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 A221940 #6 Jul 24 2025 19:41:07 %S A221940 1,489,24535,1759041,119035112,8118923949,552777330287,37650441405451, %T A221940 2564181255574899,174635497977344862,11893675692936218866, %U A221940 810026663520753630706,55167419097633745309553,3757214394165203187203557 %N A221940 Number of 4Xn arrays of occupancy after each element stays put or moves to some horizontal or antidiagonal neighbor, without move-in move-out straight through or left turns. %C A221940 Row 4 of A221937 %H A221940 R. H. Hardin, <a href="/A221940/b221940.txt">Table of n, a(n) for n = 1..24</a> %e A221940 Some solutions for n=3 %e A221940 ..1..3..0....0..1..1....2..0..1....1..0..1....0..1..1....0..1..0....1..1..0 %e A221940 ..0..0..1....3..0..1....1..0..0....1..3..0....3..0..1....2..3..0....1..4..0 %e A221940 ..1..2..0....0..2..0....2..3..0....0..1..1....1..1..3....1..1..2....0..2..0 %e A221940 ..2..1..1....2..1..1....1..1..1....2..1..1....0..1..0....1..0..1....0..2..1 %K A221940 nonn %O A221940 1,2 %A A221940 _R. H. Hardin_ Feb 01 2013