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 A208169 #9 Jul 22 2025 21:13:23 %S A208169 1,13,8469,99752346,16567902881244,38625516979536823947, %T A208169 1263976383376771173195964686,580579239138791881884932674640711196, %U A208169 3743190521310525744113304412007474906812935048 %N A208169 Number of n X n 0..3 arrays with new values 0..3 introduced in row major order and no element equal to more than one of its immediate leftward or upward neighbors. %C A208169 Diagonal of A208175 %H A208169 R. H. Hardin, <a href="/A208169/b208169.txt">Table of n, a(n) for n = 1..10</a> %e A208169 Some solutions for n=4 %e A208169 ..0..0..0..0....0..0..0..1....0..1..0..0....0..0..0..1....0..0..0..0 %e A208169 ..0..1..1..2....1..1..0..0....2..2..1..2....1..0..1..2....1..1..1..1 %e A208169 ..1..0..1..3....2..3..3..0....1..1..3..2....2..2..1..0....2..1..2..3 %e A208169 ..1..3..3..1....0..2..1..3....0..3..0..0....0..3..0..1....0..1..2..0 %K A208169 nonn %O A208169 1,2 %A A208169 _R. H. Hardin_ Feb 24 2012