  In [1] it was shown that K^, a certain differential cohomology functor
associated to complex K-theory, satisfies the Mayer-Vietoris property when the
underlying manifold is compact. It turns out that this result is quite general.
The work that follows shows the M-V property to hold on compact manifolds for
any differential cohomology functor J^ associated to any Z-graded cohomology
functor J(,Z) which, in each degree, assigns to a point a finitely generated
group. The approach is to show that the result follows from Diagram 1, the
commutative diagram we take as a definition of differential cohomology, and
Diagram 2, which combines the three Mayer-Vietoris sequences for J*(,Z), J*(,R)
and J*(,R/Z).
