Particles with spin in stationary flat spacetimes

We construct stationary flat three-dimensional Lorentzian manifolds with singularities that are obtained from Euclidean surfaces with cone singularities and closed one-forms on these surfaces. In the application to (2?+?1)-gravity, these spacetimes correspond to models containing massive particles with spin. We analyse their geometrical properties, introduce a generalised notion of global hyperbolicity and classify all stationary flat spacetimes with singularities that are globally hyperbolic in that sense. We then apply our results to (2?+?1)-gravity and analyse the causality structure of these spacetimes in terms of measurements by observers. In particular, we derive a condition on observers that excludes causality violating light signals despite the presence of closed timelike curves in these spacetimes.     

Constructions of Pisot and Salem numbers with flat palindromes

This paper introduces explicit conditions for some natural family of polynomials to define Pisot or Salem numbers, and reviews related topics as well as their references.     

Constructions of Pisot and Salem numbers with flat palindromes

This paper introduces explicit conditions for some natural family of polynomials to define Pisot or Salem numbers, and reviews related topics as well as their references.     

Submanifolds with flat normal bundle in spaces of constant curvature

In the present paper parallel submanifolds and focal points of a given submanifold with flat normal bundle are discussed provided that the ambient space has constant sectional curvature. We present shape operators of parallel submanifolds with respect to arbitrary normal vectors. Furthermore, we prove that the focal points of a submanifold with flat normal bundle form totally geodesic hypersurfaces in the normal submanifolds.Supported by Hungarian Nat. Found. for Sci. Research Grant No. 1615 (1991).Dedicated to Professor J. Strommer on the occasion of his 75th birthday     

Convolution with measures on flat curves in low dimensions

We prove Lp→Lq convolution estimates for the affine arclength measure on certain flat curves in Rd when d∈{2,3,4}. For d=2,3, we also establish certain related Lorentz space estimates.     

Electrostatics in a locally flat space with conical singularities

We calculate the potential of a pointlike source in a multicenter three-dimensional space-time and obtain general relations between the values of the regularized self-energy, force, and force moment. The self-action effects as well as the relative contribution of higher multipoles infinitely increase as the angle deficit increases. The results obtained are generalized to a system of parallel cosmic strings one of which carries a current. The case of string with a finite thickness is also considered. Translated from Teoreticheskaya i Matematicheskya Fizika, Vol. 123, No. 1, pp. 150–162, April, 2000.     

Rigidity of minimal submanifolds with flat normal bundle

Let M ⁿ (n ≥ 3) be an n-dimensional complete immersed $ \frac{{n - 2}} {n} $ \frac{{n - 2}} {n} -super-stable minimal submanifold in an (n + p)-dimensional Euclidean space ℝ ^n+p with flat normal bundle. We prove that if the second fundamental form of M satisfies some decay conditions, then M is an affine plane or a catenoid in some Euclidean subspace.     

L2 -cohomology of manifolds with flat ends

Geometric and Functional Analysis - We give a topological interpretation of the spaces of L2 -harmonic forms on manifolds with flat ends. We also prove a Chern-Gauss-Bonnet formula for the L2...     

Counting saddle connections in flat surfaces with poles of higher order

We consider smooth moduli spaces of semistable vector bundles of fixed rank and determinant on a compact Riemann surface X of genus at least 3. The choice of a Poincaré bundle for such a moduli space M induces an isomorphism between X and a component of the moduli space of semistable sheaves over M. We prove that \(\dim H^0(M,\, \text {End}({\mathcal {E}})\otimes TM)\,=\, 1\) for any vector bundle \(\mathcal {E}\) on M coming from this component. Furthermore, there are no nonzero integrable co-Higgs fields on \(\mathcal {E}\).     

Universal moduli spaces of surfaces with flat bundles and cobordism theory

For a compact, connected Lie group G, we study the moduli of pairs (Σ,E), where Σ is a genus g Riemann surface and E→Σ is a flat G-bundle. Varying both the Riemann surface Σ and the flat bundle leads to a moduli space , parametrizing families Riemann surfaces with flat G-bundles. We show that there is a stable range in which the homology of is independent of g. The stable range depends on the genus of the surface. We then identify the homology of this moduli space in the stable range, in terms of the homology of an explicit infinite loop space. Rationally, the stable cohomology of this moduli space is generated by the Mumford-Morita-Miller κ-classes, and the ring of characteristic classes of principal G-bundles, H^∗(BG). Equivalently, our theorem calculates the homology of the moduli space of semi-stable holomorphic bundles on Riemann surfaces.We then identify the homotopy type of the category of one-manifolds and surface cobordisms, each equipped with a flat G-bundle. Our methods combine the classical techniques of Atiyah and Bott, with the new techniques coming out of Madsen and Weiss's proof of Mumford's conjecture on the stable cohomology of the moduli space of Riemann surfaces.     

Complete and non-compact conformally flat manifolds with constant scalar curvature

Non-compact conformally flat manifolds with constant scalar curvature and non-compact Kaehler manifolds with vanishing Bochner curvature are studied and classified.Partially supported by TGRC-KOSEF, 1990.     

The structure of commutative rings with monomorphic flat envelopes

It is obvious that OF and Von Neumann regular rings have monomorphic flat envelopes. In this paper we completely describe the structure,in terms of OF and Von Neumann regular rings, of those commutative rings all of whose modules have a monomorphic flat envelope (m.f.e. ). For that, we introduce the notion of locally QF ring with m.f.e., whose structure is given in terms of OF rings. It turns out that a commutative ring R with m.f.e. is characterized as a (essential) subdirect product of a locally QF ring with m.f.e. and a Von Neumann regular ring, with the latter flat as an R-module.     

Convex-cocompactness of Kleinian groups and conformally flat manifolds with positive scalar curvature

We give a sufficient condition for a higher dimensional Kleinian group to be convex cocompact in terms of the critical exponent of . As a consequence, we see that the fundamental group of a compact conformally flat manifold with positive scalar curvature is hyperbolic in the sense of Gromov. We give some other applications to geometry and topology of conformally flat manifolds with positive scalar curvature.

     

Ricci flat metrics with bidimensional null orbits and non-integrable orthogonal distribution

We study Ricci flat 4-metrics of any signature under the assumption that they allow a Lie algebra of Killing fields with 2-dimensional orbits along which the metric degenerates and whose orthogonal distribution is not integrable. It turns out that locally there is a unique (up to a sign) metric which satisfies the conditions. This metric is of signature (++−−) and, moreover, homogeneous possessing a 6-dimensional symmetry algebra.