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 A268621 #7 Apr 14 2020 21:36:24 %S A268621 3,42,1086,55452,6000978,1430057208,767057092212,938662461734076, %T A268621 2643885168776274078,17245474696462104041796, %U A268621 261641272761781630830646332,9262823664147270920479995625968,767111588793900754143144516979246884 %N A268621 Number of n X n 0..2 arrays with some element plus some horizontally or vertically adjacent neighbor totaling two no more than once. %C A268621 Diagonal of A268628. %H A268621 R. H. Hardin, <a href="/A268621/b268621.txt">Table of n, a(n) for n = 1..14</a> %e A268621 Some solutions for n=4 %e A268621 ..2..1..2..2. .0..0..0..0. .0..0..1..0. .2..1..0..0. .1..1..0..0 %e A268621 ..1..0..1..2. .0..1..1..0. .0..0..0..1. .1..0..0..0. .2..2..1..0 %e A268621 ..2..1..2..1. .1..0..0..1. .1..0..1..2. .0..1..0..0. .2..2..2..1 %e A268621 ..2..2..2..0. .0..0..1..0. .0..1..2..2. .0..0..0..1. .2..2..1..0 %Y A268621 Cf. A268628. %K A268621 nonn %O A268621 1,1 %A A268621 _R. H. Hardin_, Feb 09 2016