  We study the Donaldson-Witten function in four-dimensional topological gauge
theory which is constructed from N=2 supersymmetric SU(2) gauge theory with
$N_f < 4$ massless fundamental hypermultiplets. When $N_f = 2,3$, the
strong-coupling singularities with multiple massless monopoles appear in the
moduli space (the u-plane) of the Coulomb branch. We show that the invariants
made out of such singularities exhibit a property which is similar to the one
expected for four-manifolds of generalized simple type.
