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Julien Keller

Publications and source records attributed to Julien Keller.

At least 19 recordsLinked to original sources
arXiv2024

$Z$-critical equations for holomorphic vector bundles on K\"ahler surfaces

We prove that the existence of a $Z$-positive and $Z$-critical Hermitian metric on a rank 2 holomorphic vector bundle over a compact K\"ahler surface implies that the bundle is $Z$-stable. As particular cases, we obtain stability results for the deformed Hermitian Yang-Mills equation and the almost Hermite-Einstein equation for rank 2 bundles over surfaces. We show examples of $Z$-unstable bundles and $Z$-critical metrics away from the large volume limit.

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arXiv2021

Quot-scheme limit of Fubini-Study metrics and its applications to balanced metrics

We present some results that complement our prequels [arXiv:1809.08425,arXiv:1907.05770] on holomorphic vector bundles. We apply the method of the Quot-scheme limit of Fubini-Study metrics developed therein to provide a generalisation to the singular case of the result originally obtained by X.W. Wang for the smooth case, which states that the existence of balanced metrics is equivalent to the Gieseker stability of the vector bundle. We also prove that the Bergman 1-parameter subgroups form subgeodesics in the space of hermitian metrics. This paper also contains a review of techniques developed in [arXiv:1809.08425,arXiv:1907.05770] and how they correspond to their counterparts developed in the study of the Yau-Tian-Donaldson conjecture.

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arXiv2019

A variational approach to the Hermitian-Einstein metrics and the Quot-scheme limit of Fubini-Study metrics

This is a sequel of our paper [arXiv:1809.08425] on the Quot-scheme limit and variational properties of Donaldson's functional, which established its coercivity for slope stable holomorphic vector bundles over smooth projective varieties. Assuming that the coercivity is uniform in a certain sense, we provide a new proof of the Donaldson-Uhlenbeck-Yau theorem, in such a way that the analysis involved in the proof is elementary except for the asymptotic expansion of the Bergman kernel.

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arXiv2018

Quot-scheme limit of Fubini-Study metrics and Donaldson's functional for vector bundles

For a holomorphic vector bundle $E$ over a polarised K\"ahler manifold, we establish a direct link between the slope stability of $E$ and the asymptotic behaviour of Donaldson's functional, by defining the Quot-scheme limit of Fubini-Study metrics. In particular, we provide an explicit estimate which proves that Donaldson's functional is coercive on the set of Fubini-Study metrics if $E$ is slope stable, and give a new proof of Hermitian-Einstein metrics implying slope stability.

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arXiv2017

About J-flow, J-balanced metrics, uniform J-stability and K-stability

From the work of Dervan-Keller, there exists a quantization of the critical equation for the J-flow. This leads to the notion of J-balanced metrics. We prove that the existence of J-balanced metrics has a purely algebro-geometric characterization in terms of Chow stability, complementing the result of Dervan-Keller. We also obtain various criteria that imply uniform J-stability and uniform K-stability. Eventually, we discuss the case of Kähler classes that may not be integral over a compact manifold.

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arXiv2017

A finite dimensional approach to Donaldson's J-flow

We define a quantisation of the J-flow over a projective complex manifold. As corollaries, we obtain new proofs of uniqueness of critical points of the J-flow and that these critical points achieve the absolute minimum of an associated energy functional. We show that the existence of a critical point of the J-flow implies the existence of certain canonical metrics, that we call J-balanced metrics. We define a notion of Chow stability for linear systems and relate it to the existence of J-balanced metrics. We also relate the asymptotic Chow stability of a linear system to an analogue of K-semistability that was introduced by Lejmi-Székelyhidi, which we call J-semistability. Then, we relate J-semistability to K-stability when one of the polarisation is the canonical bundle. Eventually, this gives new K-stable polarisations of surfaces of general type.

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arXiv2017

Construction of constant scalar curvature Kähler cone metrics

Over a compact Kähler manifold, we provide a Fredholm alternative result for the Lichnerowicz operator associated to a Kähler metric with conic singularities along a divisor. We deduce several existence results of constant scalar curvature Kähler metrics with conic singularities: existence result under small deformations of Kähler classes, existence result over a Fano manifold, existence result over certain ruled manifolds. In this last case, we consider the projectivisation of a parabolic stable holomorphic bundle. This leads us to prove that the existing Hermitian-Einstein metric on this bundle enjoys a regularity property along the divisor on the base.

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arXiv2017

Relative K-polystability of projective bundles over a curve

Let $P(E)$ be the projectivization of a holomorphic vector bundle $E$ over a compact complex curve $C$. We characterize the existence of an extremal Kähler metric on the ruled manifold $P(E)$ in terms of relative K-polystability and the fact that $E$ decomposes as a direct sum of stable bundles.

