arXiv2025
We consider 5d $\mathcal{N}=1$ SU(2) super Yang-Mills theory on $X\times S^1$, with $X$ a closed smooth four-manifold. A partial topological twisting along $X$ renders the theory formally independent of the metric on $X$. The theory depends on the spin structure and the circumference $R$ of $S^1$. The coefficients of the $R$-expansion of the partition function are Witten indices, which are identified with $L^2$-indices of Dirac operators on moduli spaces of instantons. The partition function encodes BPS indices for instanton particles on a spatial manifold $X$, and these indices are special cases of K-theoretic Donaldson invariants. When the 't Hooft flux of the gauge theory is nonzero and $X$ is not spin, the 5d theory can be anomalous, but this anomaly can be canceled by coupling to a line bundle with connection for the global $U(1)$ ``instanton number symmetry''. For $b_2^+(X)>0$ we can derive the partition function from integration over the Coulomb branch of the effective 4d low-energy theory. When $X$ is toric we can also use equivariant localization with respect to the $\mathbb{C}^* \times \mathbb{C}^*$ symmetry. The two methods lead to the same results for the wall-crossing formula. We also determine path integrals for four-manifolds with $b_2^+(X)>1$. Our results agree with those for algebraic surfaces by G\"ottsche, Kool, Nakajima, Yoshioka, and Williams, but apply to a larger class of manifolds. When the circumference of the circle is tuned to special values, the path integral is associated with the 5d superconformal $E_1$ theory. Topological invariants in this case involve generalizations of Seiberg-Witten invariants.