TY - JOUR A1 - Bär, Christian A1 - Bandara, Lashi T1 - Boundary value problems for general first-order elliptic differential operators JF - Journal of functional analysis N2 - 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 conditions traditionally considered in the literature fit in our framework. The regularity of the solutions up to the boundary is proven. We show that imposing elliptic boundary conditions yields a Fredholm operator if the manifold is compact. We provide examples which are conveniently treated by our methods. KW - elliptic differential operators of firstorder KW - elliptic boundary KW - conditions KW - boundary regularity KW - Fredholm property KW - H-infinity-functional calculus KW - maximal regularity KW - Rarita-Schwinger KW - operator Y1 - 2022 U6 - https://doi.org/10.1016/j.jfa.2022.109445 SN - 0022-1236 SN - 1096-0783 VL - 282 IS - 12 PB - Elsevier CY - Amsterdam [u.a.] ER - TY - JOUR A1 - Bär, Christian A1 - Hanke, Bernhard T1 - Local flexibility for open partial differential relations JF - Communications on pure and applied mathematics / issued by the Courant Institute of Mathematical Sciences, New York Univ. N2 - 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, differential forms, and lapse functions in Lorentzian geometry. The main application is a general approximation result by sections that have very restrictive local properties on open dense subsets. This shows, for instance, that given any K is an element of Double-struck capital R every manifold of dimension at least 2 carries a complete C-1,C- 1-metric which, on a dense open subset, is smooth with constant sectional curvature K. Of course, this is impossible for C-2-metrics in general. Y1 - 2021 U6 - https://doi.org/10.1002/cpa.21982 SN - 0010-3640 SN - 1097-0312 VL - 75 IS - 6 SP - 1377 EP - 1415 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Bär, Christian A1 - Mazzeo, Rafe T1 - Manifolds with many Rarita-Schwinger fields JF - Communications in mathematical physics N2 - 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 existence of a sequence of compact manifolds in any given dimension greater than or equal to 4 for which the dimension of the space of Rarita-Schwinger fields tends to infinity. These manifolds are either simply connected Kahler-Einstein spin with negative Einstein constant, or products of such spaces with flat tori. Moreover, we construct Calabi-Yau manifolds of even complex dimension with more linearly independent Rarita-Schwinger fields than flat tori of the same dimension. Y1 - 2021 U6 - https://doi.org/10.1007/s00220-021-04030-0 SN - 0010-3616 SN - 1432-0916 VL - 384 IS - 1 SP - 533 EP - 548 PB - Springer CY - Berlin ER - TY - JOUR A1 - Bär, Christian T1 - The Faddeev-LeVerrier algorithm and the Pfaffian JF - Linear algebra and its applications N2 - 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(n(beta+1)) where nis the size of the matrix and O(n(beta)) is the cost of multiplying n x n-matrices, beta is an element of [2, 2.37286). We compare its performance to that of other algorithms and show how it can be used to compute the Euler form of a Riemannian manifold using computer algebra. KW - Characteristic polynomial KW - Determinant KW - Pfaffian KW - Gauss-Bonnet-Chern KW - theorem Y1 - 2021 U6 - https://doi.org/10.1016/j.laa.2021.07.023 SN - 0024-3795 SN - 1873-1856 VL - 630 SP - 39 EP - 55 PB - Elsevier CY - New York ER - TY - JOUR A1 - Bär, Christian T1 - The curl operator on odd-dimensional manifolds JF - Journal of mathematical physics N2 - 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 consists of the eigenvalue 0 with infinite multiplicity and further real discrete eigenvalues of finite multiplicity. We compute the Weyl asymptotics and study the zeta-function. We give a sharp lower eigenvalue bound for positively curved manifolds and analyze the equality case. Finally, we compute the spectrum for flat tori, round spheres, and 3-dimensional spherical space forms. Published under license by AIP Publishing. Y1 - 2019 U6 - https://doi.org/10.1063/1.5082528 SN - 0022-2488 SN - 1089-7658 VL - 60 IS - 3 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Bär, Christian A1 - Strohmaier, Alexander T1 - An index theorem for Lorentzian manifolds with compact spacelike Cauchy boundary JF - American Journal of Mathematics N2 - 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. The index is also shown to equal that of a certain operator constructed from the evolution operator and a spectral projection on the boundary. In case the metric is of product type near the boundary a Feynman parametrix is constructed. Y1 - 2019 U6 - https://doi.org/10.1353/ajm.2019.0037 SN - 0002-9327 SN - 1080-6377 VL - 141 IS - 5 SP - 1421 EP - 1455 PB - Johns Hopkins Univ. Press CY - Baltimore ER - TY - JOUR A1 - Bär, Christian A1 - Strohmaier, Alexander T1 - A Rigorous Geometric Derivation of the Chiral Anomaly in Curved Backgrounds JF - Communications in mathematical physics N2 - 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 mathematically rigorous manner. It contains a term identical to the integrand in the Atiyah-Singer index theorem and another term involving the.