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arXiv2017

On the lower bounds of the L^2-norm of the Hermitian scalar curvature

On a pre-quantized symplectic manifold, we show that the symplectic Futaki invariant, which is an obstruction to the existence of constant Hermitian scalar curvature almost-Kähler metrics, is actually an asymptotic invariant. This allows us to deduce a lower bound for the L^2-norm of the Hermitian scalar curvature as obtained by S. Donaldson \cite{Don} in the Kähler case.

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arXiv2016

Quantization of Hitchin's equations for Higgs Bundles I

We provide an algebraic framework for quantization of Hermitian metrics that are solutions of the Hitchin equation for Higgs bundles over a projective manifold. Using Geometric Invariant Theory, we introduce a notion of balanced metrics in this context. We show that balanced metrics converge at the quantum limit towards the solution of the Hitchin equation. We relate the existence of balanced metrics to the Gieseker stability of the Higgs bundle.

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arXiv2015

Quantization of the Laplacian operator on vector bundles I

Let $(E,h)$ be a holomorphic Hermitian vector bundle over a polarized manifold. We provide a canonical quantization of the Laplacian operator acting on sections of the bundle of Hermitian endomorphisms of $E$. If $E$ is simple we obtain an approximation of the eigenvalues and eigenspaces of the Laplacian.

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arXiv2014

Quantization of Donaldson's heat flow over projective manifolds

Consider $E$ a holomorphic vector bundle over a projective manifold $X$ polarized by an ample line bundle $L$. Fix $k$ large enough, the holomorphic sections $H^0(E\otimes L^k)$ provide embeddings of $X$ in a Grassmanian space. We define the \textit{balancing flow for bundles} as a flow on the space of projectively equivalent embeddings of $X$. This flow can be seen as a flow of algebraic type hermitian metrics on $E$. At the quantum limit $k\to \infty$, we prove the convergence of the balancing flow towards the Donaldson heat flow, up to a conformal change. As a by-product, we obtain a numerical scheme to approximate the Yang-Mills flow in that context.

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arXiv2012

A note on Chow stability of the Projectivisation of Gieseker Stable Bundles

We investigate Chow stability of projective bundles P(E) where E is a strictly Gieseker stable bundle over a base manifold that has constant scalar curvature. We show that, for suitable polarisations L, the pair (P(E),L) is Chow stable and give examples for which it is not asymptotically Chow stable.

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arXiv2012

Numerical Weil-Petersson metrics on moduli spaces of Calabi-Yau manifolds

We introduce a simple and very fast algorithm that computes Weil-Petersson metrics on moduli spaces of polarized Calabi-Yau manifolds. Also, by using Donaldson's quantization link between the infinite and finite dimensional G.I.T quotients that describe moduli spaces of varieties, we define a natural sequence of Kaehler metrics. We prove that the sequence converges to the Weil-Petersson metric. We also develop an algorithm that numerically approximates such metrics, and hence the Weil-Petersson metric itself. Explicit examples are provided on a family of Calabi-Yau Quintic hypersurfaces in CP^4. The scope of our second algorithm is much broader; the same techniques can be used to approximate metrics on null spaces of Dirac operators coupled to Hermite Yang-Mills connections.

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arXiv2011

Non trivial examples of coupled equations for Kähler metrics and Yang-Mills connections

We provide non trivial examples of solutions to the system of coupled equations introduced by M. García-Fernández for the uniformization problem of a triple $(M,L,E)$ where $E$ is a holomorphic vector bundle over a polarized complex manifold $(M,L)$, generalizing the notions of both constant scalar curvature Kähler metric and Hermitian-Einstein metric.

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arXiv2011

About the Calabi problem: a finite dimensional approach

Let us consider a projective manifold and $Ω$ a volume form. We define the gradient flow associated to the problem of $Ω$-balanced metrics in the quantum formalism, the Ω$-balacing flow. At the limit of the quantization, we prove that the $Ω$-balacing flow converges towards a natural flow in Kähler geometry, the $Ω$-Kähler flow. We study the existence of the $Ω$-Kähler flow and proves its long time existence and convergence towards the solution to the Calabi problem of prescribing the volume form in a given Kähler class. We derive some natural geometric consequences of our study.

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arXiv2007

Ricci iterations on Kahler classes

In this paper we consider the dynamical system involved by the Ricci operator on the space of Kähler metrics. A. Nadel has defined an iteration scheme given by the Ricci operator for Fano manifold and asked whether it has some nontrivial periodic points. First, we prove that no such periodic points can exist. We define the inverse of the Ricci operator and consider the dynamical behaviour of its iterates for a Fano Kähler-Einstein manifold. In particular we show that the iterates do converge to the existing Kähler-Ricci soliton on a toric manifold. Finally, we define a finite dimensional procedure to give an approximation of Kähler-Einstein metrics using this iterative procedure and apply it for $\mathbb{CP}^2$ blown up in 3 points.

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