Limiting Behavior of a Class of Hermitian Yang--Mills Metrics, II: Exponential Approximation

This paper is a sequel to [9], where the first-named author constructed a family of approximate Hermitian Yang–Mills metrics $H_{0,\epsilon }$ on stable rank-two holomorphic vector bundles arising from double spectral covers over the product of two one-dimensional complex tori. We prove that these approximate metrics give an all-order, exponentially accurate asymptotic description of the exact Hermitian Yang–Mills metrics in the large Kähler limit. More precisely, the mean curvature of $H_{0,\epsilon }$ decays exponentially in every $C^k$-norm. Moreover, if $H_{1,\epsilon }$ denotes the exact Hermitian Yang–Mills metric and $$H_{\epsilon }=H_{0,\epsilon }^{-1}{H}_{1,\epsilon },$$ then, after normalization, for every nonnegative integer $k$, there exist positive constants $C_k$ and $c_k$ such that $$\mathrm{\parallel }{H}_{\epsilon }-Id{\mathrm{\parallel }}_{C^k}\le C_k{e}^{-\frac{c_k}{\epsilon }}\mathrm{.}$$ The main analytic difficulty lies in the global $C^0$-comparison. Obtaining $C^0$-estimates for the coupled nonlinear Hermitian Yang–Mills system is intrinsically difficult; moreover, the equation controls only the contraction of the curvature, and hence only certain combinations of second derivatives, whereas one needs global control of the full matrix-valued metric.