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 A221331 #8 Sep 05 2025 13:23:06 %S A221331 1,577,33873,2460824,162430981,10794156688,711858713069, %T A221331 46910596287335,3089778765626866,203426990390736036, %U A221331 13389504827335440162,881130781309712234876,57978558022828544235124,3814742757245230038689495 %N A221331 Number of 4 X n arrays of occupancy after each element stays put or moves to some horizontal or antidiagonal neighbor, with no occupancy greater than 2. %C A221331 Row 4 of A221328. %H A221331 R. H. Hardin, <a href="/A221331/b221331.txt">Table of n, a(n) for n = 1..27</a> %e A221331 Some solutions for n=3 %e A221331 ..2..0..1....1..2..0....2..1..1....1..1..1....1..1..0....2..0..1....1..1..2 %e A221331 ..2..2..0....1..1..0....1..0..0....1..0..2....2..0..0....0..2..1....1..2..1 %e A221331 ..1..0..2....1..1..0....1..2..0....0..2..0....2..2..1....1..2..1....0..1..2 %e A221331 ..0..2..0....1..2..2....0..2..2....1..2..1....1..0..2....1..1..0....0..0..1 %Y A221331 Cf. A221328. %K A221331 nonn,changed %O A221331 1,2 %A A221331 _R. H. Hardin_, Jan 11 2013