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 A221520 #12 Oct 18 2017 11:50:52 %S A221520 0,12,30,144,536,2172,8544,33960,134480,533248,2113456,8377808, %T A221520 33207936,131632288,521770784,2068227136,8198158272,32496344448, %U A221520 128810916224,510588286464,2023899853184,8022453208832,31799871553536,126050200002816 %N A221520 Number of 0..4 arrays of length n with each element differing from at least one neighbor by 2 or more. %C A221520 Column 4 of A221524. %H A221520 R. H. Hardin, <a href="/A221520/b221520.txt">Table of n, a(n) for n = 1..210</a> %F A221520 Empirical: a(n) = 2*a(n-1) +6*a(n-2) +6*a(n-3) +4*a(n-4) +4*a(n-6). %F A221520 Empirical g.f.: 2*x^2*(6 + 3*x + 6*x^2 - 2*x^3 + 4*x^4) / (1 - 2*x - 6*x^2 - 6*x^3 - 4*x^4 - 4*x^6). - _Colin Barker_, Oct 18 2017 %e A221520 Some solutions for n=6: %e A221520 ..0....1....4....3....3....0....0....3....3....3....3....0....1....0....4....1 %e A221520 ..2....4....1....0....0....2....4....1....1....0....0....2....4....3....1....3 %e A221520 ..0....1....2....0....0....1....2....1....0....2....4....2....2....3....3....4 %e A221520 ..0....0....4....4....4....4....3....3....3....4....2....0....0....0....1....2 %e A221520 ..2....4....2....3....4....0....0....0....3....1....2....1....3....0....0....4 %e A221520 ..0....1....0....0....1....3....4....4....1....3....4....3....1....2....3....0 %Y A221520 Cf. A221524. %K A221520 nonn %O A221520 1,2 %A A221520 _R. H. Hardin_, Jan 19 2013