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 A221257 #6 Jun 02 2025 08:13:36 %S A221257 0,16,30,2116,22268,488601,7356420,132273001,2175160988,37343243536, %T A221257 627439570380,10652755988736,179923226827008,3046558564618321, %U A221257 51520912019433840,871812703964410321,14747896839403221578 %N A221257 Number of 3Xn arrays of occupancy after each element moves to some horizontal or antidiagonal neighbor, with no occupancy greater than 2. %C A221257 Row 3 of A221255 %H A221257 R. H. Hardin, <a href="/A221257/b221257.txt">Table of n, a(n) for n = 1..36</a> %F A221257 Empirical: a(n) = 21*a(n-1) +21*a(n-2) -1891*a(n-3) +4713*a(n-4) +31527*a(n-5) -98973*a(n-6) -92217*a(n-7) +299577*a(n-8) -27117*a(n-9) +103383*a(n-10) -393*a(n-11) +6211*a(n-12) +69*a(n-13) +69*a(n-14) +a(n-15) for n>18 %e A221257 Some solutions for n=3 %e A221257 ..0..2..1....0..2..0....1..2..0....0..2..2....1..2..0....1..2..0....0..2..2 %e A221257 ..0..2..0....1..2..0....0..2..1....0..2..0....0..2..0....1..2..1....0..2..1 %e A221257 ..1..2..1....1..2..1....1..2..0....1..2..0....1..2..1....0..2..0....0..2..0 %K A221257 nonn %O A221257 1,2 %A A221257 _R. H. Hardin_ Jan 06 2013