Ruadhaí Dervan

I'm a Royal Society University Research Fellow at the University of Cambridge, and a Research Fellow at Gonville & Caius College. I've previously spent time in École Polytechnique, Université libre de Bruxelles and Trinity College Dublin.

My first name is pronounced "Rui" (the "dh" is silent, it's Irish).

I research complex geometry. I'm interested in both the algebro-geometric and differential-geometric sides of complex geometry, and am particularly interested in the interplay between the two sides. Most of my research focuses on the existence of canonical metrics in complex geometry (e.g. Kähler-Einstein, extremal), notions of algebro-geometric stability (e.g. K-stability, Bridgeland stability), and related aspects of moduli theory and geometric analysis. My work is mostly motivated by the "Yau-Tian-Donaldson" conjecture, which links stability notions to the existence of canonical metrics; see Donaldson's survey for an introduction.

I taught Part III Complex Manifolds in Lent Term 2020 and Lent Term 2019.

My CV. My email address is rd430@cam.ac.uk.

Papers and preprints:

• Extremal Kähler metrics on blowups (with Lars Sektnan) arXiv:2110.13579 (abstract)

  Consider a compact Kähler manifold which either admits an extremal Kähler metric, or is a small deformation of such a manifold. We show that the blowup of the manifold at a point admits an extremal Kähler metric in Kähler classes making the exceptional divisor sufficiently small if and only if it is relatively K-stable, thus proving a special case of the Yau-Tian-Donaldson conjecture. We also give a geometric interpretation of what relative K-stability means in this case in terms of finite dimensional geometric invariant theory. This gives a complete solution to a problem addressed in special cases by Arezzo, Pacard, Singer and Székelyhidi. In addition, the case of a deformation of an extremal manifold proves the first non-trivial case of a general conjecture of Donaldson.

• Stability conditions for polarised varieties arXiv:2103.03177 (video) (abstract)

  We introduce an analogue of Bridgeland's stability conditions for polarised varieties. Much as Bridgeland stability is modelled on slope stability of coherent sheaves, our notion of Z-stability is modelled on the notion of K-stability of polarised varieties. We then introduce an analytic counterpart to stability, through the notion of a Z-critical Kähler metric, modelled on the constant scalar curvature Kähler condition. Our main result shows that a polarised variety which is analytically K-semistable and asymptotically Z-stable admits Z-critical Kähler metrics in the large volume regime. We also prove a local converse, and explain how these results can be viewed in terms of local wall crossing. A special case of our framework gives a manifold analogue of the deformed Hermitian Yang-Mills equation.

• Z-critical connections and Bridgeland stability conditions (with John McCarthy and Lars Sektnan) arXiv:2012.10426 (video) (abstract)

  We associate geometric partial differential equations on holomorphic vector bundles to Bridgeland stability conditions. We call solutions to these equations Z-critical connections, with Z a central charge. Deformed Hermitian Yang-Mills connections are a special case. We explain how our equations arise naturally through infinite dimensional moment maps.

  Our main result shows that in the large volume limit, a sufficiently smooth holomorphic vector bundle admits a Z-critical connection if and only if it is asymptotically Z-stable. Even for the deformed Hermitian Yang-Mills equation, this provides the first examples of solutions in higher rank.

• Valuative stability of polarised varieties (with Eveline Legendre) arXiv:2010.04023 (to appear in Math. Ann.) (abstract)

  Fujita and Li have given a characterisation of K-stability of a Fano variety in terms of quantities associated to valuations, which has been essential to all recent progress in the area. We introduce a notion of valuative stability for arbitrary polarised varieties, and show that it is equivalent to K-stability with respect to test configurations with integral central fibre. The numerical invariant governing valuative stability is modelled on Fujita's beta invariant, but includes a term involving the derivative of the volume. We give several examples of valuatively stable and unstable varieties, including the toric case. We also discuss the role that the delta invariant plays in the study of valuative stability and K-stability of polarised varieties.

