  Let a,b,c,d be nonzero rational numbers whose product is a square, and let V
be the diagonal quartic surface in PP^3 defined by ax^4+by^4+cz^4+dw^4=0. We
prove that if V contains a rational point that does not lie on any of the 48
lines on V or on any of the coordinate planes, then the set of rational points
on V is dense in both the Zariski topology and the real analytic topology.
