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 A241233 #16 Feb 16 2025 08:33:22 %S A241233 0,1,5,13,25,42,62,86,115,150,191,234,282,334,395,455,526,601,677,762, %T A241233 855,947,1045,1152,1261,1378,1498,1619,1757,1900,2041,2176,2334,2507, %U A241233 2661,2838,3011,3174,3379,3577,3773,3967,4179,4389,4618,4848,5090,5311,5559,5792,6068 %N A241233 Number of right triangles, distinct up to congruence, on a centered hexagonal grid of size n. %C A241233 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241233 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241233 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/RightTriangle.html">Right Triangle</a>. %F A241233 a(n) = A241231(n) - A241232(n) - A241234(n). %e A241233 For n = 2 the only kind of non-congruent right triangles is the following: %e A241233 /. * %e A241233 * . * %e A241233 \. . %Y A241233 Cf. A189979, A241225. %K A241233 nonn %O A241233 1,3 %A A241233 _Martin Renner_, Apr 17 2014 %E A241233 a(7) from _Martin Renner_, May 31 2014 %E A241233 a(8)-a(21) from _Giovanni Resta_, May 31 2014 %E A241233 More terms from _Bert Dobbelaere_, Oct 17 2022