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 A239593 #7 Jun 02 2025 09:29:04 %S A239593 2,3,31,643,38824,12509870,11135057785 %N A239593 Number of n X n 0..3 arrays with no element equal to zero plus the sum of elements to its left or one plus the sum of the elements above it or one plus the sum of the elements diagonally to its northwest or one plus the sum of the elements antidiagonally to its northeast, modulo 4. %C A239593 Diagonal of A239599 %e A239593 Some solutions for n=4 %e A239593 ..3..2..2..0....3..0..0..0....3..2..2..0....3..2..3..2....3..2..2..0 %e A239593 ..2..1..2..2....3..2..3..2....2..1..2..0....2..1..2..2....2..1..2..0 %e A239593 ..3..2..2..2....2..1..1..2....3..1..3..0....3..2..0..0....3..2..3..3 %e A239593 ..2..3..2..0....3..2..2..0....3..2..2..2....2..1..0..0....2..1..2..3 %K A239593 nonn %O A239593 1,1 %A A239593 _R. H. Hardin_, Mar 22 2014