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 A208868 #7 Jul 22 2025 21:32:28 %S A208868 15,1798,307165,52947540,9130195864,1574432811027,271499374919883, %T A208868 46818078167209471,8073434644321782464,1392204668383719239473, %U A208868 240075497495975972163390,41399261047580372916745574 %N A208868 Number of nX4 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 or right-upward antidiagonal neighbors. %C A208868 Column 4 of A208872 %H A208868 R. H. Hardin, <a href="/A208868/b208868.txt">Table of n, a(n) for n = 1..210</a> %F A208868 Empirical: a(n) = 198*a(n-1) -4637*a(n-2) +40574*a(n-3) -164314*a(n-4) +321776*a(n-5) -281785*a(n-6) +85300*a(n-7) -8100*a(n-8) for n>9 %e A208868 Some solutions for n=4 %e A208868 ..0..0..1..0....0..1..1..2....0..1..0..0....0..0..1..2....0..0..0..1 %e A208868 ..1..2..2..0....0..2..0..0....2..2..1..2....1..3..3..0....1..1..2..0 %e A208868 ..2..3..3..0....0..1..3..0....1..2..3..2....1..0..1..2....0..1..1..3 %e A208868 ..1..2..3..2....1..2..2..0....0..3..0..0....1..2..0..0....3..0..3..1 %K A208868 nonn %O A208868 1,1 %A A208868 _R. H. Hardin_ Mar 02 2012