A01
Discrete Riemann Surfaces

Investigating the Facets of Discrete Complex Analysis

Riemann surfaces arise in complex analysis as the natural domain of holomorphic functions. They are oriented two-dimensional real manifolds with a conformal structure. Several discretizations of Riemann surfaces exist, e.g., involving discretized Cauchy-Riemann equations, patterns of circles, or discrete conformal equivalence of triangle meshes. Project A01 aims at developing a comprehensive theory including discrete versions of theorems such as uniformization, convergence issues and connections to mathematical physics.

Mission-

We plan to develop a comprehensive theory of discrete Riemann surfaces. The aim is to discretize the notions and theorems of complex analysis.

Scientific Details+

The theory of discrete Riemann surfaces that we envisage should be comprehensive in different respects:

Not one, but several sensible definitions of discrete holomorphic functions are known today. The oldest approach is to discretize the Cauchy-Riemann equations, and this leads to a linear theory. Other definitions, leading to nonlinear theories, originate from ideas of Thurston and involve patterns of circles. The most recent discrete model of a Riemann surface is based on a discretized notion of conformal equivalence for triangulated surfaces. We want to clarify the relationship between these linear and nonlinear theories and develop a unified theory.

The discrete theory should ideally be as rich and well developed as the classical smooth theory. We will focus on proving discrete versions of the Riemann mapping theorem and classical uniformization theorems. As it turns out, nonlinear theories of conformal maps are closely related to the theory of polyhedra in hyperbolic 3-space. Uniformization theorems for discrete Riemann surfaces are equivalent to realization theorems for hyperbolic polyhedra with prescribed dihedral angels or prescribed intrinsic metric.

As an ultimate goal, the discrete theory should contain the classical smooth theory as a limiting case. We will investigate the convergence of discrete conformal maps to smooth ones in different situations, as the discretization is refined. The aim is twofold. On the theoretical side, the discrete theory should become a source of new proofs for theorems belonging to the smooth theory. On the practical side, due to the abundance of convex variational principles, the discrete theory lends itself to numerical computation. If one could prove convergence, the discrete theory would lead to versatile new numerical methods to solve problems of conformal mapping and in Riemann surface theory.

A theory of discrete conformal maps should be accompanied by a compatible theory of discrete quasiconformal maps. There should be a notion of discrete quasiconformal distortion, which is zero only for discrete conformal maps. When considering mapping problems which are not solvable in the class of conformal maps (like mapping between conformally inequivalent Riemann surfaces, or mapping between planar domains with prescribed boundary values) one may ask for maps with least distortion. On the theoretical side one would hope for helpful characterizations of such optimal quasiconformal maps. The holy grail in this strand of research would be a discrete version of the classical Teichmüller theorem. On the practical side, one can hope for reasonable algorithms to compute optimal quasiconformal maps.

As a preliminary exercise for proving the geometrization conjecture using Ricci flow on 3-dimensional Riemannian manifolds, Richard Hamilton provided proofs of the classical uniformization theorem using Ricci flow on surfaces. Several discretized versions of Ricci flow for triangulated surfaces have been proposed, but they all suffer from the wrong scaling behavior. We will study a "correct" Ricci flow for triangulated surfaces.

We are interested in the development of a discrete theory of conformality not only for its own sake, but also because of the connections to other areas of mathematics:

Discrete conformal models in statistical physics and quantum field theory. Many 2-dimensional discrete models of statistical physics exhibit conformally invariant properties in the thermodynamic limit. This has been proved in different cases by Smirnov and Kenyon, and in each case the linear theory of discrete holomorphic functions has been instrumental. Bazhanov and others have connected the nonlinear theory of circle patterns with an important model in quantum field theory. We plan to investigate the role that the nonlinear theories of conformality play in statistical physics. In particular, there seems to be a connection between the new discrete theory of conformally equivalent metrics and the dimer model.

Publications+

Dissertations
Discrete Riemann surfaces and integrable systems

Author: Günther, Felix
Advisor: Alexander I. Bobenko
Date: Sep 2014
Download: external

Spinor representation of Bryant surfaces with catenoidal and smooth ends

Author: Pavljukevich, Tatiana
Advisor: Alexander I. Bobenko
Date: 2014


Papers
A variational principle for cyclic polygons with prescribed edge lengths

Authors: Kourimska, H. and Skuppin, L. and Springborn, B.
In Collection: Advances in Discrete Differential Geometry, Springer
Note: Preprint at arxiv
Date: 2016
Download: arXiv

Approximation of conformal mappings using confomally equivalent triangular lattices

Author: Bücking, U.
In Collection: Advances in Discrete Differential Geometry, Springer
Note: Preprint at arxiv
Date: 2016
Download: arXiv

Constructing solutions to the Björling problem for isothermic surfaces by structure preserving discretization

