a1 Department of Mathematics, University College London, London, WC1E 6BT.
a2 Department of Mathematics, University College London, London WC1E 6BT.
Packing measures have been introduced to complement the theory of Hausdorff measures in [13,14]. (For a new treatment see also [10, Chapter 5]. While Hausdorff measures are intimately connected to upper density estimates (see, e.g., [5,2.10.18]), the importance of packing measures stems from their connection to lower density estimates.
(Received January 04 1994)