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Haakan Hedenmalm

Publications and source records attributed to Haakan Hedenmalm.

At least 37 records · Page 2Linked to original sources
arXiv2019

Off-spectral analysis of Bergman kernels

The asymptotic analysis of Bergman kernels with respect to exponentially varying measures near emergent interfaces has attracted recent attention. Such interfaces typically occur when the associated limiting Bergman density function vanishes on a portion of the plane, the off-spectral region. This type of behaviour is observed when the metric is negatively curved somewhere, or when we study partial Bergman kernels in the context of positively curved metrics. In this work, we cover these two situations in a unified way, for exponentially varying planar measures on the complex plane. We obtain uniform asymptotic expansions of root functions, which are essentially normalized partial Bergman kernels at an off-spectral point, valid in the entire off-spectral component and protruding into the spectrum as well, which allows us to show error function transition behaviour of the original kernel along the interface. In contrast, previous work on asymptotic expansions of Bergman kernels is typically local, and valid only in the bulk region of the spectrum.

math.CV↗
arXiv2019

Gaussian analytic functions and operator symbols of Dirichlet type

Let $\calH$ be a separable infinite-dimensional $\C$-linear Hilbert space, with sesquilinear inner product $\langle\cdot,\cdot\rangle_\calH$. Given any two orthonormal systems $x_1,x_2,x_3,\ldots$ and $y_1,y_2,y_3,\ldots$ in $\calH$, we show that the weighted sums \[ S(l):=\sum_{j,k:j+k=l}\bigg(\frac{l}{jk}\bigg)^{\frac12}\, \langle x_j,y_k\rangle_{\calH} \] satisfy $|S(l)|^2\lessapprox2$ holds in an average sense. A construction due to Zachary Chase shows that this would not be true if the number $2$ is replaced by the smaller number $1.72$. In the construction, the system $y_1,y_2,y_3,\ldots$ is a permutation of the system $x_1,x_2,x_3,\ldots$. We interpret our bound in terms of the correlation $\expect Φ(z)Ψ(z)$ of two copies of a Gaussian analytic function with possibly intricate Gaussian correlation structure between them. The Gaussian analytic function we study arises in connection with the classical Dirichlet space, which is naturally Möbius invariant. The study of the correlations $\expectΦ(z)Ψ(z)$ leads us to introduce a new space, the \emph{mock-Bloch space}, which is slightly bigger than the standard Bloch space. Our bound has an interpretation in terms of McMullen's asymptotic variance, originally considered for functions in the Bloch space. Finally, we show that the correlations $\expectΦ(z)Ψ(w)$ may be expressed as Dirichlet symbols of contractions on $L^2(\D)$, and show that the Dirichlet symbols of Grunsky operators associated with univalent functions find a natural characterization in terms of a nonlinear wave equation.

math.CV↗
arXiv2017

Sharpening Hölder's inequality

We strengthen Hölder's inequality. The new family of sharp inequalities we obtain might be thought of as an analog of Pythagorean theorem for the $L^p$ spaces. Our reasonings rely upon Bellman functions of four variables.

math.CA↗
arXiv2017

A critical topology for $L^p$-Carleman classes with $0<p<1$

In this paper, we explain a sharp phase transition phenomenon which occurs for $L^p$-Carleman classes with exponents $0<p<1$. In principle, these classes are defined as usual, only the traditional $L^\infty$-bounds are replaced by corresponding $L^p$-bounds. To mirror the classical definition, we add the feature of dilatation invariance as well, and consider a larger soft-topology space, the $L^p$-Carleman class. A particular degenerate instance is when we obtain the $L^p$-Sobolev spaces, analyzed previously by Peetre, following an initial insight by Douady. Peetre found that these $L^p$-Sobolev spaces are highly degenerate for $0<p<1$. Essentially, the contact is lost between the function and its derivatives. Here, we analyze this degeneracy for the more general $L^p$-Carleman classes defined by a weight sequence. Under some reasonable growth and regularity properties, and a condition on the collection of test functions, we find that there is a sharp boundary, defined in terms of the weight sequence: on the one side, we get Douady-Peetre's phenomenon of "disconnexion" between the function and its derivatives, while on the other, we obtain a collection of highly smooth functions. We also look at the more standard second phase transition, between non-quasianalyticity and quasianalyticity, in the $L^p$ setting, with $0<p<1$.

