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 A268791 #7 Apr 19 2020 23:43:47 %S A268791 0,22,248,4800,168740,11138352,1384570516,325815151556, %T A268791 145913571668076,124768928833331732,204377241703937153076, %U A268791 642788798787566885184552,3889478620658657294120765368,45350632160719542617837369658508 %N A268791 Number of n X n 0..2 arrays with some element plus some horizontally, vertically or antidiagonally adjacent neighbor totalling two exactly once. %C A268791 Diagonal of A268798. %H A268791 R. H. Hardin, <a href="/A268791/b268791.txt">Table of n, a(n) for n = 1..18</a> %e A268791 Some solutions for n=4 %e A268791 ..1..0..0..0. .2..1..2..1. .2..2..1..0. .1..2..1..2. .2..2..2..1 %e A268791 ..1..0..1..0. .2..2..2..2. .1..0..0..0. .2..2..2..2. .2..1..2..2 %e A268791 ..0..0..0..0. .2..2..2..2. .0..1..0..1. .2..1..2..1. .2..2..2..1 %e A268791 ..1..0..0..0. .1..2..1..1. .0..0..0..0. .2..1..2..2. .2..2..2..1 %Y A268791 Cf. A268798. %K A268791 nonn %O A268791 1,2 %A A268791 _R. H. Hardin_, Feb 13 2016