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 A241224 #11 Feb 16 2025 08:33:22 %S A241224 0,8,204,1788,8690,30360,85194,205394,441876,870912,1601708,2783574, %T A241224 4616220,7358312,11339430,16972182,24763604,35328426,49405944, %U A241224 67873484,91762128,122276784 %N A241224 Number of acute triangles on a centered hexagonal grid of size n. %C A241224 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241224 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241224 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/AcuteTriangle.html">Acute Triangle</a>. %F A241224 a(n) = A241223(n) - A241225(n) - A241226(n). %e A241224 For n = 2 the eight acute triangles are the following: %e A241224 /. * * * * . . . . . . . * . . * %e A241224 . * * . * . * * . * * . . * . . * * . . * * . . %e A241224 \. . . . . . * . * * . * * . . * %Y A241224 Cf. A190019, A241223. %K A241224 nonn %O A241224 1,2 %A A241224 _Martin Renner_, Apr 17 2014 %E A241224 a(7) from _Martin Renner_, May 31 2014 %E A241224 a(8)-a(22) from _Giovanni Resta_, May 31 2014