# Christian BärUniversität Potsdam · Institute of Mathematics

Christian Bär

Dr.

## About

92

Publications

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Introduction

I am interested in differential geometry, global analysis, spectral geometry and applications to mathematical physics. Currently one focus of my research is on index theory in Lorentzian signature.

Additional affiliations

October 1999 - August 2003

April 1994 - September 1999

April 1994 - September 1999

Education

August 1992 - March 1994

August 1991 - July 1992

October 1990 - July 1991

## Publications

Publications (92)

We study boundary value problems for first-order elliptic differential operators on manifolds with compact boundary. The adapted boundary operator need not be selfadjoint and the boundary condition need not be pseudo-local. We show the equivalence of various characterisations of elliptic boundary conditions and demonstrate how the boundary conditio...

Based on the Atiyah-Patodi-Singer index formula, we construct an obstruction to positive scalar curvature metrics with mean convex boundaries on spin manifolds of infinite K-area. We also characterize the extremal case. Next we show a general deformation principle for boundary conditions of metrics with lower scalar curvature bounds. This implies t...

We prove a local version of the index theorem for Lorentzian Dirac-type operators on globally hyperbolic Lorentzian manifolds with Cauchy boundary. In case the Cauchy hypersurface is compact we do not assume self-adjointness of the Dirac operator on the spacetime or the associated elliptic Dirac operator on the boundary. In this case integration of...

We adapt the Faddeev-LeVerrier algorithm for the computation of characteristic polynomials to the computation of the Pfaffian of a skew-symmetric matrix. This yields a very simple, easy to implement and parallelize algorithm of computational cost O(nβ+1) where n is the size of the matrix and O(nβ) is the cost of multiplying n×n-matrices, β∈[2,2.372...

We show scalar-mean curvature rigidity of warped products of round spheres of dimension at least 2 over compact intervals equipped with strictly log-concave warping functions. This generalizes earlier results of Cecchini-Zeidler to all dimensions.
Moreover, we show scalar curvature rigidity of round spheres of dimension at least 3 minus two antipo...

Non-local boundary conditions, such as the Atiyah-Patodi-Singer (APS) conditions, for Dirac operators on Riemannian manifolds are well understood while not much is known for such operators on spacetimes with timelike boundary. We define a class of Lorentzian boundary conditions that are local in time and non-local in the spatial directions and show...

We show that local deformations, near closed subsets, of solutions to open partial differential relations can be extended to global deformations, provided all but the highest derivatives stay constant along the subset. The applicability of this general result is illustrated by a number of examples, dealing with convex embeddings of hypersurfaces, d...

The Rarita-Schwinger operator is the twisted Dirac operator restricted to 3 /2-spinors. Rarita-Schwinger fields are solutions of this operator which are in addition divergence-free. This is an overdetermined problem and solutions are rare; it is even more unexpected for there to be large dimensional spaces of solutions. In this paper we prove the e...

We show that the Dirac operator on a compact globally hyperbolic Lorentzian spacetime with spacelike Cauchy boundary is a Fredholm operator if appropriate boundary conditions are imposed. We prove that the index of this operator is given by the same expression as in the index formula of Atiyah-Patodi-Singer for Riemannian manifolds with boundary. T...

We study the spectral properties of curl, a linear differential operator of first order acting on differential forms of appropriate degree on an odd-dimensional closed oriented Riemannian manifold. In three dimensions its eigenvalues are the electromagnetic oscillation frequencies in vacuum without external sources. In general, the spectrum consist...

These volumes contain the proceedings of the conference held at Aarhus, Oxford and Madrid in September 2016 to mark the seventieth birthday of Nigel Hitchin, one of the world’s foremost geometers and Savilian Professor of Geometry at Oxford. The proceedings contain twenty-nine articles, including three by Fields medallists (Donaldson, Mori and Yau)...

We show that local deformations, near closed subsets, of solutions to open partial differential relations can be extended to global deformations, provided all but the highest derivatives stay constant along the subset. The applicability of this general result is illustrated by a number of examples, dealing with convex embeddings of hypersurfaces, d...

We review some recent results on geometric equations on Lorentzian manifolds such as the wave and Dirac equations. This includes well-posedness and stability for various initial value problems, as well as results on the structure of these equations on black-hole spacetimes (in particular, on the Kerr solution), the index theorem for hyperbolic Dira...

