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 A265871 #6 Aug 12 2016 06:53:59 %S A265871 6,42,340,3136,30456,307464,3182784,33588630,359679572,3896854004, %T A265871 42621505752,469855280984,5214136065200,58192269283872, %U A265871 652647993512352,7351118301680664,83112561412246944,942832221757264300 %N A265871 Number of 3Xn arrays containing n copies of 0..3-1 with no element plus any antidiagonal neighbor equal to 3-1. %C A265871 Row 3 of A265870. %H A265871 R. H. Hardin, <a href="/A265871/b265871.txt">Table of n, a(n) for n = 1..87</a> %e A265871 Some solutions for n=4 %e A265871 ..2..0..1..1....2..1..1..2....0..1..0..1....2..0..2..1....0..1..2..1 %e A265871 ..1..0..2..0....0..0..1..0....2..0..2..2....1..1..0..0....0..2..0..1 %e A265871 ..1..2..0..2....1..2..0..2....0..2..1..1....2..0..1..2....2..0..2..1 %Y A265871 Cf. A265870. %K A265871 nonn %O A265871 1,1 %A A265871 _R. H. Hardin_, Dec 17 2015