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 A215921 #6 Jun 22 2021 14:37:23 %S A215921 1,2,4,42,136,2906,12840,433252,2387180,113503692,742181112, %T A215921 46538017584,350038464728,27700815674032,234578644301776, %U A215921 22702140562948192,213078984368820288,24626774613248937584,253140626416265149920 %N A215921 Number of diagonal and antidiagonal neighbor colorings of the odd squares of an n X 3 array with new integer colors introduced in row major order. %C A215921 Column 3 of A215924. %H A215921 R. H. Hardin, <a href="/A215921/b215921.txt">Table of n, a(n) for n = 1..98</a> %e A215921 Some solutions for n=4 %e A215921 ..x..0..x....x..0..x....x..0..x....x..0..x....x..0..x....x..0..x....x..0..x %e A215921 ..1..x..2....1..x..1....1..x..2....1..x..1....1..x..2....1..x..2....1..x..1 %e A215921 ..x..3..x....x..2..x....x..0..x....x..2..x....x..0..x....x..3..x....x..2..x %e A215921 ..1..x..1....3..x..1....3..x..4....1..x..1....2..x..2....4..x..5....0..x..1 %Y A215921 Cf. A215924. %K A215921 nonn %O A215921 1,2 %A A215921 _R. H. Hardin_ Aug 27 2012