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 A241237 #16 Feb 16 2025 08:33:22 %S A241237 0,3,15,35,69,106,162,222,300,382,486,587,715,840,997,1147,1313,1491, %T A241237 1700,1890,2129,2341,2598,2842,3126,3394,3711,3995,4341,4641,5024, %U A241237 5349,5750,6128,6540,6959,7381,7772,8255,8722,9252,9688,10220,10698,11277,11806,12381,12905 %N A241237 Number of isosceles triangles, distinct up to congruence, on a centered hexagonal grid of size n. %C A241237 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241237 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241237 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/IsoscelesTriangle.html">Isosceles Triangle</a>. %F A241237 a(n) = A241231(n) - A241236(n). %e A241237 For n = 2 the three kinds of non-congruent isosceles triangles are the following: %e A241237 /. * * * * . %e A241237 . * * . . * . . * %e A241237 \. . . . * . %Y A241237 Cf. A189978, A241228. %K A241237 nonn %O A241237 1,2 %A A241237 _Martin Renner_, Apr 17 2014 %E A241237 a(7) from _Martin Renner_, May 31 2014 %E A241237 a(8)-a(21) from _Giovanni Resta_, May 31 2014 %E A241237 More terms from _Bert Dobbelaere_, Oct 17 2022