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 A268881 #7 Jan 15 2019 16:11:26 %S A268881 2,14,84,462,2418,12252,60666,295230,1417452,6732102,31690914, %T A268881 148080468,687592338,3175567374,14597507076,66827528094,304831251762, %U A268881 1386004252620,6283722000714,28414577975934,128187044049948,577056144993366 %N A268881 Number of n X 3 binary arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling two exactly once. %H A268881 R. H. Hardin, <a href="/A268881/b268881.txt">Table of n, a(n) for n = 1..210</a> %F A268881 Empirical: a(n) = 10*a(n-1) - 31*a(n-2) + 30*a(n-3) - 9*a(n-4). %F A268881 Empirical g.f.: 2*x*(1 - 2*x)*(1 - x + x^2) / (1 - 5*x + 3*x^2)^2. - _Colin Barker_, Jan 15 2019 %e A268881 Some solutions for n=4: %e A268881 ..0..1..0. .0..0..0. .1..0..0. .0..1..0. .0..1..1. .1..0..1. .1..0..0 %e A268881 ..0..1..0. .0..1..0. .1..1..0. .0..1..0. .0..0..0. .0..0..0. .1..0..0 %e A268881 ..1..0..0. .0..0..1. .0..0..1. .1..0..0. .1..0..0. .1..1..0. .1..1..0 %e A268881 ..0..0..1. .0..1..0. .1..0..0. .1..0..1. .1..0..0. .0..0..0. .0..1..0 %Y A268881 Column 3 of A268886. %K A268881 nonn %O A268881 1,1 %A A268881 _R. H. Hardin_, Feb 15 2016