Das Werk bietet eine Einführung in die lineare Algebra und die analytische Geometrie und enthält Material für eine zweisemestrige Vorlesung. Es beginnt mit einem Kapitel, das allgemein in die mathematische Denkweise und Beweistechniken einführt, um dann über lineare Gleichungssysteme zur linearen Algebra überzuleiten. Besonderer Wert wird auf eine...

On a compact globally hyperbolic Lorentzian spin manifold with smooth spacelike Cauchy boundary the (hyperbolic) Dirac operator is known to be Fredholm when Atiyah-Patodi-Singer boundary conditions are imposed. In this paper we investigate to what extent these boundary conditions can be replaced by more general ones and how the index then changes....

These are the lecture notes of a course on geometric wave equations which I taught at the University of Potsdam in the winter term 2015/2016. The course gave an introduction to linear hyperbolic PDEs on Lorentzian manifolds. The geometric setup allows to apply the theory in general relativity, for instance.
Some basic knowledge of differential geom...

We present an introduction to boundary value problems for Dirac-type operators on complete Riemannian manifolds with compact boundary. We introduce a very general class of boundary conditions which contain local elliptic boundary conditions in the sense of Lopatinski and Shapiro as well as the Atiyah–Patodi–Singer boundary conditions. We discuss bo...

We discuss the chiral anomaly for a Weyl field in a curved background and show that a novel index theorem for the Lorentzian Dirac operator can be applied to describe the gravitational chiral anomaly. A formula for the total charge generated by the gravitational and gauge field background is derived directly in Lorentzian signature and in a mathema...

We study the global theory of linear wave equations for sections of vector bundles over globally hyperbolic Lorentz manifolds. We introduce spaces of finite energy sections and show well-posedness of the Cauchy problem in those spaces. These spaces depend in general on the choice of a time function but it turns out that certain spaces of finite ene...

A Riemannian manifold is called geometrically formal if the wedge product of any two harmonic forms is again harmonic. We classify geometrically formal compact 4-manifolds with nonnegative sectional curvature. If the sectional curvature is strictly positive, the manifold must be homeomorphic to S^4 or diffeomorphic to CP^2. This conclusion stills h...

Green-hyperbolic operators are linear differential operators acting on
sections of a vector bundle over a Lorentzian manifold which possess advanced
and retarded Green's operators. The most prominent examples are wave operators
and Dirac-type operators. This paper is devoted to a systematic study of this
class of differential operators. For instanc...

These are the lecture notes of an introductory course on gauge theory which I taught at Potsdam University in 2009. The aim was to develop the mathematical underpinnings of gauge theory such as bundle theory, characteristic classes etc. and to give applications both in physics (electrodynamics, Yang-Mills fields) as well as in mathematics (theory o...

These are the lecture notes of an introductory course on differential geometry given in 2013.

These are the lecture notes of an introductory course on special and general relativity given in 2013. It does not contain an explanation of differential geometric concepts. Participants without prior knowledge of differential geometry were expected to attend the course on differential geometry that was offered in parallel.

This text provides a systematic introduction to differential characters, as introduced by Cheeger and Simons. Differential characters form a model of what is nowadays called differential cohomology. In degree 2, integral cohomology of a space X classifies U(1)-bundles over X via the first Chern class, while differential characters correspond to U(1...

We study Cheeger-Simons differential characters and provide geometric
descriptions of the ring structure and of the fiber integration map. The
uniqueness of differential cohomology (up to unique natural transformation) is
proved by deriving an explicit formula for any natural transformation between a
differential cohomology theory and the model giv...

A linear different operator L is called weakly hypoelliptic if any local
solution u of Lu=0 is smooth. We allow for systems, that is, the coefficients
may be matrices, not necessarily of square size. This is a huge class of
important operators which cover all elliptic, overdetermined elliptic,
subelliptic and parabolic equations.
We extend several...

In the last few years, it has been strongly emphasized the need to use new mathematical tools and structures which are not part of the traditional pool of expertise of the community working on the analysis of the mathematical and structural properties of classical and quantum field theory. Goal of the workshop has been to bring together some of the...

We introduce renormalized integrals which generalize conventional measure theoretic integrals. One approximates the integration domain by measure spaces and defines the integral as the limit of integrals over the approximating spaces. This concept is implicitly present in many mathematical contexts such as Cauchy's principal value, the determinant...

