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 A221519 #10 Oct 18 2017 13:28:40 %S A221519 0,6,10,36,94,274,768,2182,6170,17476,49470,140066,396544,1122694, %T A221519 3178538,8999012,25477790,72132146,204218880,578179846,1636929594, %U A221519 4634437764,13120914558,37147634242,105171535360,297759253766 %N A221519 Number of 0..3 arrays of length n with each element differing from at least one neighbor by 2 or more. %C A221519 Column 3 of A221524. %H A221519 R. H. Hardin, <a href="/A221519/b221519.txt">Table of n, a(n) for n = 1..210</a> %F A221519 Empirical: a(n) = a(n-1) +4*a(n-2) +3*a(n-3) +a(n-4). %F A221519 Empirical g.f.: 2*x^2*(3 + 2*x + x^2) / ((1 + x)*(1 - 2*x - 2*x^2 - x^3)). - _Colin Barker_, Oct 18 2017 %e A221519 Some solutions for n=6 %e A221519 ..0....2....0....2....3....0....0....3....3....2....0....1....3....1....3....1 %e A221519 ..3....0....3....0....0....2....3....1....0....0....2....3....0....3....1....3 %e A221519 ..0....0....0....2....1....1....1....3....0....2....3....2....1....0....1....2 %e A221519 ..1....3....3....3....3....3....3....2....2....0....0....0....3....1....3....0 %e A221519 ..3....3....2....0....3....2....3....0....3....2....0....3....0....3....0....3 %e A221519 ..1....1....0....3....0....0....1....3....1....0....3....1....3....1....3....0 %K A221519 nonn %O A221519 1,2 %A A221519 _R. H. Hardin_, Jan 19 2013