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 A241225 #11 Feb 16 2025 08:33:22 %S A241225 0,12,216,1104,3708,9396,20304,38868,68364,112632,176076,263832, %T A241225 381924,536424,735240,985896,1296540,1676508,2137392,2689248,3344244, %U A241225 4114020 %N A241225 Number of right triangles on a centered hexagonal grid of size n. %C A241225 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241225 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241225 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RightTriangle.html">Right Triangle</a>. %F A241225 a(n) = A241223(n) - A241224(n) - A241226(n). %Y A241225 Cf. A077435. %K A241225 nonn %O A241225 1,2 %A A241225 _Martin Renner_, Apr 17 2014 %E A241225 a(7) from _Martin Renner_, May 31 2014 %E A241225 a(8)-a(22) from _Giovanni Resta_, May 31 2014