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 A268898 #8 Jan 16 2019 09:24:00 %S A268898 3,36,240,1344,6912,33792,159744,737280,3342336,14942208,66060288, %T A268898 289406976,1258291200,5435817984,23353884672,99857989632,425201762304, %U A268898 1803886264320,7627861917696,32160715112448,135239930216448 %N A268898 Number of n X 2 0..2 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two exactly once. %H A268898 R. H. Hardin, <a href="/A268898/b268898.txt">Table of n, a(n) for n = 1..210</a> %F A268898 Empirical: a(n) = 8*a(n-1) - 16*a(n-2). %F A268898 Conjectures from _Colin Barker_, Jan 16 2019: (Start) %F A268898 G.f.: 3*x*(1 + 4*x) / (1 - 4*x)^2. %F A268898 a(n) = 4^(n-1) * (6*n-3). %F A268898 (End) %e A268898 Some solutions for n=4: %e A268898 ..2..2. .0..1. .1..0. .0..1. .2..1. .2..0. .1..0. .1..0. .0..1. .0..1 %e A268898 ..2..1. .1..0. .1..1. .0..0. .0..1. .0..0. .1..2. .0..1. .0..1. .2..2 %e A268898 ..0..2. .1..0. .2..1. .1..1. .0..1. .1..2. .2..0. .0..2. .2..2. .0..0 %e A268898 ..2..1. .0..1. .2..1. .2..2. .2..0. .1..0. .0..0. .1..2. .1..1. .0..0 %Y A268898 Column 2 of A268904. %K A268898 nonn %O A268898 1,1 %A A268898 _R. H. Hardin_, Feb 15 2016