math.CA↗
arXiv2016

Bloch functions and asymptotic tail variance

We obtain an optimal exponential square integrability theorem for the Bergman projection of a function bounded by 1 in modulus. This is interpreted as the statement that the asymptotic tail variance of such a function is at most 1. The asymptotic tail variance defines a seminorm on the Bloch space. We apply the main result to quasiconformal Teichmüller theory, and obtain an estimate of the integral means spectrum of k-quasiconformal mappings that are conformal in the exterior disk: $B(k,t)\le\frac14k^2|t|^2(1+7k)^2$. This is conjectured asymptotically sharp as k tends to 0, by Prause and Smirnov (2011).

math.CV↗
arXiv2015

An extension of ergodic theory for Gauss-type maps

We propose an extension of ergodic theory which focuses on the identification of ergodicity in terms of the uniqueness of the invariant measure. We first explain the concept for the doubling maps, which can be analyzed using Fourier methods. We then proceed to the Gauss-type maps of interest, of the form $x\mapsto -β/x$ mod $2\mathbb Z$ on the symmetric interval $[-1,1]$, for $0<β\le1$. We study an extended state space on the interval, formed as the restriction to the interval $[-1,1]$ of functions of the form $f+\mathbf{H}g$, where $f$ and $g$ are $L^1$-functions. We then look for invariant states for the Gauss-type map. We find that the standard ergodicity results available for $L^1$ extend with difficulty to the larger state space. The machinery developed involves a dynamical decomposition of the odd part of the Hilbert kernel. We apply the result to decide the issue when the nonnegative integer powers of two given atomic singular inner functions is complete in $H^\infty$ with respect to the weak-star topology.

math.DS↗
arXiv2015

Random normal matrices and Ward identities

Consider the random normal matrix ensemble associated with a potential on the plane which is sufficiently strong near infinity. It is known that, to a first approximation, the eigenvalues obey a certain equilibrium distribution, given by Frostman's solution to the minimum energy problem of weighted logarithmic potential theory. On a finer scale, one can consider fluctuations of eigenvalues about the equilibrium. In the present paper, we give the correction to the expectation of fluctuations, and we prove that the potential field of the corrected fluctuations converge on smooth test functions to a Gaussian free field with free boundary conditions on the droplet associated with the potential.

math.CV↗
arXiv2014

On the uniqueness theorem of Holmgren

We rereview the classical Cauchy-Kovalevskaya theorem and the related uniqueness theorem of Holmgren, in the simple setting of powers of the Laplacian and a smooth curve segment in the plane. As a local problem, the Cauchy-Kovalevskaya and Holmgren theorems supply a complete answer to the existence and uniqueness issues. Here, we consider a global uniqueness problem of Holmgren's type. Perhaps surprisingly, we obtain a connection with the theory of quadrature identities, which demonstrates that rather subtle algebraic properties of the curve come into play. For instance, if $Ω$ is the interior domain of an ellipse, and $I$ is a proper arc of the ellipse $\partialΩ$, then there exists a nontrivial biharmonic function $u$ in $Ω$ which vanishes to degree three on $I$ (i.e., all partial derivatives of $u$ of order $\le2$ vanish on $I$) if and only if the ellipse is a circle. Finally, we consider a three-dimensional case, and analyze it partially using analogues of the square of the 2X2 Cauchy-Riemann operator.

