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 A268965 #8 Jan 17 2019 09:20:34 %S A268965 9,60,336,1728,8448,39936,184320,835584,3735552,16515072,72351744, %T A268965 314572800,1358954496,5838471168,24964497408,106300440576, %U A268965 450971566080,1906965479424,8040178778112,33809982554112,141836999983104 %N A268965 Number of n X 2 0..2 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two not more than once. %H A268965 R. H. Hardin, <a href="/A268965/b268965.txt">Table of n, a(n) for n = 1..210</a> %F A268965 Empirical: a(n) = 8*a(n-1) - 16*a(n-2). %F A268965 Conjectures from _Colin Barker_, Jan 17 2019: (Start) %F A268965 G.f.: 3*x*(3 - 4*x) / (1 - 4*x)^2. %F A268965 a(n) = 4^(n-1) * (6*n+3). %F A268965 (End) %e A268965 Some solutions for n=4: %e A268965 ..0..0. .0..0. .0..1. .2..2. .1..2. .1..0. .2..2. .0..2. .2..1. .0..1 %e A268965 ..0..2. .1..1. .0..0. .1..1. .2..0. .0..0. .2..1. .2..1. .0..2. .2..2 %e A268965 ..2..2. .2..1. .2..2. .2..2. .0..1. .2..2. .2..1. .0..0. .1..2. .2..0 %e A268965 ..1..0. .2..1. .2..2. .2..1. .2..1. .1..0. .2..2. .0..0. .2..2. .1..2 %Y A268965 Column 2 of A268971. %K A268965 nonn %O A268965 1,1 %A A268965 _R. H. Hardin_, Feb 16 2016