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%I #11 Jan 20 2019 23:18:07
%S 4,108,1620,20412,236196,2598156,27634932,286978140,2927177028,
%T 29443957164,292889889684,2887057484028,28242953648100,
%U 274521509459532,2653707924775476,25530500379736476,244598664928443012
%N Number of n X 2 0..3 arrays with some element plus some horizontally or antidiagonally adjacent neighbor totalling three exactly once.
%H R. H. Hardin, <a href="/A269270/b269270.txt">Table of n, a(n) for n = 1..210</a>
%F Empirical: a(n) = 18*a(n-1) - 81*a(n-2).
%F Conjectures from _Colin Barker_, Jan 20 2019: (Start)
%F G.f.: 4*x*(1 + 9*x) / (1 - 9*x)^2.
%F a(n) = 9^(n-1) * (8*n-4).
%F (End)
%e Some solutions for n=4:
%e 2 3 2 2 3 3 2 2 3 0 2 2 0 0 1 1 1 3 3 1
%e 2 0 2 2 3 1 0 0 0 0 3 1 2 1 0 2 1 1 1 1
%e 3 1 0 0 0 0 1 2 1 0 0 3 0 1 0 2 2 0 3 0
%e 1 0 3 3 3 2 0 0 1 3 1 0 0 0 1 0 1 1 2 2
%Y Column 2 of A269276.
%K nonn
%O 1,1
%A _R. H. Hardin_, Feb 21 2016
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