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 A266682 #6 Aug 12 2016 06:53:59 %S A266682 2,5,15,59,257,1205,5879,29693,153097,806375,4307317,23313947, %T A266682 127420533,702562447,3900415155,21786817517,122309555899,689707853981, %U A266682 3904123962093,22174631940293,126322762595339,721552901977497 %N A266682 Number of 3Xn arrays containing n copies of 0..3-1 with no element 1 greater than its northwest or northeast neighbor modulo 3 and the upper left element equal to 0. %C A266682 Row 3 of A266681. %H A266682 R. H. Hardin, <a href="/A266682/b266682.txt">Table of n, a(n) for n = 1..72</a> %e A266682 Some solutions for n=4 %e A266682 ..0..2..1..2....0..2..1..2....0..2..0..1....0..0..2..0....0..1..2..1 %e A266682 ..1..0..1..0....2..0..2..1....1..2..1..2....0..2..2..1....1..2..0..2 %e A266682 ..2..1..2..0....0..1..0..1....2..0..1..0....1..2..1..1....1..0..2..0 %Y A266682 Cf. A266681. %K A266682 nonn %O A266682 1,1 %A A266682 _R. H. Hardin_, Jan 02 2016