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 A233019 #8 Jul 23 2025 07:47:15 %S A233019 1,15,849,179999,149128385,487651369463,6338787392715565, %T A233019 328918675019714663095,68332288847776936953486283, %U A233019 56955693489584720827393016043135 %N A233019 Number of n X n 0..3 arrays with no element x(i,j) adjacent to value 3-x(i,j) horizontally, vertically, diagonally or antidiagonally, and top left element zero. %C A233019 Diagonal of A233026 %H A233019 R. H. Hardin, <a href="/A233019/b233019.txt">Table of n, a(n) for n = 1..11</a> %e A233019 Some solutions for n=4 %e A233019 ..0..0..2..3....0..0..2..3....0..0..2..2....0..2..2..0....0..2..0..0 %e A233019 ..0..2..2..2....2..0..2..2....2..0..0..2....0..2..0..2....2..0..2..2 %e A233019 ..0..2..3..3....2..2..0..2....0..0..0..0....0..2..2..2....0..2..0..2 %e A233019 ..0..2..2..3....0..0..2..2....0..0..0..0....0..2..0..2....2..2..0..0 %K A233019 nonn %O A233019 1,2 %A A233019 _R. H. Hardin_, Dec 03 2013