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 A221251 #6 Jun 02 2025 08:13:01 %S A221251 4,81,2116,58081,1612900,44903401,1250753956,34842328921,970622980804, %T A221251 27039303204489,753252725702500,20983889126032561,584562920742585316, %U A221251 16284579460194091201,453650957002812576484,12637673040690140943889 %N A221251 Number of nX4 arrays of occupancy after each element moves to some horizontal or antidiagonal neighbor, with no occupancy greater than 2. %C A221251 Column 4 of A221255 %H A221251 R. H. Hardin, <a href="/A221251/b221251.txt">Table of n, a(n) for n = 1..23</a> %F A221251 Empirical: a(n) = 39*a(n-1) -356*a(n-2) +1365*a(n-3) -2739*a(n-4) +2904*a(n-5) -1683*a(n-6) +573*a(n-7) -116*a(n-8) +15*a(n-9) -a(n-10) %e A221251 Some solutions for n=3 %e A221251 ..0..1..2..1....1..1..1..0....1..2..0..0....0..2..2..2....0..1..1..1 %e A221251 ..0..2..2..0....1..2..1..1....0..2..2..1....0..0..2..0....1..2..1..0 %e A221251 ..0..1..2..1....0..1..2..1....2..1..1..0....0..1..2..1....1..1..2..1 %K A221251 nonn %O A221251 1,1 %A A221251 _R. H. Hardin_ Jan 06 2013