Dedicated to Duong H. Phong, with admiration, on the occasion of his 65th birthday.
IntroductionLet (X, ω) be a compact Kähler manifold of dimension n and (E, h) → X a C ∞ hermitian vector bundle on X. The celebrated theorem of Donaldson-Uhlenbeck-Yau states that if A is an integrable unitary connection on (E, h) that induces an ω-slope stable holomorphic structure on E, then there is a complex gauge transformation g such that g(A) satises the Hermitian-Yang-Mills (HYM) equations. The proof in [40] uses the continuity method applied to a deformation of the Hermitian-Einstein equations for the metric h. The approach in [11, 12] deforms the metric using a nonlinear parabolic equation, the Donaldson ow.Deforming the metric is equivalent to acting by a complex gauge transformation modulo unitary ones, and in this context the Donaldson ow is equivalent (up to unitary gauge transformations) to the Yang-Mills ow on the space of integrable unitary connections. The proof in [12] assumes that X is a projective algebraic manifold (more precisely, that ω is a Hodge metric) whereas the argument in [40] does not. The methods of Uhlenbeck-Yau and Donaldson were combined by Simpson [35] to prove convergence of the Yang-Mills ow for stable bundles on all compact Kähler manifolds. The Yang-Mills ow thus denes aHYM (E, h) from the space of smooth integrable connections on (E, h) inducing stable holomorphic structures to the moduli space M * HYM (E, h) of irreducible HYM connections.1 Continuity of this map follows by a comparison of Kuranishi slices (see [15,31]).When the holomorphic bundle E A = (E,∂ A ) is strictly semistable, then the Donaldson ow fails to converge unless E A splits holomorphically into a sum of stable bundles (i.e. it is polystable). If n = 1 it is still true, however, that the Yang-Mills ow converges to a smooth HYM connection on E for any semistable initial condition. This was proven by Daskalopoulos and Råde [7,32]. Moreover, the holomorphic structure of the limiting connection is isomorphic to the polystable holomorphic bundle Gr(E A ) obtained from the