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%I #4 Feb 04 2016 22:00:44
%S 4,16,61,229,852,3146,11536,42032,152254,548568,1966757,7019311,
%T 24946486,88313632,311507619,1095064315,3837407016,13407618832,
%U 46715533678,162345449332,562801287366,1946559269050,6717910370343,23136984979001
%N Number of length-n 0..3 arrays with no adjacent pair x,x+1 followed at any distance by x+1,x.
%C Column 3 of A268457.
%H R. H. Hardin, <a href="/A268452/b268452.txt">Table of n, a(n) for n = 1..210</a>
%F Empirical: a(n) = 19*a(n-1) -156*a(n-2) +730*a(n-3) -2178*a(n-4) +4446*a(n-5) -6613*a(n-6) +7587*a(n-7) -6948*a(n-8) +5152*a(n-9) -3101*a(n-10) +1503*a(n-11) -573*a(n-12) +163*a(n-13) -31*a(n-14) +3*a(n-15)
%e Some solutions for n=9
%e ..0....1....0....1....1....0....3....3....3....2....1....1....2....1....0....2
%e ..1....2....2....0....1....1....0....2....0....2....0....0....0....2....0....2
%e ..2....3....2....2....0....3....3....3....3....3....0....3....0....2....0....2
%e ..3....0....0....2....2....0....0....3....3....0....2....3....1....0....2....2
%e ..1....2....1....3....0....0....1....1....1....0....0....3....2....0....0....3
%e ..3....3....3....3....0....2....3....3....1....1....1....1....2....3....0....1
%e ..1....3....0....0....2....1....1....1....0....3....1....3....2....0....1....2
%e ..2....0....0....2....0....1....1....1....0....3....3....1....3....0....2....0
%e ..0....3....2....2....0....2....2....3....0....0....0....0....0....3....3....1
%Y Cf. A268457.
%K nonn
%O 1,1
%A _R. H. Hardin_, Feb 04 2016
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