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 A241226 #13 Feb 16 2025 08:33:22 %S A241226 0,12,480,4488,22278,78288,220374,531594,1143648,2254440,4145244, %T A241226 7202190,11940444,19029300,29318754,43872402,63999528,91288794, %U A241226 127642164,175326336,237000816,315772032 %N A241226 Number of obtuse triangles on a centered hexagonal grid of size n. %C A241226 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241226 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241226 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ObtuseTriangle.html">Obtuse Triangle</a>. %F A241226 a(n) = A241223(n) - A241224(n) - A241225(n). %Y A241226 Cf. A190020. %K A241226 nonn,more %O A241226 1,2 %A A241226 _Martin Renner_, Apr 17 2014 %E A241226 a(7) from _Martin Renner_, May 31 2014 %E A241226 a(8)-a(22) from _Giovanni Resta_, May 31 2014