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 A221831 #6 Jul 23 2025 02:16:51 %S A221831 0,64,2930,115417,4715043,192788993,7888216533,322723834959, %T A221831 13203608881221,540203150208573,22101534579211095,904249026745794715, %U A221831 36995912673327194833,1513629078095632460534,61927733346071816062294 %N A221831 Number of 4Xn arrays of occupancy after each element moves to some horizontal, vertical or antidiagonal neighbor, without 2-loops or left turns. %C A221831 Row 4 of A221828 %H A221831 R. H. Hardin, <a href="/A221831/b221831.txt">Table of n, a(n) for n = 1..25</a> %e A221831 Some solutions for n=3 %e A221831 ..1..1..1....1..1..0....1..1..1....0..2..0....0..3..1....0..2..0....1..1..1 %e A221831 ..2..0..1....2..1..1....2..2..0....1..2..0....0..1..1....2..2..0....1..2..0 %e A221831 ..1..1..0....1..0..2....2..0..0....0..2..1....1..1..1....2..1..0....1..1..1 %e A221831 ..2..1..1....1..1..1....1..1..1....1..3..0....1..2..0....1..1..1....1..2..0 %K A221831 nonn %O A221831 1,2 %A A221831 _R. H. Hardin_ Jan 26 2013