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 A221896 #6 Jul 23 2025 02:19:04 %S A221896 13,2241,236532,25821731,2814706089,307131177266,33498766685333, %T A221896 3653989849310339,398568427571535443,43474867097541833158, %U A221896 4742132126980966864298,517260183698295861792396 %N A221896 Number of nX4 arrays of occupancy after each element stays put or moves to some horizontal, vertical or antidiagonal neighbor, without move-in move-out straight through or left turns. %C A221896 Column 4 of A221898 %H A221896 R. H. Hardin, <a href="/A221896/b221896.txt">Table of n, a(n) for n = 1..58</a> %e A221896 Some solutions for n=3 %e A221896 ..1..2..1..0....1..0..2..1....0..2..2..0....0..1..0..2....1..1..0..0 %e A221896 ..0..1..1..2....0..3..1..1....1..0..0..1....2..3..0..1....0..2..4..1 %e A221896 ..1..0..3..0....0..1..2..0....0..5..0..1....0..2..0..1....1..1..0..1 %K A221896 nonn %O A221896 1,1 %A A221896 _R. H. Hardin_ Jan 30 2013