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 A268632 #7 Apr 19 2020 23:41:14 %S A268632 0,24,840,47640,5419992,1328460312,725187096504,897937832100888, %T A268632 2550310193527246056,16737195174968513278656, %U A268632 255111495114527014241534160,9064348272085612349020943764248,752834148761621410865592159869765856 %N A268632 Number of n X n 0..2 arrays with some element plus some horizontally or vertically adjacent neighbor totalling two exactly once. %C A268632 Diagonal of A268639. %H A268632 R. H. Hardin, <a href="/A268632/b268632.txt">Table of n, a(n) for n = 1..14</a> %e A268632 Some solutions for n=4 %e A268632 ..0..0..0..1. .2..2..0..1. .0..0..0..0. .0..0..0..0. .1..0..1..0 %e A268632 ..1..0..0..0. .1..2..1..2. .0..1..0..0. .1..0..0..0. .2..1..2..0 %e A268632 ..0..1..0..1. .2..1..2..1. .1..0..1..0. .1..0..0..0. .1..0..1..0 %e A268632 ..0..0..0..2. .1..0..1..0. .2..0..0..0. .2..1..0..1. .0..0..0..0 %Y A268632 Cf. A268639. %K A268632 nonn %O A268632 1,2 %A A268632 _R. H. Hardin_, Feb 09 2016