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 A268964 #7 Apr 19 2020 23:45:00 %S A268964 3,60,2016,136080,18852912,5712070032,3870355944960,5963413766987592, %T A268964 21098726553552173712,172587783760354579099176, %U A268964 3279749300107885644432555744,145303202175464965696103410891992 %N A268964 Number of n X n 0..2 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two not more than once. %C A268964 Diagonal of A268971. %H A268964 R. H. Hardin, <a href="/A268964/b268964.txt">Table of n, a(n) for n = 1..15</a> %e A268964 Some solutions for n=4 %e A268964 ..0..1..0..1. .1..0..0..1. .1..2..0..0. .2..1..2..2. .0..1..0..0 %e A268964 ..0..1..0..0. .1..0..2..1. .1..0..1..2. .2..1..2..1. .2..1..1..0 %e A268964 ..2..1..0..0. .1..2..2..1. .1..0..1..2. .2..2..2..1. .0..0..1..0 %e A268964 ..2..1..1..0. .2..2..2..2. .1..0..1..0. .1..1..2..1. .0..0..1..0 %Y A268964 Cf. A268971. %K A268964 nonn %O A268964 1,1 %A A268964 _R. H. Hardin_, Feb 16 2016