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 A241239 #13 Feb 16 2025 08:33:22 %S A241239 0,1,4,10,19,30,45,61,84,106,134,165,199,234,277,321,364,412,478,523, %T A241239 595 %N A241239 Number of obtuse isosceles triangles, distinct up to congruence, on a centered hexagonal grid of size n. %C A241239 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241239 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241239 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ObtuseTriangle.html">Obtuse Triangle</a>. %H A241239 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/IsoscelesTriangle.html">Isosceles Triangle</a>. %F A241239 a(n) = A241237(n) - A241238(n). %e A241239 For n = 2 the only kind of non-congruent obtuse isosceles triangles is the following: %e A241239 /* * %e A241239 . . * %e A241239 \. . %Y A241239 Cf. A190310, A241230. %K A241239 nonn,more %O A241239 1,3 %A A241239 _Martin Renner_, Apr 17 2014 %E A241239 a(7) from _Martin Renner_, May 31 2014 %E A241239 a(8)-a(21) from _Giovanni Resta_, May 31 2014