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 A267020 #15 Feb 02 2023 15:06:42 %S A267020 1,5,21,259,5493,255437,26320280,6071784885 %N A267020 Number of (n+1)Xn arrays containing n copies of 0..n with every element equal to or 1 greater than any northeast or northwest neighbors modulo n and the upper left element equal to 0. %C A267020 Subdiagonal 1 of A267019. %e A267020 Some solutions for n=4 %e A267020 ..0..4..1..3....0..2..4..1....0..3..1..3....0..2..0..3....0..2..4..2 %e A267020 ..4..1..4..2....3..0..2..0....4..1..3..1....3..1..3..1....2..0..2..4 %e A267020 ..2..0..2..4....0..3..1..3....2..4..1..4....1..4..1..4....0..3..0..3 %e A267020 ..0..2..0..3....4..1..3..1....0..2..4..2....0..2..4..2....3..1..3..1 %e A267020 ..3..1..3..1....2..4..2..4....3..0..2..0....2..0..3..4....1..4..1..4 %Y A267020 Cf. A267020. %K A267020 nonn,more %O A267020 1,2 %A A267020 _R. H. Hardin_, Jan 09 2016