-François Alcover_, Feb 24 2019 *) %Y A075756 Cf. A075729, A075744. %K A075756 nonn %O A075756 0,3 %A A075756 _N. J. A. Sloane_, Oct 15 2002