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 A241228 #12 Feb 16 2025 08:33:22 %S A241228 0,20,252,1296,4232,10668,22956,43832,77160,126972,198176,296976, %T A241228 429252,602876,825552,1106376,1454192,1879956,2397024,3014312,3747564, %U A241228 4609476 %N A241228 Number of isosceles triangles on a centered hexagonal grid of size n. %C A241228 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241228 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241228 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/IsoscelesTriangle.html">Isosceles Triangle</a>. %F A241228 a(n) = A241223(n) - A241227(n). %Y A241228 Cf. A186434. %K A241228 nonn,more %O A241228 1,2 %A A241228 _Martin Renner_, Apr 17 2014 %E A241228 a(7) from _Martin Renner_, May 31 2014 %E A241228 a(8)-a(22) from _Giovanni Resta_, May 31 2014