Isomonodromic deformations of logarithmic connections and stability

Abstract

Let \(X_0\) be a compact connected Riemann surface of genus g with \(D_0 \subset X_0\) an ordered subset of cardinality n, and let \(E_G\) be a holomorphic principal G-bundle on \(X_0\), where G is a reductive affine algebraic group defined over \(\mathbb C\), that is equipped with a logarithmic connection \(\nabla _0\) with polar divisor \(D_0\). Let \((\mathcal {E}_G , \nabla )\) be the universal isomonodromic deformation of \((E_G ,\nabla _0)\) over the universal Teichmuller curve \((\mathcal {X}, \mathcal {D})\,{\longrightarrow }\, \text {Teich}_{g,n}\), where \(\text {Teich}_{g,n}\) is the Teichmuller space for genus g Riemann surfaces with n–marked points. We prove the following (see Sect. 5): (1) Assume that \(g \ge 2\) and \(n= 0\). Then there is a closed complex analytic subset \(\mathcal {Y} \subset \text {Teich}_{g,n}\), of codimension at least g, such that for any \(t\in \text {Teich}_{g,n} {\setminus } \mathcal {Y}\), the principal G-bundle \(\mathcal {E}_G\vert _{{\mathcal X}_t}\) is semistable, where \({\mathcal X}_t\) is the compact Riemann surface over t. (2) Assume that \(g\ge 1\), and if \(g= 1\), then \(n > 0\). Also, assume that the monodromy representation for \(\nabla _0\) does not factor through some proper parabolic subgroup of G. Then there is a closed complex analytic subset \(\mathcal {Y}' \subset \text {Teich}_{g,n}\), of codimension at least g, such that for any \(t\in \text {Teich}_{g,n} {\setminus } \mathcal {Y}'\), the principal G-bundle \(\mathcal {E}_G\vert _{{\mathcal X}_t}\) is semistable. (3) Assume that \(g\ge 2\). Assume that the monodromy representation for \(\nabla _0\) does not factor through some proper parabolic subgroup of G. Then there is a closed complex analytic subset \(\mathcal {Y}'' \subset \text {Teich}_{g,n}\), of codimension at least \(g-1\), such that for any \(t\in \text {Teich}_{g,n} {\setminus } \mathcal {Y}'\), the principal G-bundle \(\mathcal {E}_G\vert _{{\mathcal X}_t}\) is stable. In [12], the second-named author proved the above results for \(G= \text {GL}(2,{\mathbb C})\).