%I
%S 4,16,252,64,3912,228984,256,59928,12870096,2545607472,1024,906912,
%T 696448224,465659203104,272454472598400,4096,13571712,36376835616,
%U 79102304162784,140221913588582784
%N Number of ways of 4coloring an n x m grid so that there is no rectangle with all 4 corners the same color.
%C The rectangles have sides parallel to the x and yaxes.
%H J. D. Davies <a href="http://www.jasondavies.com/17x17/">Number of unique rectanglefree 4colourings for an nxm grid</a>
%K nonn,tabl
%O 1,1
%A _Jason Davies_, Nov 14 2011
%E a(20) from _Jason Davies_, Nov 08 2012