• Uniqueness of optimal symplectic connections (with Lars Sektnan) arXiv:2003.13626 (Forum Math. Sigma. Vol 9 (2021) e18, pp. 1-37) (video) (abstract)

  Consider a holomorphic submersion between compact Kähler manifolds, such that each fibre admits a constant scalar curvature Kähler metric. When the fibres admit continuous automorphisms, a choice of fibrewise constant scalar curvature Kähler metric is not unique. An optimal symplectic connection is choice of fibrewise constant scalar curvature Kähler metric satisfying a geometric partial differential equation. The condition generalises the Hermite-Einstein condition for a holomorphic vector bundle, through the induced fibrewise Fubini-Study metric on the associated projectivisation.

  We prove various foundational analytic results concerning optimal symplectic connections. Our main result proves that optimal symplectic connections are unique, up to the action of the automorphism group of the submersion, when they exist. Thus optimal symplectic connections are canonical relatively Kähler metrics when they exist. In addition, we show that the existence of an optimal symplectic connection forces the automorphism group of the submersion to be reductive, and that an optimal symplectic connection is automatically invariant under a maximal compact subgroup of this automorphism group.

• Moduli theory, stability of fibrations and optimal symplectic connections (with Lars Sektnan) arXiv:1911.12701 (Geom. Topol. 25-5 (2021), pp. 2643-2697) (video) (abstract)

  K-polystability is, on the one hand, conjecturally equivalent to the existence of certain canonical Kähler metrics on polarised varieties, and, on the other hand, conjecturally gives the correct notion to form moduli. We introduce a notion of stability for families of K-polystable varieties, extending the classical notion of slope stability of a bundle, viewed as a family of K-polystable varieties via the associated projectivisation. We conjecture that this is the correct condition for forming moduli of fibrations.

  Our main result relates this stabiility condition to Kähler geometry: we prove that the existence of an optimal symplectic connection implies semistability of the fibration. An optimal symplectic connection is a choice of fibrewise constant scalar curvature Kähler metric, satisfying a certain geometric partial differential equation. We conjecture that the existence of such a connection is equivalent to polystability of the fibration. We prove a finite dimensional analogue of this conjecture, by describing a GIT problem for fibrations embedded in a fixed projective space, and showing that GIT polystability is equivalent to the existence of a zero of a certain moment map.

• Optimal symplectic connections on holomorphic submersions (with Lars Sektnan) arXiv:1907.11014 (Comm. Pure Appl. Math. Vol. 74 no. 10 (2021), pp. 2134-2184) (abstract)

  The main result of this paper gives a new construction of extremal Kähler metrics on the total space of certain holomorphic submersions, giving a vast generalisation and unification of results of Hong, Fine and others. The principal new ingredient is a novel geometric partial differential equation on such fibrations, which we call the optimal symplectic connection equation.

  We begin with a smooth fibration for which all fibres admit a constant scalar curvature Kähler metric. When the fibres admit automorphisms, such metrics are not unique in general, but rather are unique up to the action of the automorphism group of each fibre. We define an equation which, at least conjecturally, determines a canonical choice of constant scalar curvature Kähler metric on each fibre. When the fibration is a projective bundle, this equation specialises to asking that the hermitian metric determining the fibrewise Fubini-Study metric is Hermite-Einstein.

  Assuming the existence of an optimal symplectic connection, and the existence of an appropriate twisted extremal metric on the base of the fibration, we show that the total space of the fibration itself admits an extremal metric for certain polarisations making the fibres small.

• K-semistability of optimal degenerations arXiv:1905.11334 (Q. J. Math. Vol. 71 (2020) Issue 3, pp. 989-995) (abstract)

  K-polystability of a polarised variety is an algebro-geometric notion conjecturally equivalent to the existence of a constant scalar curvature Kähler metric. When a variety is K-unstable, it is expected to admit a "most destabilising" degeneration. In this note we show that if such a degeneration exists, then the limiting scheme is itself relatively K-semistable.

