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István Prause

Publications and source records attributed to István Prause.

17 recordsLinked to original sources
arXiv2025

Dimer Models and Conformal Structures

In this work we study the variational problem associated to dimer models, a class of models from integrable probability and statistical mechanics in dimension two which have been the focus of intense research efforts over the last decades. These models give rise to an infinite family of non-differentiable functionals on Lipschitz functions with gradient constraint, determined by solutions of the Dirichlet problem on compact convex polygons for a class of Monge-Ampère equations. We settle a number or outstanding open questions for this infinite class functionals. In particular we prove a complete classification of the regularity of minimizers, also known as height functions, for all dimer models for a natural class of polygonal (simply or multiply connected) domains much studied in numerical simulations and elsewhere. Our classification in particular implies that the Pokrovsky-Talapov law holds for all dimer models at a generic point on the frozen boundary and in addition shows a very strong local rigidity of dimer models which can be interpreted as a geometric universality result. Furthermore, we give a complete classification of the regularity of the associated free boundary, also known in the literature as frozen boundary or arctic curves and prove that they are all algebraic curves. The lack of differentiability of the functionals is intimately connected to the boundary behaviour of the solutions to the Monge-Ampère equations and we prove a complete classification for these, of independent interest.

math.AP↗
arXiv2024

On quasiconformal dimension distortion for subsets of the real line

Optimal quasiconformal dimension distortions bounds for subsets of the complex plane have been established by Astala. We show that these estimates can be improved when one considers subsets of the real line of arbitrary Hausdorff dimension. We present some explicit numerical bounds.

math.CV↗
arXiv2023

Limit shapes from harmonicity: dominos and the five vertex model

We discuss how to construct limit shapes for the domino tiling model (square lattice dimer model) and $5$-vertex model, in appropriate polygonal domains. Our methods are based on the harmonic extension method of [R. Kenyon and I. Prause, Gradient variational problems in $\mathbb{R}^2$, Duke Math J. 2022].

math.PR↗
arXiv2022

The genus-zero five-vertex model

We study the free energy and limit shape problem for the five-vertex model with periodic "genus zero" weights. We derive the exact phase diagram, free energy and surface tension for this model. We show that its surface tension has trivial potential and use this to give explicit parameterizations of limit shapes.

math.PR↗
arXiv2021

Stretching and Rotation of Planar Quasiconformal Mappings on a Line

In this article, we examine stretching and rotation of planar quasiconformal mappings on a line. We show that for almost every point on the line, the set of complex stretching exponents (describing stretching and rotation jointly) is contained in the disk $ \overline{B}(1/(1-k^4),k^2/(1-k^4))$. This yields a quadratic improvement over the known optimal estimate for general sets of Hausdorff dimension $1$. Our proof is based on holomorphic motions and estimates for dimensions of quasicircles. We also give a lower bound for the dimension of the image of a $1$-dimensional subset of a line under a quasiconformal mapping.

math.CV↗
arXiv2021

Gradient variational problems in $\mathbb{R}^2$

We prove a new integrability principle for gradient variational problems in $\mathbb{R}^2$, showing that solutions are explicitly parameterized by $κ$-harmonic functions, that is, functions which are harmonic for the laplacian with varying conductivity $κ$, where $κ$ is the square root of the Hessian determinant of the surface tension.

math.AP↗
arXiv2021

On mappings of finite distortion that are quasiconformal in the unit disk

We study quasiconformal mappings of the unit disk that have planar extension with controlled distortion. For these mappings we prove a bound for the modulus of continuity of the inverse map, which somewhat surprisingly is almost as good as for global quasiconformal maps. Furthermore, we give examples which improve the known bounds for the three point property of generalized quasidisks. Finally, we establish optimal regularity of such maps when the image of the unit disk has cusp type singularities.

math.CV↗
arXiv2021

Localized Regularity of Planar Maps of Finite Distortion

In this article we study fine regularity properties for mappings of finite distortion. Our main theorems yield strongly localized regularity results in the borderline case in the class of maps of exponentially integrable distortion. Analogues of such results were known earlier in the case of quasiconformal mappings. Moreover, we study regularity for maps whose distortion has higher exponential integrability.

math.CV↗
arXiv2017

Quasiconformal extensions, Loewner chains, and the lambda-Lemma

In 1972, J. Becker [J. Reine Angew. Math. 255] discovered a sufficient condition for quasiconformal extendibility of Loewner chains. Many known conditions for quasiconformal extendibility of holomorphic functions in the unit disk can be deduced from his result. We give a new proof of (a generalization of) Becker's result based on Slodkowski's Extended lambda-Lemma. Moreover, we characterize all quasiconformal extensions produced by Becker's (classical) construction and use that to obtain examples in which Becker's extension is extremal (i.e. optimal in the sense of maximal dilatation) or, on the contrary, fails to be extremal.

