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 A221730 #12 Oct 14 2023 13:17:57 %S A221730 0,44,464,6188,80160,1048566,13757056,180437354,2363318964, %T A221730 30930073844,404584067882,5290050421396 %N A221730 Number of 4 X n arrays of occupancy after each element stays put or moves to some horizontal, diagonal or antidiagonal neighbor, with every occupancy equal to zero or two. %e A221730 Some solutions for n=3: %e A221730 ..2..2..0....2..0..2....2..2..0....0..0..2....0..0..2....0..2..0....0..2..0 %e A221730 ..0..2..0....2..0..0....0..0..2....2..2..0....2..2..2....2..0..2....0..0..2 %e A221730 ..0..0..2....0..2..0....2..2..0....2..2..0....0..2..0....0..2..2....2..2..0 %e A221730 ..2..2..0....2..2..0....0..2..0....0..0..2....0..0..2....0..0..2....2..0..2 %Y A221730 Row 4 of A221728. %K A221730 nonn,more %O A221730 1,2 %A A221730 _R. H. Hardin_, Jan 22 2013