• Moduli of polarised manifolds via canonical Kähler metrics (with Philipp Naumann) arXiv:1810.02576 (video) (abstract)

  We construct a moduli space of polarised manifolds which admit a constant scalar curvature Kähler metric. We show that this space admits a natural Kähler metric.

• Extremal metrics on fibrations (with Lars Sektnan) arXiv:1712.05374 (Proc. Lond. Math. Soc. (3) 120 (2020) pp. 587-616) (video) (abstract)

  Consider a fibred compact Kähler manifold X endowed with a relatively ample line bundle, such that each fibre admits a constant scalar curvature Kähler metric and has discrete automorphism group. Assuming the base of the fibration admits a twisted extremal metric where the twisting form is a certain Weil-Petersson type metric, we prove that X admits an extremal metric for polarisations making the fibres small. Thus X admits a constant scalar curvature Kähler metric if and only if the Futaki invariant vanishes. This extends a result of Fine, who proved this result when the base admits no continuous automorphisms.

  As consequences of our techniques, we obtain the following analogues for maps of various fundamental results for varieties: if a map admits a twisted constant scalar curvature Kähler metric metric, then its automorphism group is reductive; a twisted extremal metric is invariant under a maximal compact subgroup of the automorphism group of the map; there is a geometric interpretation for uniqueness of twisted extremal metrics on maps.

• Stable maps in higher dimensions (with Julius Ross) arXiv:1708.09750 (Math. Ann. Vol. 374 (2019) no. 3-4, pp. 1033-1073) (abstract)

  We formulate a notion of stability for maps between polarised varieties which generalises Kontsevich's definition when the domain is a curve and Tian-Donaldson's definition of K-stability when the target is a point. We give some examples, such as Kodaira embeddings and fibrations. We prove the existence of a projective moduli space of canonically polarised stable maps, generalising the Kontsevich-Alexeev moduli space of stable maps in dimensions one and two. We also state an analogue of the Yau-Tian-Donaldson conjecture in this setting, relating stability of maps to the existence of certain canonical Kähler metrics.

• Hermitian Yang-Mills connections on blowups (with Lars Sektnan) arXiv:1707.07638 (J. Geom. Anal. Vol. 31 (2021), pp. 516-542) (abstract)

  Consider a vector bundle over a Kähler manifold which admits a Hermitian Yang-Mills connection. We show that the pullback bundle on the blowup of the Kähler manifold at a collection of points also admits a Hermitian Yang-Mills connection, for Kähler classes on the blowup which make the exceptional divisors small. Our proof uses gluing techniques, and is hence asymptotically explict. This recovers, through the Hitchin-Kobayashi correspondence, algebro-geometric results due to Buchdahl and Sibley.

• The Kähler-Ricci flow and optimal degenerations (with Gábor Székelyhidi) arXiv:1612.07299 (J. Differential Geom. Vol. 116 (2020), no. 1 pp. 187-203) (abstract)

  We prove that on Fano manifolds, the Kähler-Ricci flow produces a "most destabilising" degeneration, with respect to a new stability notion related to the H-functional. This answers questions of Chen-Sun-Wang and He.

  We give two applications of this result. Firstly, we give a purely algebro-geometric formula for the supremum of Perelman's μ-functional on Fano manifolds, resolving a conjecture of Tian-Zhang-Zhang-Zhu as a special case. Secondly, we use this to prove that if a Fano manifold admits a Kähler-Ricci soliton, then the Kähler-Ricci flow converges to it modulo the action of automorphisms, with any initial metric. This extends work of Tian-Zhu and Tian-Zhang-Zhang-Zhu, where either the manifold was assumed to admit a Kähler-Einstein metric, or the initial metric of the flow was assumed to be invariant under a maximal compact group of automorphism.

