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 A198521 #11 Feb 22 2025 14:34:10 %S A198521 1,15,16714,1084539825,1814002928583853,72341858272434900016248 %N A198521 Number of n X n 0..4 arrays with values 0..4 introduced in row major order and each element equal to no more than two horizontal and vertical neighbors. %e A198521 Some solutions for n=4: %e A198521 ..0..0..0..0....0..0..0..0....0..0..0..0....0..0..0..0....0..0..0..0 %e A198521 ..0..1..1..2....0..1..1..1....0..1..1..0....0..1..1..1....0..1..1..2 %e A198521 ..2..1..3..0....2..1..3..4....2..1..3..0....2..1..0..2....3..0..0..3 %e A198521 ..1..2..1..0....1..3..4..0....3..0..0..0....2..0..0..2....3..4..0..1 %Y A198521 Diagonal of A198528. %K A198521 nonn,more %O A198521 1,2 %A A198521 _R. H. Hardin_, Oct 26 2011