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 A197273 #12 Sep 07 2025 12:22:01 %S A197273 1,9,34,627,16890,1276986,209785488,84453386767,71088592371829 %N A197273 Number of n X n 0..4 arrays with each element x equal to the number of its horizontal and vertical neighbors equal to 2,0,1,1,1 for x=0,1,2,3,4. %C A197273 Every 0 is next to 0 2's, every 1 is next to 1 0's, every 2 is next to 2 1's, every 3 is next to 3 1's, every 4 is next to 4 1's. %e A197273 Some solutions containing all values 0 to 4 for n=4: %e A197273 ..1..0..1..2....0..1..1..0....1..3..1..0....2..1..0..1....1..0..1..2 %e A197273 ..1..1..4..1....1..4..1..0....0..1..4..1....1..4..1..3....3..1..4..1 %e A197273 ..0..1..1..0....3..1..4..1....1..4..1..1....0..1..4..1....1..4..1..0 %e A197273 ..0..1..3..1....1..0..1..2....2..1..0..0....0..1..1..0....0..1..1..0 %Y A197273 Diagonal of A197280. %K A197273 nonn,more,changed %O A197273 1,2 %A A197273 _R. H. Hardin_, Oct 12 2011