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 A154639 #11 Aug 18 2017 16:08:23 %S A154639 1,5,20,80,300,1140,4260 %N A154639 a(n) is the number of reduced words of length n (i.e., all possible length-reducing cancellations have been applied) in the generators of the "Apollonian reflection group" in three dimensions. This is a Coxeter group with five generators, satisfying the identities (S_i)^2 = (S_i S_j)^3 = I. %C A154639 ABA and BAB are equal, but are counted as distinct reduced words. %H A154639 R. L. Graham, J. C. Lagarias, C. L. Mallows, Allan Wilks and C. Yan, <a href="http://arxiv.org/abs/math/0010324">Apollonian Circle Packings: Geometry and Group Theory III. Higher Dimensions.</a>, Discrete & Computational Geometry, 35 (2006), no. 1, 37-72. %H A154639 C. L. Mallows, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL12/Mallows/mallows8.html">Growing Apollonian Packings</a>, J. Integer Sequences, 12 (2009), article 09.2.1. %e A154639 All 80 squarefree words of length 3 are counted, so a(3) = 80. %Y A154639 For other sequences relating to the 3-dimensional case, see A154638-A154645. %K A154639 more,nonn %O A154639 0,2 %A A154639 _Colin Mallows_, Jan 13 2009