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 A268972 #8 Jan 17 2019 17:20:28 %S A268972 9,60,240,912,3312,11664,40176,136080,454896,1504656,4933872,16061328, %T A268972 51963120,167226768,535692528,1709114256,5433452784,17218688400, %U A268972 54411055344,171498136464,539289320688,1692252695952,5299912289520 %N A268972 Number of 2 X n 0..2 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two not more than once. %H A268972 R. H. Hardin, <a href="/A268972/b268972.txt">Table of n, a(n) for n = 1..210</a> %F A268972 Empirical: a(n) = 6*a(n-1) - 9*a(n-2) for n>4. %F A268972 Conjectures from _Colin Barker_, Jan 17 2019: (Start) %F A268972 G.f.: 3*x*(3 - x)*(1 + x - 4*x^2) / (1 - 3*x)^2. %F A268972 a(n) = 16*3^(n-3) * (4*n+3) for n>2. %F A268972 (End) %e A268972 Some solutions for n=4: %e A268972 ..0..1..0..1. .2..2..1..2. .1..2..1..0. .2..1..0..0. .1..1..2..2 %e A268972 ..0..0..0..2. .2..2..2..2. .2..2..1..0. .2..1..2..2. .2..2..2..1 %Y A268972 Row 2 of A268971. %K A268972 nonn %O A268972 1,1 %A A268972 _R. H. Hardin_, Feb 16 2016