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 A239698 #34 Feb 16 2025 08:33:21 %S A239698 0,2,6,11,18,25,35,45,56,69,83,97,115,131,150,169,189,212,234,258,284 %N A239698 Number of equilateral triangles, distinct up to congruence, on a centered hexagonal grid of size n. %C A239698 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A239698 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A239698 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/EquilateralTriangle.html">Equilateral Triangle</a>. %e A239698 For n = 2 the two kinds of non-congruent equilateral triangles are the following: %e A239698 /. * * . %e A239698 . * * . . * %e A239698 \. . * . %Y A239698 Cf. A008893. %K A239698 nonn,more %O A239698 1,2 %A A239698 _Martin Renner_, Apr 17 2014 %E A239698 a(7) from _Martin Renner_, May 31 2014 %E A239698 a(8)-a(21) from _Giovanni Resta_, May 31 2014