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 A241238 #15 Feb 16 2025 08:33:22 %S A241238 0,2,11,25,50,76,117,161,216,276,352,422,516,606,720,826,949,1079, %T A241238 1222,1367,1534 %N A241238 Number of acute isosceles triangles, distinct up to congruence, on a centered hexagonal grid of size n. %C A241238 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241238 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241238 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/AcuteTriangle.html">Acute Triangle</a>. %H A241238 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/IsoscelesTriangle.html">Isosceles Triangle</a>. %F A241238 a(n) = A241237(n) - A241239(n). %e A241238 For n = 2 the two kinds of non-congruent acute isosceles triangles are the following: %e A241238 /. * * . %e A241238 . * * . . * %e A241238 \. . * . %Y A241238 Cf. A190309, A241229. %K A241238 nonn,more %O A241238 1,2 %A A241238 _Martin Renner_, Apr 17 2014 %E A241238 a(7) from _Martin Renner_, May 31 2014 %E A241238 a(8)-a(21) from _Giovanni Resta_, May 31 2014