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David Witt Nyström

Publications and source records attributed to David Witt Nyström.

At least 19 recordsLinked to original sources

Transcendental Okounkov bodies

We show that the volume of transcendental big $(1,1)$-classes on compact Kähler manifolds can be realized by convex bodies, thus answering questions of Lazarsfeld-Mustaţă and Deng. In our approach we use an approximation process by partial Okounkov bodies together with properties of the restricted volume, and we study the extension of Kähler currents, as well as the bimeromorphic behavior of currents with analytic singularities. We also establish a connection between transcendental Okounkov bodies and toric degenerations.

math.DG↗

Infinite-dimensional flats in the space of positive metrics on an ample line bundle

We show that any continuous positive metric on an ample line bundle L lies at the apex of many infinite-dimensional Mabuchi-flat cones. More precisely, given any bounded graded filtration F of the section ring of L, the set of bounded decreasing convex functions on the support of the Duistermaat--Heckman measure of F embeds L^p-isometrically into the space of bounded positive metrics on L with respect to Darvas' d_p distance for all p\in[1,\infty), and in particular with respect to the Mabuchi metric (p=2).

math.DG↗

Two Calabi-Yau theorems for degenerations of compact Kähler manifolds

We discuss two closely related Calabi-Yau theorems for degenerations of compact Kähler manifolds. The first is a Calabi-Yau theorem for big test configurations, that generalizes a result in [WN24]. It follows from recent joint work with Mesquita-Piccione [MW25], but is here given a more direct proof. The second result is a Calabi-Yau theorem for a wider class of degenerations, formulated in the language of non-Archimedean Kähler geometry. It was first proved in the algebraic setting by Boucksom-Jonsson [BJ22], building on earlier work of Boucksom-Favre-Jonsson [BFJ15], while the general Kähler case was established in [MW25]. Our main focus here is on the connection between these results and the theory of big cohomology classes and their volumes.

math.DG↗

A transcendental non-Archimedean Calabi--Yau Theorem with applications to the cscK problem

Let $X$ be a compact Kähler manifold and $α$ a Kähler class on $X$. We prove that if $(X,α)$ is uniformly K-stable for models, then there is a unique cscK metric in $α$. This was first proved in the algebraic case by Chi Li, and it strengthens a related result in an article of Mesquita-Piccione. K-stability for models is defined in terms of big test configurations, but we also give a valuative criterion as in the work of Boucksom--Jonsson together with an explicit formula for the associated $β$-invariant. To accomplish this we further develop the non-Archimedean pluripotential theory in the transcendental setting, as initiated in the works of Darvas--Xia--Zhang and Mesquita-Piccione. In particular we prove the continuity of envelopes and orthogonality properties, and using that, we are able to extend the non-Archimedean Calabi-Yau Theorem found in an article of Boucksom--Jonsson to the general Kähler setting.

math.AG↗

Competitive Hele-Shaw flow and quadratic differentials

We introduce and investigate a generalization of the Hele-Shaw flow with injection where several droplets compete for space as they try to expand due to internal pressure while still preserving their topology. Droplets are described by their closed non-crossing interface curves in $\mathbb{C}$ or more generally in a Riemann surface of finite type. Our main focus is on stationary solutions which we show correspond to the critical vertical trajectories of a particular quadratic differential with second order poles at the source points. The quadratic differentials that arise in this way have a simple description in terms of their associated half-translation surfaces. Existence of stationary solutions is proved in some generality by solving an extremal problem involving an electrostatic energy functional, generalizing a classic problem studied by Teichmüller, Jenkins, Strebel and others. We study several special cases, including stationary Jordan curves on the Riemann sphere. We also introduce a discrete random version of the dynamics closely related to Propp's competitive erosion model, and conjecture that realizations of the lattice model will converge towards a corresponding solution to the competitive Hele-Shaw problem as the mesh size tends to zero.

math.CV↗

Harmonic Interpolation and a Brunn-Minkowski Theorem for Random Determinants

We describe the harmonic interpolation of convex bodies, and prove a strong form of the Brunn-Minkowski inequality and characterize its equality case. As an application we improve a theorem of Berndtsson on the volume of slices of a pseudoconvex domain. We furthermore apply this to prove subharmonicity of the expected absolute value of the determinant of a matrix of random vectors through the connection with zonoids.

