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 A241234 #16 Feb 16 2025 08:33:22 %S A241234 0,1,15,72,220,528,1076,1965,3314,5254,7954,11589,16306,22349,29939, %T A241234 39305,50738,64462,80775,100096,122602,148748,178834,213268,252439, %U A241234 296857,346878,402778,465346,534966,611966,697143,790749,893638,1006128,1129134,1262932,1408116,1565674 %N A241234 Number of obtuse triangles, distinct up to congruence, on a centered hexagonal grid of size n. %C A241234 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241234 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241234 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ObtuseTriangle.html">Obtuse Triangle</a>. %F A241234 a(n) = A241231(n) - A241232(n) - A241233(n) %e A241234 For n = 2 the only kind of non-congruent obtuse triangles is the following: %e A241234 /* * %e A241234 . . * %e A241234 \. . %Y A241234 Cf. A190022, A241226. %K A241234 nonn %O A241234 1,3 %A A241234 _Martin Renner_, Apr 17 2014 %E A241234 a(7) from _Martin Renner_, May 31 2014 %E A241234 a(8)-a(14) from _Giovanni Resta_, May 31 2014 %E A241234 More terms from _Bert Dobbelaere_, Oct 17 2022