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 A280853 #7 Mar 22 2018 09:47:20 %S A280853 2,9,29,110,442,1708,6596,25624,99432,385584,1495696,5802080,22506144, %T A280853 87301312,338644032,1313606016,5095497344,19765519104,76670777600, %U A280853 297407202816,1153647430144,4475017382912,17358666073088,67334551345152 %N A280853 Number of n X 2 0..2 arrays with no element equal to more than one of its king-move neighbors and with new values introduced in order 0 sequentially upwards. %C A280853 Column 2 of A280859. %H A280853 R. H. Hardin, <a href="/A280853/b280853.txt">Table of n, a(n) for n = 1..210</a> %F A280853 Empirical: a(n) = 4*a(n-1) - 2*a(n-2) + 8*a(n-3) - 8*a(n-4) for n>5. %F A280853 Empirical g.f.: x*(1 - x)*(2 + 3*x - 4*x^3) / (1 - 4*x + 2*x^2 - 8*x^3 + 8*x^4). - _Colin Barker_, Mar 22 2018 %e A280853 Some solutions for n=4: %e A280853 ..0..0. .0..1. .0..1. .0..1. .0..1. .0..1. .0..0. .0..1. .0..0. .0..0 %e A280853 ..1..2. .2..1. .2..2. .0..2. .0..2. .2..2. .1..2. .1..2. .1..1. .1..2 %e A280853 ..1..0. .2..0. .1..0. .1..1. .1..1. .1..1. .0..0. .0..0. .0..2. .0..2 %e A280853 ..0..2. .1..1. .0..2. .0..2. .2..2. .0..0. .1..1. .1..1. .1..0. .0..1 %Y A280853 Cf. A280859. %K A280853 nonn %O A280853 1,1 %A A280853 _R. H. Hardin_, Jan 09 2017