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 A268905 #8 Jan 16 2019 15:03:36 %S A268905 0,36,168,696,2664,9720,34344,118584,402408,1347192,4461480,14644152, %T A268905 47711592,154472184,497428776,1594323000,5089079016,16185567096, %U A268905 51311691432,162200044728,511395045480,1608569870328,5048863812648 %N A268905 Number of 2 X n 0..2 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two exactly once. %H A268905 R. H. Hardin, <a href="/A268905/b268905.txt">Table of n, a(n) for n = 1..210</a> %F A268905 Empirical: a(n) = 6*a(n-1) - 9*a(n-2) for n>4. %F A268905 Conjectures from _Colin Barker_, Jan 16 2019: (Start) %F A268905 G.f.: 12*x^2*(3 - x)*(1 - x) / (1 - 3*x)^2. %F A268905 a(n) = 8*3^(n-3) * (8*n-3) for n>2. %F A268905 (End) %e A268905 Some solutions for n=4: %e A268905 ..0..2..1..2. .2..2..2..1. .0..0..2..1. .1..0..1..0. .1..1..0..1 %e A268905 ..2..2..2..2. .1..2..1..0. .0..1..0..0. .1..2..0..0. .0..1..2..2 %Y A268905 Row 2 of A268904. %K A268905 nonn %O A268905 1,2 %A A268905 _R. H. Hardin_, Feb 15 2016