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 A221135 #6 Jun 02 2025 08:11:17 %S A221135 4,289,12544,643204,31203396,1539699121,75512292025,3709710975721, %T A221135 182118668668225,8941989810789025,439009303948483984, %U A221135 21553427021714858449,1058163003670012270144,51950295646215686851681 %N A221135 Number of nX4 arrays of occupancy after each element moves to some horizontal or vertical neighbor, with no occupancy greater than 2. %C A221135 Column 4 of A221139 %H A221135 R. H. Hardin, <a href="/A221135/b221135.txt">Table of n, a(n) for n = 1..23</a> %e A221135 Some solutions for n=3 %e A221135 ..2..2..2..1....2..1..1..1....1..2..2..0....0..1..2..0....2..2..1..0 %e A221135 ..1..0..1..1....2..1..0..1....1..0..1..2....2..2..1..0....0..1..1..1 %e A221135 ..1..0..0..1....0..2..1..0....1..1..0..1....0..1..2..1....1..2..0..1 %K A221135 nonn %O A221135 1,1 %A A221135 _R. H. Hardin_ Jan 02 2013