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 A208304 #7 May 18 2020 19:27:43 %S A208304 1,15,877,433252,656404264,2227156082842,14216420563705724, %T A208304 152834703739876070716,2562708954761302163168116, %U A208304 63338792205478132108785337770,2209363673047740561377241754826088,105080298194482216982100203776114994146,6626004995903793205763315247117544349120572 %N A208304 Number of 4 X n nonnegative integer arrays with new values 0 upwards introduced in row major order and no element equal to any horizontal or antidiagonal neighbor (colorings ignoring permutations of colors). %H A208304 Andrew Howroyd, <a href="/A208304/b208304.txt">Table of n, a(n) for n = 0..100</a> %e A208304 Some solutions for n=4: %e A208304 ..0..1..0..1....0..1..0..1....0..1..0..1....0..1..0..1....0..1..0..1 %e A208304 ..0..1..2..0....0..1..0..1....0..1..0..1....0..1..0..1....0..1..0..2 %e A208304 ..0..1..2..0....0..1..0..1....0..1..0..1....2..1..0..1....2..1..0..1 %e A208304 ..0..1..2..0....2..1..0..1....0..1..0..1....0..1..0..1....0..1..0..2 %Y A208304 Row 4 of A208301. %K A208304 nonn %O A208304 0,2 %A A208304 _R. H. Hardin_, Feb 25 2012 %E A208304 a(0)=1 prepended and terms a(8) and beyond from _Andrew Howroyd_, May 18 2020