math.AP↗
arXiv2014

Weighted integrability of polyharmonic functions

To address the uniqueness issues associated with the Dirichlet problem for the $N$-harmonic equation on the unit disk $\D$ in the plane, we investigate the $L^p$ integrability of $N$-harmonic functions with respect to the standard weights $(1-|z|^2)^α$. The question at hand is the following. If $u$ solves $Δ^N u=0$ in $\D$, where $Δ$ stands for the Laplacian, and [\int_\D|u(z)|^p (1-|z|^2)^α\diff A(z)<+\infty,] must then $u(z)\equiv0$? Here, $N$ is a positive integer, $α$ is real, and $0 β(N,p)$ there exist non-trivial functions $u$ with $Δ^N u=0$ of the given integrability, while for $α\leβ(N,p)$, only $u(z)\equiv0$ is possible. We also investigate the obstruction to uniqueness for the Dirichlet problem, that is, we study the structure of the functions in $\mathrm{PH}^p_{N,α}(\D)$ when this space is nontrivial. We find a fascinating structural decomposition of the polyharmonic functions -- the cellular (Almansi) expansion -- which decomposes the polyharmonic weighted $L^p$ in a canonical fashion. Corresponding to the cellular expansion is a tiling of part of the $(p,α)$ plane into cells. A particularly interesting collection of cells form the entangled region.

math.AP↗
arXiv2014

On Hörmander's solution of the dbar-equation

We explain how Hörmander's classical solution of the dbar-equation in the plane with a weight which permits growth near infinity carries over to the rather opposite situation when we ask for decay near infinity. Here, however, a natural condition on the datum needs to be imposed. The condition is not only natural but also necessary to have the result at least in the Fock weight case.

math.AP↗
arXiv2013

Asymptotic expansion of polyanalytic Bergman kernels

We consider mainly the Hilbert space of bianalytic functions on a given domain in the plane, square integrable with respect to a weight. We show how to obtain the asymptotic expansion of the corresponding bianalytic Bergman kernel for power weights, under the standard condition on those weights. This is known only in the analytic setting, from the work of e.g. Tian, Yau, Zelditch, Catlin, et al. We remark that a bianalytic function may be identified with a vector-valued analytic function, supplied with a locally singular metric on the vectors. We also apply our findings to two bianalytic Bergman metrics introduced here.

math.CV↗
arXiv2012

Coulomb gas ensembles and Laplacian growth

We consider the normal matrix ensemble under a general confining potential. We find that the eigenvalues condensate on a compact set in the plane, which we call the spectral droplet. We also study the evolution of incrementally adding a dimension, i.e., adding an extra electron in this fermionic model.

math.PR↗
arXiv2012

Heisenberg's uncertainty principle in the sense of Beurling

We shed new light on Heisenberg's uncertainty principle in the sense of Beurling, by offering an essentially different proof which permits us to weaken the assumptions substantially, and examples show that the result is sharp. The proof involves Fourier and Mellin transforms. We alo extend to a setting of two functions. A higher-dimensional analogue is considered as well.

math.FA↗
arXiv2012

The Gaussian free field and Hadamard's variational formula

We relate the Gaussian free field on a planar domain to the variational formula of Hadamard which explains the change of the Green function under a perturbation of the domain. This is accomplished by means of a natural integral operator related to Hadamard's formula.

math.PR↗
arXiv2012

Perron-Frobenius operators and the Klein-Gordon equation

We study the non-uniqueness sets for solutions to the Klein-Gordon equation in 1 space dimension, for solutions whose Fourier transform is a finite complex measure absolutely continuous with respect to arc length. We show that generally, in the non-unique case for lattice-crosses, the space of solutions that vanish there is infinite-dimensional.

math.DS↗
arXiv2011

Fourier uniqueness sets and the Klein-Gordon equation

We generalize the study of Heisenberg uniqueness pairs considered in earlier work with Alfonso Montes-Rodriguez. We also find the critical density in the case of one branch of the hyperbola. In the critical case there is non-uniqueness for a one-dimensional space of solutions. These solutions cannot vanish at any other point of the axes of the characteristic lattice-cross, due to an unexpected connection with the Nielsen spiral.

math.AP↗
arXiv2011

The polyanalytic Ginibre ensembles

We consider a polyanalytic generalization of the Ginibre ensemble. This models allowing higher Landau levels (the Ginibre ensemble corresponds to the lowest Landau level). We study the local behavior of this point process under blow-ups.

math.PR↗