  This paper considers a class C(Z_p) of closed sets of the p-adic integers
obtained by graph-directed constructions analogous to those of Mauldin and
Williams over the real numbers. These sets are characterized as collections of
those p-adic integers whose p-adic expansions are describeed by paths in the
graph of a finite automaton issuing from a distinguished initial vertex. This
paper shows that this class of sets is closed under the arithmetic operations
of addition and multiplication by p-integral rational numbers. In addition the
Minkowski sum (under p-adic addition) of two set in the class is shown to also
belong to this class. These results represent purely p-adic phenomena in that
analogous closure properties do not hold over the real numbers. We also show
the existence of computable formulas for the Hausdorff dimensions of such sets.
