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 A240692 #11 May 12 2014 11:52:44 %S A240692 0,4,9,13,21,25,28,36,39,43,49,57,63,64,67,76,81,84,91,93,97,109,111, %T A240692 117,121,124,129,133,144,147,148,156,157,163,171,175,183,189,193,196, %U A240692 199,201 %N A240692 Squared radii of circles which exactly encircle clusters in the A2 lattice in increasing order. %H A240692 Rajan Murthy, <a href="/A240692/a240692.txt">Generating code for the sequence (Scilab)</a> %e A240692 For n = 1, a(1) = 4, the squared distance to the corners furthest from the deep hole of three hexagons which share the deep hole as a corner. %e A240692 For n= 3, a(3) = 13, the squared distance to the furthest corners of the 6 hexagons third most distant from the deep hole - which, when added to the 3 that are closest and the 3 that are second-closest, yields a total of 12, which is A038588(3). %Y A240692 A038588(n) gives the corresponding cluster size starting at n = 1 (the first positive radius circle); %Y A240692 A000404 is the analog for a Cartesian lattice. %K A240692 nonn %O A240692 0,2 %A A240692 _Rajan Murthy_, Apr 10 2014