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A133129
Number of black/white colorings of a 3 X n rectangle which have no monochromatic 2 by 2 subsquares.
Empirical G.f.: x*(8+2*x-2*x^2)/(1-6*x-3*x^2+2*x^3). - Colin Barker, Jan 04 2012
EXAMPLE
a(2) = 50 because if the middle row is not monochromatic, the top and bottom rows are unconstrained, contributing 2*4*4. if the middle row is monochromatic, the top and bottom rows can each take on only 3 values contributing 2*3*3.