-invariant of the Cauchy hypersurfaces. Y1 - 2016 U6 - https://doi.org/10.1007/s00220-016-2664-1 SN - 0010-3616 SN - 1432-0916 VL - 347 SP - 703 EP - 721 PB - Springer CY - New York ER - TY - JOUR A1 - Bär, Christian A1 - Wafo, Roger Tagne T1 - Initial value problems for wave equations on manifolds JF - Mathematical physics, analysis and geometry : an international journal devoted to the theory and applications of analysis and geometry to physics N2 - 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 energy solutions are independent of this choice and hence invariantly defined. We also show existence and uniqueness of solutions for the Goursat problem where one prescribes initial data on a characteristic partial Cauchy hypersurface. This extends classical results due to Hormander. KW - Wave equation KW - Globally hyperbolic Lorentz manifold KW - Cauchy problem KW - Goursat problem KW - Finite energy sections Y1 - 2015 U6 - https://doi.org/10.1007/s11040-015-9176-7 SN - 1385-0172 SN - 1572-9656 VL - 18 IS - 1 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Bär, Christian T1 - Green-Hyperbolic Operators on Globally Hyperbolic Spacetimes JF - Communications in mathematical physics N2 - 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 instance, we show that this class is closed under taking restrictions to suitable subregions of the manifold, under composition, under taking "square roots", and under the direct sum construction. Symmetric hyperbolic systems are studied in detail. Y1 - 2015 U6 - https://doi.org/10.1007/s00220-014-2097-7 SN - 0010-3616 SN - 1432-0916 VL - 333 IS - 3 SP - 1585 EP - 1615 PB - Springer CY - New York ER - TY - JOUR A1 - Bär, Christian T1 - Geometrically formal 4-manifolds with nonnegative sectional curvature JF - Communications in analysis and geometry N2 - 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 CP2. This conclusion stills holds true if the sectional curvature is strictly positive and we relax the condition of geometric formality to the requirement that the length of harmonic 2-forms is not too nonconstant. In particular, the Hopf conjecture on S-2 x S-2 holds in this class of manifolds. Y1 - 2015 SN - 1019-8385 SN - 1944-9992 VL - 23 IS - 3 SP - 479 EP - 497 PB - International Press of Boston CY - Somerville ER - TY - JOUR A1 - Bär, Christian A1 - Becker, Christian T1 - Differential characters and geometric chains JF - Lecture notes in mathematics : a collection of informal reports and seminars JF - Lecture Notes in Mathematics N2 - 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 given by differential characters. Fiber integration for fibers with boundary is treated in the context of relative differential characters. As applications we treat higher-dimensional holonomy, parallel transport, and transgression. Y1 - 2014 SN - 978-3-319-07034-6; 978-3-319-07033-9 U6 - https://doi.org/10.1007/978-3-319-07034-6_1 SN - 0075-8434 VL - 2112 SP - 1 EP - 90 PB - Springer CY - Berlin ER - TY - JOUR A1 - Bär, Christian A1 - Pfaeffle, Frank T1 - Asymptotic heat kernel expansion in the semi-classical limit N2 - Let H-h = h(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-h as h SE arrow 0. As a consequence we get an asymptotic expansion for the quantum partition function and we see that it is asymptotic to the classical partition function. Moreover, we show how to bound the quantum partition function for positive h by the classical partition function. Y1 - 2010 UR - http://www.springerlink.com/content/100467 U6 - https://doi.org/10.1007/s00220-009-0973-3 SN - 0010-3616 ER - TY - JOUR A1 - Bär, Christian A1 - Bessa, C. Pacelli T1 - Stochastic completeness and volume growth N2 - It was 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 general. We also give counterexamples to a converse implication. Y1 - 2010 UR - http://www.ams.org/proc/ U6 - https://doi.org/10.1090/S0002-9939-10-10281-0 SN - 0002-9939 ER - TY - JOUR A1 - Bär, Christian T1 - Spectral bounds for Dirac operators on open manifolds N2 - 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. Y1 - 2009 UR - http://www.springerlink.com/content/100233 U6 - https://doi.org/10.1007/s10455-008-9149-1 SN - 0232-704X ER - TY - JOUR A1 - Bär, Christian A1 - Schopka, Sven T1 - The dirac determinant of spherical space forms N2 - The zeta-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. Y1 - 2003 UR - http://users.math.uni-potsdam.de/~baer/preprints/determinante.html ER - TY - JOUR A1 - Bär, Christian A1 - Dahl, Matthias T1 - Small eigenvalues of the conformal laplacian N2 - 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 give lower and upper bounds in terms of the alpha-genus. Y1 - 2003 UR - http://xxx.uni-augsburg.de/abs/math.DG/0204200 ER - TY - JOUR A1 - Bär, Christian A1 - Moroianu, Sergiu T1 - Heat kernel asymptotics for roots of generalized laplacians N2 - We describe the heat kernel asymptotics for roots of a Laplace type operator on a closed manifold. A previously known relation between the Wodzicki residue and heat trace asymptotics is shown to hold pointwise for the corresponding densities. Y1 - 2003 UR - http://users.math.uni-potsdam.de/~baer/preprints/hta.html ER - TY - JOUR A1 - Bär, Christian A1 - Dahl, Matthias T1 - The first dirac eigenvalue on manifolds with positive scalar curvature N2 - 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. Y1 - 2004 UR - http://xxx.uni-augsburg.de/abs/math.DG/0305277 ER - TY - JOUR A1 - Bär, Christian A1 - Gouduchon, Paul A1 - Moroianu, Andrei T1 - Generalized Cylinders in Semi-Riemannian and Spin Geometry N2 - 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 and to derive the variation formula for the Dirac operator. Moreover, we show that generalized Killing spinors for Codazzi tensors are restrictions of parallel spinors. Finally, we study the space of Lorentzian metrics and give a criterion when two Lorentzian metrics on a manifold can be joined in a natural manner by a 1-parameter family of such metrics. Y1 - 2005 UR - http://xxx.uni-augsburg.de/abs/math.DG/0303095 ER -