348 research outputs found
On the Geroch-Traschen class of metrics
We compare two approaches to semi-Riemannian metrics of low regularity. The maximally 'reasonable' distributional setting of Geroch and Traschen is shown to be consistently contained in the more general setting of nonlinear distributional geometry in the sense of Colombea
On the completeness of impulsive gravitational wave space-times
We consider a class of impulsive gravitational wave space-times, which
generalize impulsive pp-waves. They are of the form ,
where is a Riemannian manifold of arbitrary dimension and carries
the line element with the line
element of and the Dirac measure. We prove a completeness result
for such space-times with complete Riemannian part .Comment: 13 pages, minor changes suggested by the referee
Concurrent Design of Railway Vehicles by Simulation Model Reuse
This paper describes a concurrent design approach to railway vehicle design. Current railway vehicles use many different concepts that are combined into the final design concept. The design support for such systems is based on reusing components from previous design cases. The key part of the railway vehicle design concept is its simulation model. Therefore the support is based on support for reuse of previous simulation models. The simulation models of different railway component concepts are stored using the methodology from the EU CLOCKWORK project. The new concept usually combines stored components
Geodesics in spacetimes with expanding impulsive gravitational waves
We study geodesic motion in expanding spherical impulsive gravitational waves
propagating in a Minkowski background. Employing the continuous form of the
metric we find and examine a large family of geometrically preferred geodesics.
For the special class of axially symmetric spacetimes with the spherical
impulse generated by a snapping cosmic string we give a detailed physical
interpretation of the motion of test particles.Comment: 12 pages, Revtex, final versio
The wave equation on singular space-times
We prove local unique solvability of the wave equation for a large class of
weakly singular, locally bounded space-time metrics in a suitable space of
generalised functions.Comment: Latex, 19 pages, 1 figure. Discussion of class of metrics covered by
our results and some examples added. Conclusion more detailed. Version to
appear in Communications in Mathematical Physic
Isomorphisms of algebras of Colombeau generalized functions
We show that for smooth manifolds X and Y, any isomorphism between the
special algebra of Colombeau generalized functions on X, resp. Y is given by
composition with a unique Colombeau generalized function from Y to X. We also
identify the multiplicative linear functionals from the special algebra of
Colombeau generalized functions on X to the ring of Colombeau generalized
numbers. Up to multiplication with an idempotent generalized number, they are
given by an evaluation map at a compactly supported generalized point on X.Comment: 10 page
Aichelburg-Sexl boost of an isolated source in general relativity
A study of the Aichelburg--Sexl boost of the Schwarzschild field is described
in which the emphasis is placed on the field (curvature tensor) with the metric
playing a secondary role. This is motivated by a description of the Coulomb
field of a charged particle viewed by an observer whose speed relative to the
charge approaches the speed of light. Our approach is exemplified by carrying
out an Aichelburg-- Sexl type boost on the Weyl vacuum gravitational field due
to an isolated axially symmetric source. Detailed calculations of the boosts
transverse and parallel to the symmetry axis are given and the results, which
differ significantly, are discussed.Comment: 25 pages, LateX2
Dispersal syndromes are poorly associated with climatic niche differences in the Azorean seed plants
Aim: Environmental niche tracking is linked to the species ability to disperse. While
well investigated on large spatial scales, dispersal constraints also influence small-scale
processes and may explain the difference between the potential and the realized
niche of species at small scales. Here we test whether niche size and niche fill differ
systematically according to dispersal syndrome within isolated oceanic islands. We
expect that species with higher dispersal abilities (anemochorous or endozoochorous)
will have a higher niche fill, despite their environmental niche size.
Location: Azores archipelago.
Taxon: Native seed plants.
Methods: We combined a georeferenced database of the species distribution within
the archipelago (Azorean Biodiversity Portal/GBIF) with an expert-based
dispersal
syndrome categorization and a high-resolution
climatic grid (CIELO model). Using
four climatic variables (Annual Mean Temperature, Mean Diurnal Range, Annual
Precipitation, Precipitation Seasonality), we calculated a four-dimensional
hypervolume
to estimate the niche size of each species. Niche fill was quantified as the suitable
climatic space of the island that was occupied by the focal species.
Results: We found a significant relationship between dispersal syndromes and niche
size, and also between dispersal syndromes and niche fill. Such relationships presented
no phylogenetic signal. Endozoochorous species display higher niche fill compared to epizoochorous and hydrochorous species, and larger niches than anemochorous and
epizoochorous. Differences among the remaining groups are not significant for either
niche size or for niche fill.
Main conclusions: The ability of a species to track its niche at small scales is not tightly
related to its dispersal syndrome, although endozoochorous species track their niche
more efficiently than the rest of groups. Despite being intuitively appealing, dispersal
syndrome classifications might not be the most appropriate tools for understanding
dispersal processes at small scales.info:eu-repo/semantics/acceptedVersio
Symmetries and geodesics in (anti-)de Sitter spacetimes with nonexpanding impulsive waves
We consider a class of exact solutions which represent nonexpanding impulsive
waves in backgrounds with nonzero cosmological constant. Using a convenient
5-dimensional formalism it is shown that these spacetimes admit at least three
global Killing vector fields. The same geometrical approach enables us to find
all geodesics in a simple explicit form and describe the effect of impulsive
waves on test particles. Timelike geodesics in the axially-symmetric
Hotta-Tanaka spacetime are studied in detail. It is also demonstrated that for
vanishing cosmological constant, the symmetries and geodesics reduce to those
for well-known impulsive pp-waves.Comment: 16 pages, 3 figures, LaTeX 2e. To appear in Class. Quantum Gra
Remarks on the distributional Schwarzschild geometry
This work is devoted to a mathematical analysis of the distributional Schwarzschild geometry. The Schwarzschild solution is extended to include the singularity; the energy momentum tensor becomes a delta-distribution supported at r=0. Using generalized distributional geometry in the sense of Colombeau's (special) construction the nonlinearities are treated in a mathematically rigorous way. Moreover, generalized function techniques are used as a tool to give a unified discussion of various approaches taken in the literature so far; in particular we comment on geometrical issues
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