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 A229840 #7 Jun 02 2025 08:38:42 %S A229840 0,15,987,453837,1296261759,23643209345079,2745117942652795617 %N A229840 Number of n X n 0..2 arrays x(i,j) with each element horizontally or vertically next to at least one element with value 2-x(i,j). %C A229840 Diagonal of A229847 %e A229840 Some solutions for n=3 %e A229840 ..0..2..0....1..1..2....0..0..0....2..0..2....1..1..1....1..2..0....1..0..0 %e A229840 ..1..0..0....1..1..0....2..2..2....1..1..2....0..2..1....1..0..2....1..2..2 %e A229840 ..1..2..2....1..1..2....2..0..0....2..0..0....0..2..0....1..2..0....2..0..0 %K A229840 nonn %O A229840 1,2 %A A229840 _R. H. Hardin_, Oct 01 2013