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 A221901 #5 Apr 22 2021 14:33:23 %S A221901 13,2305,235332,25821731,2815434576,307126807101,33499434763000, %T A221901 3654072363322660,398576690257243663,43475776796985596792, %U A221901 4742231413650315959473 %N A221901 Number of 4 X n 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 A221901 Row 4 of A221898. %e A221901 Some solutions for n=3 %e A221901 ..1..0..1....0..2..0....0..1..2....0..2..1....0..0..2....0..0..0....0..2..1 %e A221901 ..0..3..2....1..1..2....2..0..1....2..0..1....4..0..0....2..2..2....1..1..0 %e A221901 ..1..0..1....0..0..3....1..0..2....0..2..4....1..0..0....1..0..0....2..2..2 %e A221901 ..2..0..1....2..1..0....0..3..0....0..0..0....0..5..0....2..1..2....0..0..1 %Y A221901 Cf. A221898. %K A221901 nonn %O A221901 1,1 %A A221901 _R. H. Hardin_ Jan 30 2013