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 A241230 #12 Feb 16 2025 08:33:22 %S A241230 0,12,108,528,1674,4104,8622,16206,28332,46200,71484,106746,153552, %T A241230 214860,293166,391470,512592,660546,840840,1054704,1308696,1606776 %N A241230 Number of obtuse isosceles triangles on a centered hexagonal grid of size n. %C A241230 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241230 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241230 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ObtuseTriangle.html">Obtuse Triangle</a>. %H A241230 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/IsoscelesTriangle.html">Isosceles Triangle</a>. %F A241230 a(n) = A241228(n) - A241229(n). %Y A241230 Cf. A190318. %K A241230 nonn,more %O A241230 1,2 %A A241230 _Martin Renner_, Apr 17 2014 %E A241230 a(7) from _Martin Renner_, May 31 2014 %E A241230 a(8)-a(22) from _Giovanni Resta_, May 31 2014