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 A241231 #16 Feb 16 2025 08:33:22 %S A241231 0,4,34,134,379,866,1718,3085,5149,8095,12188,17664,24781,33861,45269, %T A241231 59327,76461,97017,121458,150379,184053,223137,268117,319578,378132, %U A241231 444455,519178,602675,696102,800051,914995,1042094,1181858,1335414,1503251,1686811,1886417,2103007 %N A241231 Number of triangles, distinct up to congruence, on a centered hexagonal grid of size n. %C A241231 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241231 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241231 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Triangle.html">Triangle</a>. %F A241231 a(n) = A241232(n) + A241233(n) + A241234(n) = A241236(n) + A241237(n). %e A241231 For n = 2 the four kinds of non-congruent triangles are the following: %e A241231 /. * * * . * * . %e A241231 . * * . . * * . * . . * %e A241231 \. . . . . . * . %Y A241231 Cf. A028419, A241223. %K A241231 nonn %O A241231 1,2 %A A241231 _Martin Renner_, Apr 17 2014 %E A241231 a(7) from _Martin Renner_, May 31 2014 %E A241231 a(8)-a(14) from _Giovanni Resta_, May 31 2014 %E A241231 More terms from _Bert Dobbelaere_, Oct 17 2022