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%I #8 Jul 11 2018 06:18:03
%S 7,20,61,191,603,1909,6049,19173,60777,192665,610761,1936161,6137793,
%T 19457329,61681409,195535393,619864097,1965022785,6229292161,
%U 19747394881,62600949633,198450424449,629105008769,1994317286913,6322158281217
%N 1/4 the number of (n+1) X 2 0..3 arrays with every 2 X 2 subblock having exactly two distinct clockwise edge differences.
%C Column 1 of A209553.
%H R. H. Hardin, <a href="/A209546/b209546.txt">Table of n, a(n) for n = 1..210</a>
%F Empirical: a(n) = 5*a(n-1) - 6*a(n-2) + 2*a(n-4).
%F Empirical g.f.: x*(7 - 15*x + 3*x^2 + 6*x^3) / ((1 - x)*(1 - 4*x + 2*x^2 + 2*x^3)). - _Colin Barker_, Jul 11 2018
%e Some solutions for n=4:
%e ..0..1....0..1....3..2....3..0....2..3....0..2....0..1....1..3....1..2....0..1
%e ..3..2....1..2....0..1....0..3....3..2....2..0....3..2....3..1....0..1....1..2
%e ..0..1....2..1....1..2....1..2....2..3....0..2....2..1....1..3....1..2....0..3
%e ..3..2....1..2....0..1....2..3....3..2....2..0....1..2....3..1....2..3....3..0
%e ..2..3....0..1....1..0....1..2....2..3....0..2....2..1....1..3....3..2....2..1
%Y Cf. A209553.
%K nonn
%O 1,1
%A _R. H. Hardin_, Mar 10 2012