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 A221536 #9 Oct 18 2017 18:22:28 %S A221536 0,2,4,10,22,54,134,334,822,2014,4934,12110,29750,73086,179494,440750, %T A221536 1082262,2657630,6526342,16026766,39356662,96646974,237332966, %U A221536 582812014,1431196758,3514554142,8630600774,21193942094,52045411766,127806567678 %N A221536 Number of 0..2 arrays of length n with each element differing from at least one neighbor by something other than 1, starting with 0. %C A221536 Column 2 of A221542. %H A221536 R. H. Hardin, <a href="/A221536/b221536.txt">Table of n, a(n) for n = 1..210</a> %F A221536 Empirical: a(n) = 3*a(n-1) -2*a(n-2) +4*a(n-4). %F A221536 Empirical g.f.: 2*x^2*(1 - x + x^2) / (1 - 3*x + 2*x^2 - 4*x^4). - _Colin Barker_, Oct 18 2017 %e A221536 Some solutions for n=6 %e A221536 ..0....0....0....0....0....0....0....0....0....0....0....0....0....0....0....0 %e A221536 ..2....2....2....2....2....2....0....0....2....0....0....0....2....0....2....0 %e A221536 ..2....0....2....2....2....2....2....0....0....2....2....1....0....2....0....2 %e A221536 ..0....2....0....2....2....2....0....0....0....0....2....1....0....0....2....1 %e A221536 ..2....2....2....1....2....0....2....1....1....1....0....2....2....0....0....1 %e A221536 ..0....0....2....1....0....0....0....1....1....1....0....2....0....0....2....1 %K A221536 nonn %O A221536 1,2 %A A221536 _R. H. Hardin_, Jan 19 2013