%N Number of ways of 4-coloring 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 y-axes.
%H J. D. Davies <a href="http://www.jasondavies.com/17x17/">Number of unique rectangle-free 4-colourings for an nxm grid</a>
%A _Jason Davies_, Nov 14 2011
%E a(20) from _Jason Davies_, Nov 08 2012