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 A274889 #6 Apr 21 2020 12:40:44 %S A274889 1,4,16,104,913,12217,237807,6843349,292253909,18485204565, %T A274889 1735778832010,242007116498916,50034244634072936,15365321608796091811 %N A274889 Number of n X n 0..2 arrays with no element equal to any value at offset (0,-1), (-1,-1) or (-2,0) and new values introduced in order 0..2. %C A274889 Diagonal of A274895. %e A274889 Some solutions for n=4 %e A274889 ..0..1..0..1. .0..1..2..0. .0..1..2..0. .0..1..2..0. .0..1..2..1 %e A274889 ..1..2..0..1. .0..1..2..0. .1..2..0..1. .0..1..2..0. .0..2..0..1 %e A274889 ..1..0..1..2. .1..2..0..1. .2..0..1..2. .1..2..0..1. .1..2..1..2 %e A274889 ..2..0..1..2. .1..2..1..2. .2..0..1..0. .2..0..1..2. .2..0..1..2 %Y A274889 Cf. A274895. %K A274889 nonn %O A274889 1,2 %A A274889 _R. H. Hardin_, Jul 10 2016