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 A208019 #8 Jun 25 2017 17:13:51 %S A208019 5,87,27093,30066912,80318704605,421673189900658,3840411377316733859, %T A208019 55738840149772892317910,1212109474086951274342594323, %U A208019 37684550247867169399313491413883,1614317526939802578784333734266333582,92481709137707355217408169152041892948306 %N A208019 Number of n X 4 nonnegative integer arrays with new values 0 upwards introduced in row major order and no element equal to any horizontal, vertical, diagonal or antidiagonal neighbor (colorings ignoring permutations of colors). %H A208019 Andrew Howroyd, <a href="/A208019/b208019.txt">Table of n, a(n) for n = 1..50</a> %e A208019 Some solutions for n=4 %e A208019 ..0..1..2..1....0..1..0..1....0..1..0..1....0..1..0..1....0..1..0..1 %e A208019 ..2..3..0..3....2..3..2..3....2..3..2..3....2..3..2..3....2..3..2..3 %e A208019 ..0..4..1..2....4..0..5..0....0..1..0..1....0..1..0..4....0..1..4..0 %e A208019 ..1..2..3..4....1..2..3..1....4..2..3..5....2..3..2..1....2..3..2..1 %Y A208019 Column 4 of A208021. %K A208019 nonn %O A208019 1,1 %A A208019 _R. H. Hardin_, Feb 22 2012 %E A208019 Terms a(8) and beyond from _Andrew Howroyd_, Jun 25 2017