math.CV↗
arXiv2017

Quasidisks and twisting of the Riemann map

Consider a conformal map from the unit disk onto a quasidisk. We determine a range of critical complex powers with respect to which the derivative is integrable. The results fit into the picture predicted by a circular analogue of Brennan's conjecture.

math.CV↗
arXiv2015

Asymptotic variance of the Beurling transform

We study the interplay between infinitesimal deformations of conformal mappings, quasiconformal distortion estimates and integral means spectra. By the work of McMullen, the second derivative of the Hausdorff dimension of the boundary of the image domain is naturally related to asymptotic variance of the Beurling transform. In view of a theorem of Smirnov which states that the dimension of a $k$-quasicircle is at most $1+k^2$, it is natural to expect that the maximum asymptotic variance $Σ^2 = 1$. In this paper, we prove $0.87913 \le Σ^2 \le 1$. For the lower bound, we give examples of polynomial Julia sets which are $k$-quasicircles with dimensions $1+ 0.87913 \, k^2$ for $k$ small, thereby showing that $Σ^2 \ge 0.87913$. The key ingredient in this construction is a good estimate for the distortion $k$, which is better than the one given by a straightforward use of the $λ$-lemma in the appropriate parameter space. Finally, we develop a new fractal approximation scheme for evaluating $Σ^2$ in terms of nearly circular polynomial Julia sets.

math.CV↗
arXiv2014

A hunt for sharp $L ^p$-estimates and rank-one convex variational integrals

Learning how to figure out sharp $L^p$-estimates of nonlinear differential expressions, to prove and use them, is a fundamental part of the development of PDEs and Geometric Function Theory (GFT). Our survey presents, among what is known to date, some notable recent efforts and novelties made in this direction. We focus attention here on the historic Morrey's Conjecture and Burkholder's martingale inequalities for stochastic integrals. Some of these topics have already been discussed by the present authors [5] and by Rodrigo Bañuelos [10]. Nevertheless, there is always something new to add.

math.CV↗
arXiv2013

Bilipschitz and quasiconformal rotation, stretching and multifractal spectra

We establish sharp bounds for simultaneous local rotation and Hölder-distortion of planar quasiconformal maps. In addition, we give sharp estimates for the corresponding joint quasiconformal multifractal spectrum, based on new estimates for Burkholder functionals with complex parameters. As a consequence, we obtain optimal rotation estimates also for bi-Lipschitz maps.

math.CV↗
arXiv2010

Burkholder integrals, Morrey's problem and quasiconformal mappings

Inspired by Morrey's Problem (on rank-one convex functionals) and the Burkholder integrals (of his martingale theory) we find that the Burkholder functionals $B_p$, $p \ge 2$, are quasiconcave, when tested on deformations of identity $f\in Id + C^\infty_0(Ω)$ with $B_p(Df(x)) \ge 0$ pointwise, or equivalently, deformations such that $|Df|^2 \leq \frac{p}{p-2} J_f$. In particular, this holds in explicit neighbourhoods of the identity map. Among the many immediate consequences, this gives the strongest possible $L^p$- estimates for the gradient of a principal solution to the Beltrami equation $\f_{\bar{z}} = μ(z) f_z$, for any $p$ in the critical interval $2 \leq p \leq 1+1/\|μ_f\|_\infty$. Examples of local maxima lacking symmetry manifest the intricate nature of the problem.

math.CA↗
arXiv2009

Hausdorff measure of quasicircles

S. Smirnov proved recently that the Hausdorff dimension of any K-quasicircle is at most 1+k^2, where k=(K-1)/(K+1). In this paper we show that if $Γ$ is such a quasicircle, then $H^{1+k^2}(B(x,r)\cap Γ)\leq C(k) r^{1+k^2}$ for all x in \C and r>0, where H^s stands for the s-Haudorff measure. On a related note we derive a sharp weak-integrability of the derivative of the Riemann map of a quasidisk.

math.CV↗
arXiv2009

Quasisymmetric distortion spectrum

We give improved bounds for the distortion of the Hausdorff dimension under quasisymmetric maps in terms of the dilatation of their quasiconformal extension. The sharpness of the estimates remains an open question and is shown to be closely related to the fine structure of harmonic measure.

math.CV↗