Logarithmic Connections, WZNW Action, and Moduli of Parabolic Bundles on the Sphere

Moduli spaces of stable parabolic bundles of parabolic degree 0 over the Riemann sphere are stratified according to the Harder–Narasimhan filtration of underlying vector bundles. Over a Zariski open subset $$\mathscr {N}_{0}$$ N 0 of the open stratum depending explicitly on a choice of parabolic weights, a real-valued function $$\mathscr {S}$$ S is defined as the regularized critical value of the non-compact Wess–Zumino–Novikov–Witten action functional. The definition of $$\mathscr {S}$$ S depends on a suitable notion of parabolic bundle ‘uniformization map’ following from the Mehta–Seshadri and Birkhoff–Grothendieck theorems. It is shown that $$-\mathscr {S}$$ - S is a primitive for a (1,0)-form $$\vartheta $$ ϑ on $$\mathscr {N}_{0}$$ N 0 associated with the uniformization data of each intrinsic irreducible unitary logarithmic connection. Moreover, it is proved that $$-\mathscr {S}$$ - S is a Kähler potential for $$(\Omega -\Omega _{\mathrm {T}})|_{\mathscr {N}_{0}}$$ ( Ω - Ω T ) | N 0 , where $$\Omega $$ Ω is the Narasimhan–Atiyah–Bott Kähler form in $$\mathscr {N}$$ N and $$\Omega _{\mathrm {T}}$$ Ω T is a certain linear combination of tautological (1, 1)-forms associated with the marked points. These results provide an explicit relation between the cohomology class $$[\Omega ]$$ [ Ω ] and tautological classes, which holds globally over certain open chambers of parabolic weights where $$\mathscr {N}_{0} = \mathscr {N}$$ N 0 = N .

Stability of the parabolic Poincaré bundle

Given a compact Riemann surface [Formula: see text] and a moduli space [Formula: see text] of parabolic stable bundles on it of fixed determinant of complete parabolic flags, we prove that the Poincaré parabolic bundle on [Formula: see text] is parabolic stable with respect to a natural polarization on [Formula: see text].

THETA FUNCTIONS ON THE MODULI SPACE OF PARABOLIC BUNDLES

In this paper we extend the result on base point freeness of the powers of the determinant bundle on the moduli space of vector bundles on a curve. We describe the parabolic analogues of parabolic theta functions, then we determine a uniform bound depending only on the rank of the parabolic bundles. In order to get this bound, we construct a parabolic analogue of Grothendieck's scheme of quotients, which parametrizes quotient bundles of a parabolic bundle, of fixed parabolic Hilbert polynomial. We prove an estimate for its dimension, which extends the result of Popa and Roth on the dimension of the Quot scheme. As an application of the theorem on base point freeness, we characterize parabolic semistability on the algebraic stack of quasi-parabolic bundles as the base locus of the linear system of the parabolic determinant bundle.

The Narasimhan—Seshadri theorem for parabolic bundles: an orbifold approach

Given a stable parabolic bundle over a Riemann surface, we study the problem of finding a compatible Yang-Mills connexion. When the parabolic weights are rational there is an equivalent problem on an orbifold bundle. When the weights are irrational our method is to choose a sequence of approximating rational weights, obtain a corresponding sequence of Yang-Mills connexions on the resulting orbifold bundles and obtain the solution as the limit of this sequence: we need to consider mildly singular connexions which locally about a marked point take the form d — Aid# + a . Here A is a constant diagonal matrix whose entries depend on the weights and their rational approximations, 0 = arg(z) for z a local uniformizing (orbifold) coordinate centred on the marked point and a is an L 2 1 connexion matrix. In this context we find all the necessary gauge-theoretic tools to prove the theorem, including a version of Uhlenbeck’s weak compactness theorem, provided | A| is sufficiently small. (One of the advantages of this approach is that we do analysis on a compact orbifold rather than on the punctured surface.) Our methods also allow us to consider the analogous problem for stable parabolic Higgs bundles.

PROLONGEMENT D’UN FIBRE HOLOMORPHE HERMITIEN A COURBURE Lp SUR UNE COURBE OUVERTE

Let X be a Riemann surface and S a finite set of marked points on X. If [Formula: see text] is a hermitian holomorphic vector bundle with Lp-curvature for some p>1, we study the asymptotic behaviour of the Chern connection around the marked points; by solving directly a [Formula: see text]-problem in a weighted Sobolev space, we extend the holomorphic structure of [Formula: see text] over S to get a parabolic bundle. We deduce a proof of the classification of these hermitian metrics.