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 A280955 #9 Feb 15 2019 04:29:38 %S A280955 2,9,42,205,997,4850,23593,114769,558298,2715861,13211405,64267362, %T A280955 312630929,1520804569,7398009354,35987886621,175064388469, %U A280955 851607109714,4142673879353,20152188344737,98031056006074,476875651282597 %N A280955 Number of n X 2 0..2 arrays with no element equal to more than one of its horizontal, vertical and antidiagonal neighbors and with new values introduced in order 0 sequentially upwards. %H A280955 R. H. Hardin, <a href="/A280955/b280955.txt">Table of n, a(n) for n = 1..210</a> %F A280955 Empirical: a(n) = 4*a(n-1) + 4*a(n-2) + a(n-3) for n>4. %F A280955 Empirical g.f.: x*(1 - x)*(1 + x)*(2 + x) / (1 - 4*x - 4*x^2 - x^3). - _Colin Barker_, Feb 15 2019 %e A280955 Some solutions for n=4: %e A280955 ..0..1. .0..1. .0..1. .0..1. .0..1. .0..1. .0..0. .0..0. .0..1. .0..1 %e A280955 ..0..2. .2..0. .1..2. .2..1. .2..2. .2..2. .1..1. .1..2. .2..2. .2..0 %e A280955 ..1..0. .1..2. .0..2. .0..2. .1..0. .1..0. .2..2. .2..0. .1..0. .0..1 %e A280955 ..0..1. .0..0. .1..1. .1..0. .1..0. .2..2. .0..1. .1..1. .0..2. .2..1 %Y A280955 Column 2 of A280961. %K A280955 nonn %O A280955 1,1 %A A280955 _R. H. Hardin_, Jan 11 2017