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 A268803 #8 Jan 15 2019 09:23:24 %S A268803 9,34,104,290,772,1972,4914,12010,28922,68836,162274,379524,881712, %T A268803 2036734,4681646,10714994,24430816,55516546,125777028,284188780, %U A268803 640549034,1440572818,3233256010,7243375068,16199546178,36172883716 %N A268803 Number of n X 2 0..2 arrays with some element plus some horizontally, vertically or antidiagonally adjacent neighbor totalling two not more than once. %H A268803 R. H. Hardin, <a href="/A268803/b268803.txt">Table of n, a(n) for n = 1..210</a> %F A268803 Empirical: a(n) = 2*a(n-1) + 3*a(n-2) - 2*a(n-3) - 6*a(n-4) - 4*a(n-5) - a(n-6) for n>7. %F A268803 Empirical g.f.: x*(9 + 16*x + 9*x^2 - 2*x^3 + 2*x^4 + 6*x^5 + 3*x^6) / (1 - x - 2*x^2 - x^3)^2. - _Colin Barker_, Jan 15 2019 %e A268803 Some solutions for n=4: %e A268803 ..0..1. .1..2. .1..0. .1..0. .2..2. .1..1. .0..0. .0..2. .0..0. .2..2 %e A268803 ..0..1. .2..1. .2..1. .0..1. .1..2. .0..0. .1..2. .1..2. .0..0. .2..2 %e A268803 ..0..0. .2..2. .2..2. .2..2. .2..1. .0..0. .2..2. .2..2. .0..0. .1..2 %e A268803 ..1..0. .1..2. .2..2. .2..1. .0..0. .1..0. .1..2. .2..1. .0..0. .0..1 %Y A268803 Column 2 of A268809. %K A268803 nonn %O A268803 1,1 %A A268803 _R. H. Hardin_, Feb 13 2016