We provide a systematic construction of bosonic and fermionic locally covariant quantum field theories on curved backgrounds for large classes of free fields. It turns out that bosonic quantization is possible under much more general assumptions than fermionic quantization.

We construct bosonic and fermionic locally covariant quantum field theories
on curved backgrounds for large classes of fields. We investigate the quantum
field and n-point functions induced by suitable states.

We study boundary value problems for linear elliptic differential operators
of order one. The underlying manifold may be noncompact, but the boundary is
assumed to be compact. We require a symmetry property of the principal symbol
of the operator along the boundary. This is satisfied by Dirac type operators,
for instance.
We provide a selfcontained...

This is an introduction to Wiener measure and the Feynman-Kac formula on
general Riemannian manifolds for Riemannian geometers with little or no
background in stochastics. We explain the construction of Wiener measure based
on the heat kernel in full detail and we prove the Feynman-Kac formula for
Schr\"odinger operators with $L^\infty$-potentials....

This is a survey on the analytic theory of linear wave equations on globally hyperbolic Lorentzian manifolds. There is no claim of originality. Comment: 15 pages, 6 figures

Let $${H_\hbar = \hbar^{2}L +V}$$, where L is a self-adjoint Laplace type operator acting on sections of a vector bundle over a compact Riemannian manifold and V is a symmetric endomorphism field. We derive an asymptotic expansion for the heat kernel of $${H_\hbar}$$ as $${\hbar \searrow 0}$$. As a consequence we get an asymptotic expansion for the...

The link between the physical world and its visualization is geometry. This easy-to-read, generously illustrated textbook presents an elementary introduction to differential geometry with emphasis on geometric results. Avoiding formalism as much as possible, the author harnesses basic mathematical skills in analysis and linear algebra to solve inte...

In this chapter we will collect those basic concepts and facts related to C*-algebras that will be needed later on. We give complete proofs. In Sects. 1, 2, 3, and 6 we follow closely the presentation
in [1]. For more information on C*-algebras, see, e.g. [2–6].

It has been suggested in 1999 that a certain volume growth condition for geodesically complete Riemannian manifolds might imply that the manifold is stochastically complete. This is motivated by a large class of examples and by a known analogous criterion for recurrence of Brownian motion. We show that the suggested implication is not true in gener...

We extend several classical eigenvalue estimates for Dirac operators on compact manifolds to noncompact, even incomplete manifolds.
This includes Friedrich’s estimate for manifolds with positive scalar curvature as well as the author’s estimate on surfaces.

This article explains the Clay problem on quantum Yang-Mills theory to non-mathematicians.

As long as the back reaction of quantum fields on the spacetime metric
can be ignored, quantum field theory on a curved background is an
excellent approximation to a theory combining quantum effects with
gravitational interactions. Famous predictions have been obtained in
this framework, in particular Hawking's prediction of thermal radiation
of bl...

Let M be a compact Riemannian manifold without boundary and let H be a
self-adjoint generalized Laplace operator acting on sections in a bundle over
M. We give a path integral formula for the solution to the corresponding heat
equation. This is based on approximating path space by finite dimensional
spaces of geodesic polygons. We also show a unifo...

We start by examining the analysis of so-called generalized Laplacians. A detailed exposition can be found in [12]. Throughout this section let M be a compact Riemannian manifold, let E → M be a Riemannian or Hermitian vector bundle over M. Let ∇ be a metric connection on E, i.e. for smooth sections ϕ and ψ in E and X ∈ TM we have $$
\partial _X \l...

It is well-known that a compact Riemannian spin manifold can be reconstructed
from its canonical spectral triple which consists of the algebra of smooth
functions, the Hilbert space of square integrable spinors and the Dirac
operator. It seems to be a folklore fact that the metric can be reconstructed
up to conformal equivalence if one replaces the...

This book provides a detailed introduction to linear wave equations on Lorentzian manifolds (for vector-bundle valued fields). After a collection of preliminary material in the first chapter one finds in the second chapter the construction of local fundamental solutions together with their Hadamard expansion. The third chapter establishes the exist...

In this lecture we give a brief introduction to the theory of manifolds and related basic concepts of differential geometry.

We show that on every compact spin manifold admitting a Riemannian metric of
positive scalar curvature Friedrich's eigenvalue estimate for the Dirac
operator can be made sharp up to an arbitrarily small given error by choosing
the metric suitably.