math.CV↗

Non-pluripolar energy and the complex Monge-Ampère operator

Given a domain $Ω\subset \mathbf C^n$ we introduce a class of plurisubharmonic (psh) functions $\mathcal G(Ω)$ and Monge-Ampère operators $u\mapsto [dd^c u]^p$, $p\leq n$, on $\mathcal G(Ω)$ that extend the Bedford-Taylor-Demailly Monge-Ampère operators. Here $[dd^c u]^p$ is a closed positive current of bidegree $(p,p)$ that dominates the non-pluripolar Monge-Ampère current $\langle dd^c u\rangle^p$. We prove that $[dd^c u]^p$ is the limit of Monge-Ampère currents of certain natural regularizations of $u$. On a compact Kähler manifold $(X, ω)$ we introduce a notion of non-pluripolar energy and a corresponding finite energy class $\mathcal G(X, ω)\subset \text{PSH}(X, ω)$ that is a global version of $\mathcal G(Ω)$. From the local construction we get global Monge-Ampère currents $[dd^c φ+ ω]^p$ for $φ\in \mathcal G(X,ω)$ that only depend on the current $dd^c φ+ ω$. The limits of Monge-Ampère currents of certain natural regularizations of $φ$ can be expressed in terms of $[dd^c φ+ ω]^j$ for $j\leq p$. We get a mass formula involving the currents $[dd^c φ+ω]^p$ that describes the loss of mass of the non-pluripolar Monge-Ampère measure $\langle dd^c φ+ω\rangle^n$. The class $\mathcal G(X, ω)$ includes $ω$-psh functions with analytic singularities and the class $\mathcal E(X, ω)$ of $ω$-psh functions of finite energy and certain other convex energy classes, although it is not convex itself.

math.CV↗

Interpolation, Prekopa and Brunn-Minkowski for $F$-subharmonicity

We extend Prekopa's Theorem and the Brunn-Minkowski Theorem from convexity to $F$-subharmonicity. We apply this to the interpolation problem of convex functions and convex sets introducing a new notion of "harmonic interpolation" that we view as a generalization of Minkowski-addition.

math.MG↗

The Minimum Principle for Convex Subequations

A subequation on an open subset $X\subset \mathbb R^n$ is a subset $F$ of the space of $2$-jets on $X$ with certain properties. A smooth function is said to be $F$-subharmonic if all of its $2$-jets lie in $F$, and using the viscosity technique one can extend the notion of $F$-subharmonicity to any upper-semicontinuous function. Let $\mathcal P$ denote the subequation consisting of those $2$-jets whose Hessian part is semipositive. We introduce a notion of product subequation $F\#\mathcal P$ on $X\times \mathbb R^{m}$ and prove, under suitable hypotheses, that if $F$ is convex and $f(x,y)$ is $F\#\mathcal P$-subharmonic then the marginal function $$ g(x):= \inf_y f(x,y)$$ is $F$-subharmonic. This generalises the classical statement that the marginal function of a convex function is again convex. We also prove a complex version of this result that generalises the Kiselman minimum principle for the marginal function of a plurisubharmonic function.

math.AP↗

Deformations of Kähler manifolds to normal bundles and restricted volumes of big classes

The deformation of a variety $X$ to the normal cone of a subvariety $Y$ is a classical construction in algebraic geometry. In this paper we study the case when $(X,ω)$ is a compact Kähler manifold and $Y$ is a submanifold. The deformation space $\mathcal{X}$ is fibered over $\mathbb{P}^1$ and all the fibers $X_τ$ are isomorphic to $X$, except the zero-fiber, which has the projective completion of the normal bundle $N_{Y|X}$ as one of its components. The first main result of this paper is that one can find Kähler forms on modifications of $\mathcal{X}$ which restricts to $ω$ on $X_1$ and which makes the volume of the normal bundle in the zero-fiber come arbitrarily close to the volume of $X$. Phrased differently, we find Kähler deformations of $(X,ω)$ such that almost all of the mass ends up in the normal bundle. The proof relies on a general result on the volume of big cohomology classes, which is the other main result of the paper. A $(1,1)$ cohomology class on a compact Kähler manifold $X$ is said to be big if it contains the sum of a Kähler form and a closed positive current. A quantative measure of bigness is provided by the volume function, and there is also a related notion of restricted volume along a submanifold. We prove that if $Y$ is a smooth hypersurface which intersects the Kähler locus of a big class $α$ then up to a dimensional constant, the restricted volume of $α$ along $Y$ is equal to the derivative of the volume at $α$ in the direction of the cohomology class of $Y$. This generalizes the corresponding result on the volume of line bundles due to Boucksom-Favre-Jonsson and independently Lazarsfeld-Mustaţă.

