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 A275137 #7 Apr 19 2020 23:50:02 %S A275137 1,6,80,1088,9744,165568,4055152,147762240,7887576208,624224888096, %T A275137 72861217834416,12595738987019680,3224446583629649792, %U A275137 1224344356792821362688,689079095818717776787184 %N A275137 Number of n X n 0..2 arrays with no element equal to any value at offset (-2,-2) (-1,-2) or (0,-1) and new values introduced in order 0..2. %C A275137 Diagonal of A275142. %H A275137 R. H. Hardin, <a href="/A275137/b275137.txt">Table of n, a(n) for n = 1..19</a> %e A275137 Some solutions for n=5 %e A275137 ..0..1..2..0..1. .0..1..2..1..0. .0..1..2..1..0. .0..1..2..1..0 %e A275137 ..2..0..1..2..1. .2..0..1..0..1. .2..0..1..2..0. .2..0..1..2..1 %e A275137 ..1..2..1..2..0. .1..0..1..2..0. .1..2..1..2..0. .1..2..1..2..0 %e A275137 ..0..2..0..1..0. .1..2..0..2..0. .1..2..0..1..0. .0..2..0..1..0 %e A275137 ..2..1..2..1..2. .1..0..2..1..2. .0..1..2..1..2. .2..0..2..0..2 %Y A275137 Cf. A275142. %K A275137 nonn %O A275137 1,2 %A A275137 _R. H. Hardin_, Jul 17 2016