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 A208401 #9 May 12 2023 15:08:06 %S A208401 1,15,10682,163422914,39216100214432,146298576850884166522, %T A208401 8484183047877022205116428184,7648479804732030712438650610523878146, %U A208401 107185313935414834818609318544471943157967376388 %N A208401 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 two of its immediate leftward or upward or right-upward antidiagonal neighbors. %C A208401 Diagonal of A208408. %H A208401 R. H. Hardin, <a href="/A208401/b208401.txt">Table of n, a(n) for n = 1..13</a> %e A208401 Some solutions for n=4 %e A208401 ..0..1..1..2....0..0..0..1....0..1..0..0....0..1..1..0....0..0..1..0 %e A208401 ..0..1..0..0....0..1..1..0....2..0..3..0....0..0..2..0....2..0..1..2 %e A208401 ..0..0..3..0....0..1..1..0....2..0..3..1....0..2..1..1....1..3..0..0 %e A208401 ..1..1..2..0....0..0..0..1....1..2..0..2....3..1..0..0....1..3..2..2 %Y A208401 Cf. A208408. %K A208401 nonn %O A208401 1,2 %A A208401 _R. H. Hardin_, Feb 26 2012