We use a construction which we call generalized cylinders to give a new proof of the fundamental theorem of hypersurface theory. It has the advantage of being very simple and the result directly extends to semi-Riemannian manifolds and to embeddings into spaces of constant curvature. We also give a new way to identify spinors for different metrics...

The ζ-regularized determinants of the Dirac operator and of its square are computed on spherical space forms. On S
2 the determinant of Dirac operators twisted by a complex line bundle is also calculated.

We show that for generic Riemannian metrics on a simply-connected closed spin manifold of dimension greater than or equal to5 the dimension of the space of harmonic spinors is not larger than it must be by the index theorem. The same result holds for periodic fundamental groups of odd order. The proof is based on a surgery theorem for the Dirac spe...

We describe the heat kernel asymptotics for roots of a Laplace type operator Δ on a closed manifold. A previously known relation between the Wodzicki residue of Δ and heat trace asymptotics is shown to hold pointwise for the corresponding densities.

We introduce a differential topological invariant for compact differentiable
manifolds by counting the small eigenvalues of the Conformal Laplace operator.
This invariant vanishes if and only if the manifold has a metric of positive
scalar curvature. We show that the invariant does not increase under surgery of
codimension at least three and we giv...

We prove lower Dirac eigenvalue bounds for closed surfaces with a spin structure whose Arf invariant equals 1. Besides the area only one geometric quantity enters in these estimates, the spin-cut-diameter \(\delta(M)\) which depends on the choice of spin structure. It can be expressed in terms of various distances on the surfaces or, alternatively,...

Let M be a connected Riemannian manifold and let D be a Dirac type operator acting on smooth compactly supported sections in a Hermitian vector bundle over M. Suppose D has a self-adjoint extension A in the Hilbert space of square-integrable sections. We show that any $L^2$-section $\phi$ contained in a closed A-invariant subspace onto which the re...

We study the spectrum of the Dirac operator on hyperbolic manifolds of finite volume. Depending on the spin structure it is either discrete or the whole real line. For link complements in S^3 we give a simple criterion in terms of linking numbers for when essential spectrum can occur. We compute the accumulation rate of the eigenvalues of a sequenc...

We study the spectrum of the Dirac operator on hyperbolic manifolds of finite volume. Depending on the spin structure it is either discrete or the whole real line. For link complements in S^3 we give a simple criterion in terms of linking numbers for when essential spectrum can occur. We compute the accumulation rate of the eigenvalues of a sequenc...

The theme is the influence of the spin structure on the Dirac spectrum
of a spin manifold. We survey examples and results related to this
question.

It has recently been conjectured that the eigenvalues $\lambda$ of the Dirac operator on a closed Riemannian spin manifold $M$ of dimension $n\ge 3$ can be estimated from below by the total scalar curvature: $$ \lambda^2 \ge \frac{n}{4(n-1)} \cdot \frac{\int_M S}{vol(M)}. $$ We show by example that such an estimate is impossible. Comment: 9 pages,...

Let D be a self-adjoint differential operator of Dirac type acting on sections in a vector bundle over a closed Riemannian manifold M. Let H be a closed D-invariant subspace of the Hilbert space of square integrable sections. Suppose D restricted to H is semibounded. We show that every element u in H has the weak unique continuation property, i.e....

Consider a nontrivial smooth solution to a semilinear elliptic system of first order with smooth coefficients defined over
an n-dimensional manifold. Assume the operator has the strong unique continuation property. We show that the zero set of the solution
is contained in a countable union of smooth (n−2)-dimensional submanifolds. Hence it is count...

We survey relations between the dimension of the solution space of the Dirac equation and the topology of the underlying manifold. It is shown that in certain dimensions existence of metrics with harmonic spinors is not topologically obstructed. In this respect the Dirac operator behaves very differently from the Laplace-Beltrami operator.

The Bochner-Lichnerowicz formula and the Atiyah-Singer Index Formula for the Dirac operator have been used to find an obstruction (the $\widehatA$-genus) to producing metrics of positive scalar curvature on spin manifolds. Here the technique is applied to twisted Dirac operators in order to obtain upper bounds on the minimum of the scalar curvature...

The theme is the influence of the spin structure on the Dirac spectrum of a spin manifold. We survey examples and results related to this question. Résumé (Dépendance du Spectre de l'Opérateur de Dirac de la Structure Spinorielle) Sur une variété spinorielle, nousétudionsnousétudions la dépendance du spectre de l'opérateur de Dirac par rapportàrapp...

