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 A221256 #9 Aug 04 2018 14:32:54 %S A221256 0,4,8,81,392,2500,14112,83521,484128,2829124,16473800,96059601, %T A221256 559785800,3262922884,19017180288,110841719041,646029804672, %U A221256 3765346202500,21946028005832,127910874833361,745519107896648 %N A221256 Number of 2 X n arrays of occupancy after each element moves to some horizontal or antidiagonal neighbor, with no occupancy greater than 2. %C A221256 Row 2 of A221255. %H A221256 R. H. Hardin, <a href="/A221256/b221256.txt">Table of n, a(n) for n = 1..41</a> %F A221256 Empirical: a(n) = 6*a(n-1) + 6*a(n-2) - 42*a(n-3) + 42*a(n-5) - 6*a(n-6) - 6*a(n-7) + a(n-8). %F A221256 Empirical g.f.: x^2*(4 - 16*x + 9*x^2 + 26*x^3 - 2*x^4 - 6*x^5 + x^6) / ((1 - x)*(1 + x)*(1 - 6*x + x^2)*(1 + 2*x - x^2)*(1 - 2*x - x^2)). - _Colin Barker_, Aug 04 2018 %e A221256 Some solutions for n=3: %e A221256 0 2 2 1 2 0 0 2 1 0 2 1 0 2 0 1 2 0 1 2 1 %e A221256 0 2 0 0 2 1 0 2 1 1 2 0 1 2 1 1 2 0 0 2 0 %Y A221256 Cf. A221255. %K A221256 nonn %O A221256 1,2 %A A221256 _R. H. Hardin_, Jan 06 2013