%I #10 Jun 18 2022 23:37:48
%S 20,40,68,136,236,472,836,1672,3020,6040,11108,22216,41516,83032,
%T 157316,314632,603020,1206040,2333348,4666696,9097196,18194392,
%U 35680196,71360392,140595020,281190040,556002788,1112005576,2204879276,4409758552
%N Number of (n+1) X (1+1) 0..3 arrays with every 2 X 2 subblock having its diagonal sum differing from its antidiagonal sum by 4 (constant-stress 1 X 1 tilings).
%H R. H. Hardin, <a href="/A235282/b235282.txt">Table of n, a(n) for n = 1..210</a>
%F Empirical: a(n) = 2*a(n-1) + 3*a(n-2) - 6*a(n-3).
%F Conjectures from _Colin Barker_, Oct 18 2018: (Start)
%F G.f.: 4*x*(5 - 18*x^2) / ((1 - 2*x)*(1 - 3*x^2)).
%F a(n) = 2^(2+n) + 2*3^((-1+n)/2)*(3-3*(-1)^n + 2*sqrt(3) + 2*(-1)^n*sqrt(3)).
%F (End)
%e Some solutions for n=4:
%e 3 1 1 3 2 3 3 1 2 0 3 0 2 1 0 3 1 2 2 0
%e 1 3 2 0 3 0 1 3 0 2 2 3 0 3 2 1 3 0 1 3
%e 2 0 1 3 2 3 3 1 2 0 3 0 3 2 0 3 2 3 2 0
%e 0 2 3 1 3 0 0 2 1 3 1 2 0 3 1 0 3 0 1 3
%e 2 0 0 2 1 2 2 0 3 1 3 0 1 0 0 3 2 3 2 0
%Y Column 1 of A235289.
%K nonn
%O 1,1
%A _R. H. Hardin_, Jan 05 2014