• Relative K-stability for Kähler manifolds arXiv:1611.00569 (Math. Ann. Vol. 372 (2018), no. 3-4, pp. 859-889) (addendum) (abstract)

  We study the existence of extremal Kähler metrics on Kähler manifolds. After introducing a notion of relative K-stability for Kähler manifolds, we prove that Kähler manifolds admitting extremal Kähler metrics are relatively K-stable. Along the way, we prove a general L^p lower bound on the Calabi functional involving test configurations and their associated numerical invariants, answering a question of Donaldson.

  When the Kähler manifold is projective, our definition of relative K-stability is stronger than the usual definition given by Székelyhidi. In particular our result strengthens the known results in the projective case (even for constant scalar curvature Kähler metrics), and rules out a well known counterexample to the "naïve" version of the Yau-Tian-Donaldson conjecture in this setting.

I wrote an appendix to Sjöström Dyrefelt's "On K-polystability of cscK manifolds with transcendental cohomology class", which removes a technical assumption from the main result of this paper, see arXiv:1711.11482. In particular the existence of an extremal metric on a Kähler manifold implies relative K-stability, without any assumptions on the norms of the objects involved.

• K-stability for Kähler manifolds (with Julius Ross) arXiv:1602.08983 (Math. Res. Lett. Vol. 24, No. 3 (2017), pp. 689-739) (abstract)

  We formulate a notion of K-stability for Kähler manifolds, and prove one direction of the Yau-Tian-Donaldson conjecture in this setting. More precisely, we prove that the Mabuchi functional being bounded below (resp. coercive) implies K-semistability (resp. uniformly K-stable). In particular this shows that the existence of a constant scalar curvature Kähler metric implies K-semistability, and K-stability if one assumes the automorphism group is discrete. We also show how Stoppa's argument holds in the Kähler case, giving a simpler proof of this K-stability statement.

• A finite dimensional approach to Donaldson's J-flow (with Julien Keller) arXiv:1507.03461 (Comm. Anal. Geom. (2019) Vol. 27, no. 5. pp. 1025-1085) (abstract)

  Consider a projective manifold with two distinct polarisations L₁ and L₂. From this data, Donaldson has defined a natural flow on the space of Kähler metrics in c₁(L₁), called the J-flow. The existence of a critical point of this flow is closely related to the existence of a constant scalar curvature Kähler metric in c₁(L₁) for certain polarisations L₂.

  Associated to a quantum parameter k>>0, we define a flow over Bergman type metrics, which we call the J-balancing flow. We show that in the quantum limit of k→∞, the rescaled J-balancing flow converges towards the J-flow. As corollaries, we obtain new proofs of uniqueness of critical points of the J-flow and also 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 J-balanced metrics for k >> 0. Defining a notion of Chow stability for linear systems, we show that this in turn implies the linear system |L₂| is asymptotically Chow stable. Asymptotic Chow stability of |L₂| implies an analogue of K-semistability for the J-flow introduced by Lejmi-Székelyhidi, which we call J-semistability. We prove also that J-stability holds automatically in a certain numerical cone around L₂, and that if L₂ is the canonical class of the manifold that J-semistability implies K-stability. Eventually, this leads to new K-stable polarisations of surfaces of general type.

• On K-stability of finite covers arXiv:1505.07754 (Bull. London Math. Soc. (2016) 48 (4) pp. 717-728) (abstract)

  We show that certain Galois covers of K-semistable Fano varieties are K-stable. We use this to give some new examples of Fano manifolds admitting Kähler-Einstein metrics, including hypersurfaces, double solids and threefolds.

• Non-reductive automorphism groups, the Loewy filtration and K-stability (with Giulio Codogni) arXiv:1501.03372 (Annales de l'institut Fourier 66 no. 5 (2016) pp. 1895-1921) (corrigendum) (abstract)

  We study the K-stability of a polarised variety with non-reductive automorphism group. We associate a canonical filtration of the co-ordinate ring to each variety of this kind, which destabilises the variety in several examples which we compute. We conjecture this holds in general. This is an algebro-geometric analogue of Matsushima's theorem regarding the existence of constant scalar curvature Kähler metrics. As an application, we give an example of an orbifold del Pezzo surface without a Kähler-Einstein metric.

