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 A269270 #11 Jan 20 2019 23:18:07 %S A269270 4,108,1620,20412,236196,2598156,27634932,286978140,2927177028, %T A269270 29443957164,292889889684,2887057484028,28242953648100, %U A269270 274521509459532,2653707924775476,25530500379736476,244598664928443012 %N A269270 Number of n X 2 0..3 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling three exactly once. %H A269270 R. H. Hardin, <a href="/A269270/b269270.txt">Table of n, a(n) for n = 1..210</a> %F A269270 Empirical: a(n) = 18*a(n-1) - 81*a(n-2). %F A269270 Conjectures from _Colin Barker_, Jan 20 2019: (Start) %F A269270 G.f.: 4*x*(1 + 9*x) / (1 - 9*x)^2. %F A269270 a(n) = 9^(n-1) * (8*n-4). %F A269270 (End) %e A269270 Some solutions for n=4: %e A269270 2 3 2 2 3 3 2 2 3 0 2 2 0 0 1 1 1 3 3 1 %e A269270 2 0 2 2 3 1 0 0 0 0 3 1 2 1 0 2 1 1 1 1 %e A269270 3 1 0 0 0 0 1 2 1 0 0 3 0 1 0 2 2 0 3 0 %e A269270 1 0 3 3 3 2 0 0 1 3 1 0 0 0 1 0 1 1 2 2 %Y A269270 Column 2 of A269276. %K A269270 nonn %O A269270 1,1 %A A269270 _R. H. Hardin_, Feb 21 2016