Our main results are the construction of polynomial approximation schemes for
the above problems which, given the negative result above, are the best possible
results of this type. We present a unified methodology that is helpful for numerous
geometric covering and packing problems and could potentially be applicable to
problems beyond this context. We call this fundamental technique the shifting
strategy and outline the necessary conditions for its applicability. A similar technique
has been independently discovered by Baker [I]. Her technique is applicable
to planar graphs, whereas ours applies to problems defined in Euclidean space; but
our concepts of strips of bounded width and shifting are analogous to the concepts
of bounded outerplanarity and shifting used by Baker.