If G is the structure group of a manifold M it is shown how a certain ideal in the character ring of G corresponds to the set of geometric elliptic operators on M. This provides a simple method to construct these operators. For classical structure groups like G = O(n) (Riemannian manifolds), G = SO(n) (oriented Riemannian manifolds), G = U(m) (almo...

In Kaluza–Klein theory one usually computes the scalar curvature of the principal bundle manifold using the Levi–Civita connection. Here we consider a natural family of invariant connections on a soldered principal bundle which is then parallelizable and hence spinable. This 3-parameter family includes the Levi–Civita connection and the flat connec...

Using Kato's comparison principle for heat semi-groups we derive estimates for the trace of the heat operator on surfaces with variable curvature. This estimate is from above for positively curved surfaces of genus 0 and from below for genus g S 2. It is shown that the estimates are asymptotically sharp for small time and in the case of positive cu...

We derive upper eigenvalue bounds for the Dirac operator of a closed hypersurface in a manifold with Killing spinors such as Euclidean space, spheres or hyperbolic space. The bounds involve the Willmore functional. Relations with the Willmore inequality are briefly discussed. In higher codimension we obtain bounds on the eigenvalues of the Dirac op...

We compute the spectrum of the Dirac operator on 3-dimensional Heisenberg manifolds. The behavior under collapse to the 2-torus is studied. Depending on the spin structure either all eigenvalues tend to ± ∞ or there are eigenvalues converging to those of the torus. This is shown to be true in general for collapsing circle bundles with totally geode...

Dedicated to Andrzej Trautman in honour of his 64 th birthday We study the question to what extend classical Hodge–deRham theory for harmonic differential forms carries over to harmonic spinors. Despite some special phenomena in very low dimensions and despite the Atiyah– Singer index theorem which provides a link between harmonic spinors and the t...

We show that for a suitable class of ``Dirac-like'' operators there holds a Gluing Theorem for connected sums. More precisely, if $M_1$ and $M_2$ are closed Riemannian manifolds of dimension $n\ge 3$ together with such operators, then the connected sum $M_1 # M_2$ can be given a Riemannian metric such that the spectrum of its associated operator is...

We prove that the nodal set (zero set) of a solution of a generalized Dirac equation on a Riemannian manifold has codimension 2 at least. If the underlying manifold is a surface, then the nodal set is discrete. We obtain a quick proof of the fact that the nodal set of an eigenfunction for the Laplace-Beltrami operator on a Riemannian manifold consi...

We show that every closed spin manifold of dimensionn 3 mod 4 with a fixed spin structure can be given a Riemannian metric with harmonic spinors, i.e. the corresponding Dirac operator has a non-trivial kernel (Theorem A). To prove this we first compute the Dirac spectrum of the Berger spheresS
n
,n odd (Theorem 3.1). The second main ingredient is...

Let M be an oriented connected compact Riemannian 4-manifold. We show that if the sectional curvature satisfies K⩾1 and the covariant differential of the curvature tensor satisfies ‖▿R‖L∲⩽2⧸π, then the intersection form of M is definite.

We give a description of all complete simply connected Riemannian manifolds carrying real Killing spinors. Furthermore, we present a construction method for manifolds with the exceptional holonomy groupsG
2 and Spin(7).

O. Introduction. The spin structures of an oriented Riemannian homogeneous space M = G/H can be characterized by lifts of the isotropy representation. These basics are studied in the first section. In the second section we calculate the Dirac operator in algebraic terms, generalizing a well-known formula for symmetric spaces. Using Frobenius recipr...

We derive upper eigenvalue estimates for generalized Dirac operators on closed Riemannian manifolds. In the case of the classical Dirac operator the estimates on the first eigenvalues are sharp for spheres of constant curvature.

We calculate the dimension of the space of harmonic spinors on hyperelliptic Riemann surfaces for all spin structures. Furthermore, we present non-hype relliptic examples of genus 4 and 6 on which the maximal possible number of linearly independent harmonic spinors is achieved.

We show that the fundamental tone of D
2 is zero, where D is the classical Dirac operator on the hyperbolic space.

This is Christian Bär's doctoral thesis. It deals with the Dirac operator on Riemannian spin manifolds and covers the following topics: basics, spectra, homogeneous spaces, coverings and Killing spinors. The spectrum is computed for the following examples: flat tori, Heisenberg manifolds, round spheres and their quotients, Berger spheres and lens s...