  The u-plane integral is the contribution of the Coulomb branch to correlation
functions of N=2 gauge theory on a compact four-manifold. We consider the
u-plane integral for correlators of point and surface observables of
topologically twisted theories with gauge group SU(2), for an arbitrary
four-manifold with (b1,b2+)=(0,1). The u-plane contribution equals the full
correlator in the absence of Seiberg-Witten contributions at strong coupling,
and coincides with the mathematically defined Donaldson invariants in such
cases. We demonstrate that the u-plane correlators are efficiently determined
using mock modular forms for point observables, and Appell-Lerch sums for
surface observables. We use these results to discuss the asymptotic behavior of
correlators as function of the number of observables. Our findings suggest that
the vev of exponentiated point and surface observables is an entire function of
the fugacities.
