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This is the third in our series of papers relating gauge theoretic
invariants of certain 4–manifolds with invariants of
3–manifolds derived from Rohlin's theorem. Such relations
are well-known in dimension three, starting with Casson's integral
lift of the Rohlin invariant of a homology sphere. We consider two
invariants of a spin 4–manifold that has the integral homology
of a 4–torus. The first is a degree zero Donaldson invariant,
counting flat connections on a certain SO(3)–bundle. The second,
which depends on the choice of a 1–dimensional cohomology class,
is a combination of Rohlin invariants of a 3–manifold carrying the
dual homology class. We prove that these invariants, suitably normalized,
agree modulo 2, by showing that they coincide with the quadruple cup
product of 1–dimensional cohomology classes.
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