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 A221272 #7 Jul 23 2025 01:47:50 %S A221272 1,52,596,7252,94659,1250015,16474842,217194702,2863102280, %T A221272 37731750319,497133557199,6548899949298,86261110576994, %U A221272 1136131886838454 %N A221272 Number of nX4 arrays of occupancy after each element moves to some king-move neighbor, with every occupancy equal to zero or two. %C A221272 Column 4 of A221273 %e A221272 Some solutions for n=4 %e A221272 ..0..2..2..0....0..2..0..2....0..2..2..2....0..2..2..0....0..2..2..2 %e A221272 ..2..0..0..2....0..0..2..0....0..0..0..0....2..0..2..0....0..0..0..0 %e A221272 ..0..2..2..0....2..2..0..2....2..0..2..0....0..2..0..2....0..2..2..2 %e A221272 ..0..2..2..0....2..0..0..2....2..2..2..0....0..2..2..0....0..0..2..2 %K A221272 nonn %O A221272 1,2 %A A221272 _R. H. Hardin_ Jan 08 2013