Authors: Bücking, U. and Matthes, D.
In Collection: Advances in Discrete Differential Geometry, Springer
Note: Preprint at arxiv
Date: 2016
Download: arXiv

Discrete complex analysis on planar quad-graphs

Authors: Bobenko, A. I. and Günther, F.
In Collection: Advances in Discrete Differential Geometry, Springer
Note: Preprint at arxiv
Date: 2016
Download: arXiv

Discrete conformal maps: Boundary value problems, circle domains, Fuchsian and Schottky uniformization

Authors: Bobenko, A. I. and Sechelmann, S. and Springborn, B.
In Collection: Advances in Discrete Differential Geometry, Springer
Note: Preprint
Date: 2016
Download: internal

Numerical Methods for the Discrete Map $Z^a$

Authors: Bornemann, F. and Its, A. and Olver, S. and Wechslberger, G.
In Collection: Advances in Discrete Differential Geometry, Springer
Date: 2016
Download: arXiv

Discrete Riemann surfaces based on quadrilateral cellular decompositions

Authors: Bobenko, A. I. and Günther, F.
Date: 2015

Discrete uniformization of finite branched covers over the Riemann sphere via hyper-ideal circle patterns

Authors: Bobenko, A. I. and Dimitrov, N. and Sechelmann, S.
Date: 2015
Download: arXiv

Hyper-ideal Circle Patterns with Cone Singularities

Author: Dimitrov, N.
Journal: Results in Mathematics, pages 1-45
Date: 2015
DOI: 10.1007/s00025-015-0453-3
Download: external arXiv

The asymptotic behaviour of the discrete holomorphic map $Z^a$ via the Riemann-Hilbert method

Authors: Bobenko, A. I. and Its, A.
Journal: Duke Math.~J.
Note: accepted
Date: 2015
Download: arXiv

Discrete Riemann surfaces: linear discretization and its convergence

Authors: Bobenko, Alexander and Skopenkov, Mikhail
Journal: J. reine und angew. Math.
Date: Oct 2014
DOI: 10.1515/crelle-2014-0065
Download: external arXiv

Diskretisierung in Geometrie und Dynamik - Elastische Stäbe und Rauchringe

Authors: Bobenko, A.I. and Springborn, B.
Journal: Mitteilungen der DMV, 21(1):218-224
Date: Dec 2013
Download: external

There is no triangulation of the torus with vertex degrees 5, 6, ... , 6, 7 and related results: geometric proofs for combinatorial theorems

Authors: Izmestiev, Ivan and Kusner, Robert B. and Rote, Günter and Springborn, Boris and Sullivan, John M.
Journal: Geometriae Dedicata, 166(1):15-29
Date: Oct 2013
DOI: 10.1007/s10711-012-9782-5
Download: external arXiv

Discrete complex analysis – the medial graph approach

Authors: Bobenko, Alexander I. and Günther, Felix
Journal: Actes des rencontres du CIRM 3 no. 1: Courbure discrète: théorie et applications, pages 159-169
Date: 2013
DOI: 10.5802/acirm.65
Download: external


Team+

Prof. Dr. Alexander I. Bobenko   +

Projects: A01, A02, C01, B02, CaP
University: TU Berlin, Institut für Mathematik, MA 881
Address: Strasse des 17. Juni 136, 10623 Berlin, Germany
Tel: +49 (30) 314 24655
E-Mail: bobenko[at]math.tu-berlin.de
Website: http://page.math.tu-berlin.de/~bobenko/


Dr. Ulrike Bücking   +

Projects: A01
University: TU Berlin
E-Mail: buecking[at]math.tu-berlin.de
Website: http://page.math.tu-berlin.de/~scheerer/


Prof. Dr. Boris Springborn   +

Projects: A01, A11
University: TU Berlin
E-Mail: springb[at]math.TU-Berlin.DE
Website: http://page.math.tu-berlin.de/~springb/


Niklas Affolter   +

Projects: A01
University: TU Berlin
E-Mail: affolter[at]math.tu-berlin.de


Dr. Felix Günther   +

Projects: A01
University: TU Berlin
E-Mail: fguenth[at]math.tu-berlin.de


Hana Kourimska   +

Projects: A01
University: TU Berlin
E-Mail: kourim[at]math.tu-berlin.de


Isabella Retter   +

Projects: A01
University: TU Berlin
E-Mail: thiesen[at]math.tu-berlin.de


Dr. Stefan Sechelmann   +

Projects: A01
University: TU Berlin
E-Mail: sechel[at]math.tu-berlin.de


Lara Skuppin   +

Projects: A01
University: TU Berlin
E-Mail: skuppin[at]math.tu-berlin.de


Dr. Ananth Sridhar   +

Projects: A01
University: TU Berlin
E-Mail: sridhar[at]math.tu-berlin.de