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 A241223 #18 Feb 16 2025 08:33:22 %S A241223 0,32,900,7380,34676,118044,325872,775856,1653888,3237984,5923028, %T A241223 10249596,16938588,26924036,41393424,61830480,90059672,128293728, %U A241223 179185500,245889068,332107188,442162836,581060024,754545360,969196896,1232477192,1552824900 %N A241223 Number of triangles on a centered hexagonal grid of size n. %C A241223 A centered hexagonal grid of size n is a grid with A003215(n-1) points forming a hexagonal lattice. %H A241223 Andrew Howroyd, <a href="/A241223/b241223.txt">Table of n, a(n) for n = 1..200</a> %H A241223 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HexNumber.html">Hex Number</a>. %H A241223 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Triangle.html">Triangle</a>. %F A241223 a(n) = A240826(n) - A241222(n). %F A241223 a(n) = A241224(n) + A241225(n) + A241226(n) = A241227(n) + A241228(n). %e A241223 For n = 2 the 32 triangles are the following: %e A241223 /. * * * * . . . . . . . . * * * %e A241223 . * * . * . * * . * * . . * . . * * * . * . . . %e A241223 \. . . . . . * . * * . * . . * . %e A241223 - %e A241223 /* . . . . * * . . * * * * . . * %e A241223 * . . * . * . . . . . * . . * . . . * . * * . . %e A241223 \. * * . * * . * * . . * . . * . %e A241223 - %e A241223 /* . . . * * * * * . . . . . . * %e A241223 . . . * . * . . * * . . * . . * . . . . * . . * %e A241223 \* * . * . . . . * . * * * * . * %e A241223 - %e A241223 /* . . * * . . . . . . * * . . * %e A241223 . * * * * . . * . * * . . * * . * . . . * * . . %e A241223 \. . . . * . . * * . . * * . . * %Y A241223 Cf. A045996. %K A241223 nonn %O A241223 1,2 %A A241223 _Martin Renner_, Apr 17 2014 %E A241223 a(7) from _Martin Renner_, May 31 2014 %E A241223 a(8)-a(22) from _Giovanni Resta_, May 31 2014 %E A241223 Terms a(23) and beyond from _Andrew Howroyd_, Sep 18 2017