arXiv subjects

Florent Schaffhauser

Publications and source records attributed to Florent Schaffhauser.

25 records · Page 2Linked to original sources

arXiv2011

Real points of coarse moduli schemes of vector bundles on a real algebraic curve

We examine a moduli problem for real and quaternionic vector bundles on a smooth complex projective curve with a fixed real structure, and we give a gauge-theoretic construction of moduli spaces for semi-stable such bundles with fixed topological type. These spaces embed onto connected subsets of real points inside a complex projective variety. We relate our point of view to previous work by Biswas, Huisman and Hurtubise (arxiv:0901.3071), and we use this to study the Galois action induced on moduli varieties of stable holomorphic bundles on a complex curve by a given real structure on the curve. We show in particular a Harnack-type theorem, bounding the number of connected components of the fixed-point set of that action by $2^g +1$, where $g$ is the genus of the curve. In fact, taking into account all the topological invariants of the real structure, we give an exact count of the number of connected components, thus generalising to rank $r > 1$ the results of Gross and Harris on the Picard scheme of a real algebraic curve.

math.AG↗

arXiv2010

Moduli spaces of vector bundles over a Klein surface

A compact topological surface S, possibly non-orientable and with non-empty boundary, always admits a Klein surface structure (an atlas whose transition maps are dianalytic). Its complex cover is, by definition, a compact Riemann surface M endowed with an anti-holomorphic involution which determines topologically the original surface S. In this paper, we compare dianalytic vector bundles over S and holomorphic vector bundles over M, devoting special attention to the implications that this has for moduli varieties of semistable vector bundles over M. We construct, starting from S, totally real, totally geodesic, Lagrangian submanifolds of moduli varieties of semistable vector bundles of fixed rank and degree over M. This relates the present work to the constructions of Ho and Liu over non-orientable compact surfaces with empty boundary (arXiv:math/0605587) .

math.DG↗

arXiv2009

Un théorème de convexité réel pour les applications moment à valeurs dans un groupe de Lie

In this note, we state and give the main ideas of the proof of a real convexity theorem for group-valued momentum maps. This result is a quasi-Hamiltonian analogue of the O'Shea-Sjamaar theorem in the usual Hamiltonian setting. We prove here that the image under the momentum map of the fixed-point set of a form-reversing involution defined on a quasi-Hamiltonian space is a convex polytope, that we describe as a subpolytope of the momentum polytope.

math.SG↗

arXiv2008

Representations of the fundamental group of an L-punctured sphere generated by products of Lagrangian involutions

In this paper, we characterize unitary representations of the fundamental group of a punctured sphere whose generators can be decomposed into products of two Lagrangian involutions. Our main result is that such representations are exactly the elements of the fixed-point set of an anti-symplectic involution defined on the moduli space of unitary representations of this fundamental group. Consequently, it is a Lagrangian submanifold of this representation space.

math.SG↗

arXiv2008

Decomposable representations and Lagrangian submanifolds of moduli spaces associated to surface groups

In this paper, we construct a Lagrangian submanifold of the moduli space associated to the fundamental group of a punctured Riemann surface (the space of representations of this fundamental group into a compact connected Lie group). This Lagrangian submanifold is obtained as the fixed-point set of an anti-symplectic involution defined on the moduli space. The notion of decomposable representation provides a geometric interpretation of this Lagrangian submanifold.

math.SG↗

arXiv2008

Quasi-Hamiltonian quotients as disjoint unions of symplectic manifolds

We show that the quotient associated to a quasi-Hamiltonian space has a symplectic structure even when 1 is not a regular value of the momentum map: it is a disjoint union of symplectic manifolds of possibly different dimensions, which generalizes a result of Alekseev, Malkin and Meinrenken. We illustrate this theorem with the example of representation spaces of surface groups. As an intermediary step, we show that the isotropy submanifolds of a quasi-Hamiltonian space are quasi-Hamiltonian spaces themselves.

math.SG↗

arXiv2007

A real convexity theorem for quasi-hamiltonian actions

The main result of this paper is a quasi-hamiltonian analogue of a special case of the O'Shea-Sjamaar convexity theorem for usual momentum maps. We denote by U a simply connected compact connected Lie group and we fix an involutive automorphism of maximal rank on this Lie group (such an automorphism always exists). We then denote by M a quasi-hamiltonian U-space and we prove that the image under the momentum map of the fixed-point set of a form-reversing compatible involution of M is a convex polytope, which is in fact equal to the full momentum polytope. This theorem was announced in arXiv:math/0609517v1. As an application, we obtain an example of lagrangian subspace in representation spaces of surface groups.

math.SG↗