4,129 research outputs found
Degeneracy of the b-boundary in General Relativity
The b-boundary construction by B. Schmidt is a general way of providing a
boundary to a manifold with connection. It has been shown to have undesirable
topological properties however. C. J. S. Clarke gave a result showing that for
space-times, non-Hausdorffness is to be expected in general, but the argument
contains some errors. We show that under somewhat different conditions on the
curvature, the b-boundary will be non-Hausdorff, and illustrate the degeneracy
by applying the conditions to some well known exact solutions of general
relativity.Comment: 25 pages, AMS-LaTeX v1.2, AMSFonts, submitted to Commun. Math. Phy
Planted tree fallows and their influence on soil fertility and maize production in East Africa
Soil fertility depletion is a main constraint to food production in sub-Saharan Africa. This thesis concerns the potential of N2-fixing trees to increase nitrogen inputs to agroforestry systems and accordingly to improve crop production. The suitability of five tropical tree species (including two N2-fixing species, Leucaena leucocephala and Prosopis chilensis) for tree fallows in Tanzania were evaluated by comparing their leaf chemistry, their effects on soil properties and on maize growth. After five years fallow, the per cent total soil N was higher under Prosopis compared to under other tree species. Maize biomass production was higher on soils from Leucaena or Prosopis compared to grass fallow. Prosopis contributed 11% to the total soil C over a period of 8 years. Field experiments in Kenya were performed to test a low-level 15N-tracer technique to estimate biological nitrogen fixation in Sesbania sesban over an 18-months period, and to compare the effects of short-duration tree fallows on two subsequent maize harvests with natural fallow and continuous cropping. We estimated the N derived from atmosphere by Sesbania after 18 months to between 500 and 600 kg ha-1, depending on which plant parts were used for 15N data and on the choice of reference species. We consider the 15N dilution method to be appropriate for quantifying N2 fixation in improved fallows in studies of young trees with high N2-fixing ability. In an experiment examining the effects of tree fallows on subsequent maize crops approximately 70-90% of the N in Sesbania, and 50-70% in Calliandra calothyrsus, was derived from N2-fixation. The quantity of N added by N2-fixation, 280-360 kg N ha-1 for Sesbania and 120-170 kg N ha-1 for Calliandra, resulted in a positive N balance after two cropping seasons of 170-250 kg N ha-1 and 90-140 kg N ha-1 respectively. Both the content of inorganic N in the topsoil and the quantity of N mineralised during rainy seasons were higher after the Sesbania fallows than after the other treatments. The substantial accumulation of N in planted Sesbania demonstrated its potential to increase the sustainability of crop production on N-limited soils
The Geometry of the Frame Bundle over Spacetime
One of the known mathematical descriptions of singularities in General
Relativity is the b-boundary, which is a way of attaching endpoints to
inextendible endless curves in a spacetime. The b-boundary of a manifold M with
connection is constructed by forming the Cauchy completion of the frame bundle
LM equipped with a certain Riemannian metric, the b-metric G. We study the
geometry of (LM,G) as a Riemannian manifold in the case when the connection is
the Levi-Civita connection of a Lorentzian metric g on M. In particular, we
give expressions for the curvature and discuss the isometries and the geodesics
of (LM,G) in relation to the geometry of (M,g).Comment: 14 pages, no figures, LaTeX 2e with AMSLaTeX 1.2 and AMSFonts,
submitted to J. Math. Phy
On Imprisoned Curves and b-length in General Relativity
This paper is concerned with two themes: imprisoned curves and the b-length
functional. In an earlier paper by the author, it was claimed that an endless
incomplete curve partially imprisoned in a compact set admits an endless null
geodesic cluster curve. Unfortunately, the proof was flawed. We give an outline
of the problem and remedy the situation by providing a proof by different
methods. Next, we obtain some results concerning the structure of b-length
neighbourhoods, which gives a clue to how the geometry of a spacetime is
encoded in the pseudo-orthonormal frame bundle equipped with the b-metric. We
also show that a previous result by the author, proving total degeneracy of a
b-boundary fibre in some cases, does not apply to imprisoned curves. Finally,
we correct some results in the literature linking the b-lengths of general
curves in the frame bundle with the b-length of the corresponding horizontal
curves.Comment: 26 pages, 7 figures, LaTeX 2e with AMSLaTeX 1.2 and AMSFonts,
submitted to J. Math. Phy
Gotzmann's persistence theorem for finite modules
We prove a generalization of Gotzmann's persistence theorem in the case of
modules with constant Hilbert polynomial. As a consequence, we show that the
defining equations that give the embedding of a Quot scheme of points into a
Grassmannian are given by a single Fitting ideal.Comment: 13 pages; restructured the paper and clarified some proof
Rees algebras of modules and coherent functors
We show that several properties of the theory of Rees algebras of modules
become more transparent using the category of coherent functors rather than
working directly with modules. In particular, we show that the Rees algebra is
induced by a canonical map of coherent functors.Comment: 15 pages. This is an extended part of an old version of
arXiv:1402.3219 - v2: Clarified some statements and made minor notation
change
A Web-based Distance Learning System in Business Administration Experiences from an Inter-Nordic Course
The paper presents a specially constructed Web-based tool, WebGPSS, developed in Sweden, for teaching business students simulation. The tool includes a Java Applet that provides a Graphical Users Interface, by which the student can build a simulation model of a system, such as an inventory system. The model is then run on a remote server, which then sends result graphs and tables over the Web to the student computer. The paper furthermore discusses the experiences made when using this tool in a distance-learning course at the Norwegian School of Economics and Business Administration (NHH) in Bergen, in 2000 â 2002.Web-based; Distance learning; Simulation; GPSS; Java
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