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We revisit the low-energy effective U(1) action of topologically twisted $${\mathcal {N}}=2$$ N = 2 SYM theory with gauge group of rank one on a generic oriented smooth four-manifold X with nontrivial fundamental group. After including a specific new set of $$\mathcal Q$$ Q -exact operators to the known action, we express the integrand of the path integral of the low-energy U(1) theory as an anti-holomorphic derivative. This allows us to use the theory of mock modular forms and indefinite theta functions for the explicit evaluation of correlation functions of the theory, thus facilitating the computations compared to previously used methods. As an explicit check of our results, we compute the path integral for the product ruled surfaces $$X=\Sigma _g \times \mathbb {CP}^1$$ X = Σ g × CP 1 for the reduction on either factor and compare the results with existing literature. In the case of reduction on the Riemann surface $$\Sigma _g$$ Σ g , via an equivalent topological A-model on $$\mathbb {CP}^1$$ CP 1 , we will be able to express the generating function of genus zero Gromov–Witten invariants of the moduli space of flat rank one connections over $$\Sigma _g$$ Σ g in terms of an indefinite theta function, whence we would be able to make concrete numerical predictions of these enumerative invariants in terms of modular data, thereby allowing us to derive results in enumerative geometry from number theory.

We consider topological twists of four-dimensional N = 2 supersymmetric QCD with gauge group SU(2) and N f ≤ 3 fundamental hypermultiplets. The twists are labelled by a choice of background fluxes for the flavour group, which provides an infinite family of topological partition functions. In this Part I, we demonstrate that in the presence of such fluxes the theories can be formulated for arbitrary gauge bundles on a compact four-manifold. Moreover, we consider arbitrary masses for the hypermultiplets, which introduce new intricacies for the evaluation of the low-energy path integral on the Coulomb branch. We develop techniques for the evaluation of these path integrals. In the forthcoming Part II, we will deal with the explicit evaluation.