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 A268900 #8 Jan 16 2019 09:23:37 %S A268900 36,696,9720,118584,1347192,14644152,154472184,1594323000,16185567096, %T A268900 162200044728,1608569870328,15816054042936,154394813276280, %U A268900 1498006261495224,14458132831535352,138907883786523192 %N A268900 Number of n X 4 0..2 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two exactly once. %H A268900 R. H. Hardin, <a href="/A268900/b268900.txt">Table of n, a(n) for n = 1..210</a> %F A268900 Empirical: a(n) = 18*a(n-1) - 81*a(n-2) for n>3. %F A268900 Conjectures from _Colin Barker_, Jan 16 2019: (Start) %F A268900 G.f.: 12*x*(3 + 4*x + 9*x^2) / (1 - 9*x)^2. %F A268900 a(n) = 8 * 3^(2*n-3) * (16*n-3) for n>1. %F A268900 (End) %e A268900 Some solutions for n=4: %e A268900 ..0..0..0..1. .1..0..1..2. .2..0..0..0. .2..1..0..0. .1..0..0..1 %e A268900 ..0..1..0..0. .1..2..2..1. .1..0..1..2. .1..0..0..0. .1..0..2..2 %e A268900 ..0..1..1..0. .2..1..0..1. .1..0..1..2. .0..1..0..0. .1..2..1..2 %e A268900 ..0..0..1..2. .2..1..2..0. .1..2..1..2. .2..1..0..0. .2..2..2..1 %Y A268900 Column 4 of A268904. %K A268900 nonn %O A268900 1,1 %A A268900 _R. H. Hardin_, Feb 15 2016