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 A268899 #8 Jan 16 2019 09:23:47 %S A268899 12,168,1584,12960,98496,715392,5038848,34712064,235146240,1572120576, %T A268899 10400182272,68205846528,444063596544,2873352683520,18493942726656, %U A268899 118486616113152,756057455198208,4807171282305024,30467987000524800 %N A268899 Number of n X 3 0..2 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two exactly once. %H A268899 R. H. Hardin, <a href="/A268899/b268899.txt">Table of n, a(n) for n = 1..210</a> %F A268899 Empirical: a(n) = 12*a(n-1) - 36*a(n-2). %F A268899 Conjectures from _Colin Barker_, Jan 16 2019: (Start) %F A268899 G.f.: 12*x*(1 + 2*x) / (1 - 6*x)^2. %F A268899 a(n) = 2^(n+1) * 3^(n-1) * (4*n-1). %F A268899 (End) %e A268899 Some solutions for n=4: %e A268899 ..2..1..1. .1..0..1. .2..2..2. .0..1..0. .0..0..0. .1..0..2. .0..0..0 %e A268899 ..0..0..0. .0..2..2. .1..1..0. .2..1..0. .0..0..1. .0..1..0. .1..2..1 %e A268899 ..0..0..0. .1..2..1. .2..1..2. .1..0..1. .1..0..2. .0..1..2. .1..0..1 %e A268899 ..0..0..0. .2..2..2. .2..2..2. .1..0..1. .0..1..0. .2..1..0. .1..0..1 %Y A268899 Column 3 of A268904. %K A268899 nonn %O A268899 1,1 %A A268899 _R. H. Hardin_, Feb 15 2016