math.AG↗

The Dirichlet Problem for the Complex Homogeneous Monge-Ampère Equation

We survey the Dirichlet problem for the complex Homogeneous Monge-Ampère Equation, both in the case of domains in $\mathbb C^n$ and the case of compact Kähler manifolds parametrized by a Riemann surface with boundary. We then give a self-contained account of previous work of the authors that connects this with the Hele-Shaw flow, and give several concrete examples illustrating various phenomena that solutions to this problem can display.

math.CV↗

Canonical growth conditions associated to ample line bundles

We propose a new construction which associates to any ample (or big) line bundle $L$ on a projective manifold $X$ a canonical growth condition (i.e. a choice of a psh function well-defined up to a bounded term) on the tangent space $T_p X$ of any given point $p$. We prove that it encodes such classical invariants as the volume and the Seshadri constant. Even stronger, it allows you to recover all the infinitesimal Okounkov bodies of $L$ at $p$. The construction is inspired by toric geometry and the theory of Okounkov bodies; in the toric case the growth condition is "equivalent" to the moment polytope. As in the toric case the growth condition says a lot about the Kähler geometry of the manifold. We prove a theorem about Kähler embeddings of large balls, which generalizes the well-known connection between Seshadri constants and Gromov width established by McDuff and Polterovich.

math.AG↗

Coupled Kähler-Einstein metrics

We propose new types of canonical metrics on Kähler manifolds, called coupled Kähler-Einstein metrics, generalizing Kähler-Einstein metrics. We prove existence and uniqueness results in the cases when the canonical bundle is ample and when the manifold is Kähler-Einstein Fano. In the Fano case we also prove that existence of coupled Kähler-Einstein metrics imply a certain algebraic stability condition, generalizing $K$-polystability.

math.DG↗

Monotonicity of non-pluripolar Monge-Ampère masses

We prove that on a compact Kähler manifold, the non-pluripolar Monge-Ampère mass of a $θ$-psh function decreases as the singularities increase. This was conjectured by Boucksom-Eyssidieux-Guedj-Zeriahi who proved it under the additional assumption of the functions having small unbounded locus. As a corollary we get a comparison principle for $θ$-psh functions, analogous to the comparison principle for psh functions due to Bedford-Taylor.

math.CV↗

Duality between the pseudoeffective and the movable cone on a projective manifold

We prove a conjecture of Boucksom-Demailly-Păun-Peternell, namely that on a projective manifold $X$ the cone of pseudoeffective classes in $H^{1,1}_{\mathbb{R}}(X)$ is dual to the cone of movable classes in $H^{n-1,n-1}_{\mathbb{R}}(X)$ via the Poincaré pairing. This is done by establishing a conjectured transcendental Morse inequality for the volume of the difference of two nef classes on a projective manifold. As a corollary the movable cone is seen to be equal to the closure of the cone of balanced metrics. In an appendix by Boucksom it is shown that the Morse inequality also implies that the volume function is differentiable on the big cone, and one also gets a characterization of the prime divisors in the non-Kähler locus of a big class via intersection numbers.

math.CV↗

Homogeneous Monge-Ampère Equations and Canonical Tubular Neighbourhoods in Kähler Geometry

We prove the existence of canonical tubular neighbourhoods around complex submanifolds of Kähler manifolds that are adapted to both the holomorphic and symplectic structure. This is done by solving the complex Homogeneous Monge-Ampère equation on the deformation to the normal cone of the submanifold. We use this to establish local regularity for global weak solutions, giving local smoothness to the (weak) geodesic rays in the space of (weak) Kähler potentials associated to a given complex submanifold. We also use it to get an optimal regularity result for naturally defined plurisubharmonic envelopes and for the boundaries of their associated equilibrium sets.

math.CV↗