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 A232956 #8 Oct 06 2018 11:37:57 %S A232956 4,21,129,771,4629,27771,166629,999771,5998629,35991771,215950629, %T A232956 1295703771,7774222629,46645335771,279872014629,1679232087771, %U A232956 10075392526629,60452355159771,362714130958629,2176284785751771 %N A232956 Number of 2 X n 0..3 arrays with no element x(i,j) adjacent to value 3-x(i,j) horizontally, diagonally or antidiagonally, and top left element zero. %H A232956 R. H. Hardin, <a href="/A232956/b232956.txt">Table of n, a(n) for n = 1..210</a> %F A232956 Empirical: a(n) = 5*a(n-1) + 6*a(n-2). %F A232956 Conjectures from _Colin Barker_, Oct 06 2018: (Start) %F A232956 G.f.: x*(4 + x) / ((1 + x)*(1 - 6*x)). %F A232956 a(n) = (25*6^n - 18) / 42 for n even. %F A232956 a(n) = (25*6^n + 18) / 42 for n odd. %F A232956 (End) %e A232956 Some solutions for n=5: %e A232956 ..0..1..0..2..3....0..0..0..0..1....0..0..0..1..0....0..1..1..1..0 %e A232956 ..3..2..0..2..2....0..1..1..0..2....0..0..1..1..0....0..0..0..1..0 %Y A232956 Row 2 of A232955. %K A232956 nonn %O A232956 1,1 %A A232956 _R. H. Hardin_, Dec 02 2013