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 A154640 #6 Aug 18 2017 16:06:19 %S A154640 5,5,20,60,210,690,3330 %N A154640 a(n) is the number of spheres that are added in the n-th generation of Apollonian packing of three-dimensional spheres, starting with five mutually tangent spheres and using "strategy (a)" to count them (see the reference). %C A154640 In strategy (a) we do not count spheres that are generated (by reflection) from quintuples that were not generated in the previous generation. %H A154640 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 A154640 For a(3), we apply reflection only to the 20 quintuples that were generated in the second generation, ignoring the 10 "extra" quintuples (which will appear as ABA = BAB in the third generation). %Y A154640 For other sequences relating to the 3-dimensional case, see A154638-A154645. %K A154640 hard,more,nonn %O A154640 0,1 %A A154640 _Colin Mallows_, Jan 13 2009