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 A268897 #7 Apr 19 2020 23:44:42 %S A268897 0,36,1584,118584,17194680,5344944120,3679171151832,5729028666080112, %T A268897 20420870331077074152,167959998252207918469944, %U A268897 3205098856567970748329302728,142456755446316393140675874983544 %N A268897 Number of n X n 0..2 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two exactly once. %C A268897 Diagonal of A268904. %H A268897 R. H. Hardin, <a href="/A268897/b268897.txt">Table of n, a(n) for n = 1..15</a> %e A268897 Some solutions for n=4 %e A268897 ..2..0..1..0. .0..0..0..0. .2..1..2..1. .0..2..2..1. .1..0..1..2 %e A268897 ..1..0..0..1. .1..0..0..0. .0..1..2..0. .1..2..2..2. .1..0..1..0 %e A268897 ..1..0..0..0. .1..1..0..0. .2..2..1..2. .1..2..1..2. .1..0..1..2 %e A268897 ..1..0..0..0. .0..0..0..0. .2..2..1..0. .2..2..2..2. .1..2..1..1 %Y A268897 Cf. A268904. %K A268897 nonn %O A268897 1,2 %A A268897 _R. H. Hardin_, Feb 15 2016