• Alpha invariants and coercivity of the Mabuchi functional on Fano manifolds arXiv:1412.1426 (Ann. Fac. Sci. Toulouse Sér. 6, 25 no. 4 (2016), p. 919-934) (abstract)

  We give a criterion for the coercivity of the Mabuchi functional for general Kähler classes on Fano manifolds in terms of Tian's alpha invariant. This generalises a result of Tian in the anti-canonical case implying the existence of a Kähler-Einstein metric. We also prove the alpha invariant is a continuous function on the Kähler cone. As an application, we provide new Kähler classes on a general degree one del Pezzo surface for which the Mabuchi functional is coercive.

• Uniform stability of twisted constant scalar curvature Kähler metrics arXiv:1412.0648 (Int. Math. Res. Notices (2016) Vol 15 pp. 4728-4783) (abstract)

  We introduce a norm on the space of test configurations, which we call the minimum norm. We conjecture that uniform K-stability with respect to this norm is equivalent to the existence of a constant scalar curvature Kähler metric. This notion of uniform K-stability is analogous to coercivity of the Mabuchi functional. We characterise the triviality of test configurations, by showing that a test configuration has zero minimum norm if and only if it has zero L²-norm, if and only if it is almost trivial.

  We prove that the existence of a twisted constant scalar curvature Kähler metric implies uniform twisted K-stability with respect to the minimum norm, when the twisting is ample.

  We give algebro-geometric proofs of uniform K-stability in the general type and Calabi-Yau cases, as well as in the Fano case under an alpha invariant condition. Our results hold for line bundles sufficiently close to the (anti)-canonical line bundle, and also in the twisted setting. We show that log K-stability implies twisted K-stability, and also that twisted K-semistability of a variety implies that the variety has mild singularities.

• Alpha invariants and K-stability for general polarisations of Fano varieties arXiv:1307.6527 (Int. Math. Res. Notices (2015) Vol 16 pp. 7162-7189) (abstract)

  We provide a sufficient condition for polarisations of Fano varieties to be K-stable in terms of Tian's alpha invariant, which uses the log canonical threshold to measure singularities of divisors in the linear system associated to the polarisation. This generalises a result of Odaka-Sano in the anti-canonically polarised case, which is the algebraic counterpart of Tian's analytic criterion implying the existence of a Kähler-Einstein metric. As an application, we give new K-stable polarisations of a general degree one del Pezzo surface. We also prove a corresponding result for log K-stability.

Students:

• Annamaria Ortu, PhD student at SISSA Trieste, co-supervised by Jacopo Stoppa (2020 - present)
• John McCarthy, PhD student at Imperial College, co-supervised by Simon Donaldson (2019 - present)
• Michael Hallam, DPhil student at the University of Oxford, co-supervised by Frances Kirwan (2018 - 2022)
• Alexia Corradini, visiting M1 master's student from École Polytechnique (spring/summer 2022)
• Ho Leung Fong, undergraduate summer research student (summer 2021)
• Qiangru Kuang, undergraduate summer research student (summer 2019)

Postdocs:

• Rémi Reboulet (2022 - present)

I have organised conferences in Cambridge (K-stability and Kähler geometry, 2021), Newcastle (Newcastle Complex Geometry Workshop, 2018), Rome (Moduli of K-stable varieties, 2017) and Cambridge (Postgraduate Conference in Complex Geometry, 2015). The Rome conference has an associated conference proceedings, also edited by Codogni and Viviani.

I ran the Cambridge Junior Geometry Seminar (called "Geometry Tea") in 2013-2014.

Slightly outdated list of conferences I've been to or am going to:

Pickles