24,387 research outputs found
A Portfolio Theory of International Capital Flows
This paper constructs a model in which the currency composition of national portfolios is an essential element in facilitating capital ‡ows between countries. In a two country environment, each country chooses optimal nominal bond portfolios in face of real and nominal risk.Current account deficits are financed by increases in domestic currency debt, but balanced by increases in foreign currency credit. This is combined with an evolution of risk-premiums such that the rate of return on the debtor country’s gross liabilities is lower than the return on its gross assets. This ensures stability of the world wealth distribution.
Chirality Selection in Open Flow Systems and in Polymerization
As an attempt to understand the homochirality of organic molecules in life, a
chemical reaction model is proposed where the production of chiral monomers
from achiral substrate is catalyzed by the polymers of the same enatiomeric
type. This system has to be open because in a closed system the enhanced
production of chiral monomers by enzymes is compensated by the associated
enhancement in back reaction, and the chiral symmetry is conserved. Open flow
without cross inhibition is shown to lead to the chirality selection in a
general model. In polymerization, the influx of substrate from the ambience and
the efflux of chiral products for purposes other than the catalyst production
make the system necessarily open. The chiral symmetry is found to be broken if
the influx of substrate lies within a finite interval. As the efficiency of the
enzyme increases, the maximum value of the enantiomeric excess approaches unity
so that the chirality selection becomes complete.Comment: 8 pages, 4 figure
Cosmic ray intensity and the tilt of the neutral sheet
Recent publications have related long-term variations in cosmic ray intensity at the Earth with long term variations in the tilt of the neutral sheet in the inner heliosphere. The tilt of the neutral sheet from 1971 to 1974 is compared with the cosmic ray intensity at Earth, recorded by the Mt. Washington neutron monitor. The remarkable large decreases in cosmic ray intensity which occurred in 1973 and 1974 correlate well with excursions in the tilt of the neutral sheet which occurred earlier during these same two years
Genus Ranges of Chord Diagrams
A chord diagram consists of a circle, called the backbone, with line
segments, called chords, whose endpoints are attached to distinct points on the
circle. The genus of a chord diagram is the genus of the orientable surface
obtained by thickening the backbone to an annulus and attaching bands to the
inner boundary circle at the ends of each chord. Variations of this
construction are considered here, where bands are possibly attached to the
outer boundary circle of the annulus. The genus range of a chord diagram is the
genus values over all such variations of surfaces thus obtained from a given
chord diagram. Genus ranges of chord diagrams for a fixed number of chords are
studied. Integer intervals that can, and cannot, be realized as genus ranges
are investigated. Computer calculations are presented, and play a key role in
discovering and proving the properties of genus ranges.Comment: 12 pages, 8 figure
Theoretical studies of spin-dependent electrical transport through carbon nanotbes
Spin-dependent coherent quantum transport through carbon nanotubes (CNT) is
studied theoretically within a tight-binding model and the Green's function
partitioning technique. End-contacted metal/nanotube/metal systems are modelled
and next studied in the magnetic context, i.e. either with ferromagnetic
electrodes or at external magnetic fields. The former case shows that quite a
substantial giant magnetoresistance (GMR) effect occurs () for
disorder-free CNTs. Anderson-disorder averaged GMR, in turn, is positive and
reduced down to several percent in the vicinity of the charge neutrality point.
At parallel magnetic fields, characteristic Aharonov-Bohm-type oscillations are
revealed with pronounced features due to a combined effect of:
length-to-perimeter ratio, unintentional electrode-induced doping, Zeeman
splitting, and energy-level broadening. In particular, a CNT is predicted to
lose its ability to serve as a magneto-electrical switch when its length and
perimeter become comparable. In case of perpendicular geometry, there are
conductance oscillations approaching asymptotically the upper theoretical limit
to the conductance, . Moreover in the ballistic transport regime,
initially the conductance increases only slightly with the magnetic field or
remains nearly constant because spin up- and spin down-contributions to the
total magnetoresistance partially compensate each other.Comment: 15 pages, 6 figures (to apppear in Semicond. Sci. Technol.
On the acquisition and representation of procedural knowledge
Historically knowledge acquisition has proven to be one of the greatest barriers to the development of intelligent systems. Current practice generally requires lengthy interactions between the expert whose knowledge is to be captured and the knowledge engineer whose responsibility is to acquire and represent knowledge in a useful form. Although much research has been devoted to the development of methodologies and computer software to aid in the capture and representation of some of some types of knowledge, little attention has been devoted to procedural knowledge. NASA personnel frequently perform tasks that are primarily procedural in nature. Previous work is reviewed in the field of knowledge acquisition and then focus on knowledge acquisition for procedural tasks with special attention devoted to the Navy's VISTA tool. The design and development is described of a system for the acquisition and representation of procedural knowledge-TARGET (Task Analysis and Rule Generation Tool). TARGET is intended as a tool that permits experts to visually describe procedural tasks and as a common medium for knowledge refinement by the expert and knowledge engineer. The system is designed to represent the acquired knowledge in the form of production rules. Systems such as TARGET have the potential to profoundly reduce the time, difficulties, and costs of developing knowledge-based systems for the performance of procedural tasks
Finite-dimensional analogs of string s <-> t duality and pentagon equation
We put forward one of the forms of functional pentagon equation (FPE), known
from the theory of integrable models, as an algebraic explanation to the
phenomenon known in physics as st duality. We present two simple geometrical
examples of FPE solutions, one of them yielding in a particular case the
well-known Veneziano expression for 4-particle amplitude. Finally, we interpret
our solutions of FPE in terms of relations in Lie groups.Comment: LaTeX, 12 pages, 6 eps figure
Quark-Meson Coupling Model for a Nucleon
The quark-meson coupling model for a nucleon is considered. The model
describes a nucleon as an MIT bag, in which quarks are coupled to scalar and
vector mesons. A set of coupled equations for the quark and the meson fields
are obtained and are solved in a self-consistent way. It is shown that the mass
of a nucleon as a dressed MIT bag interacting with sigma- and omega-meson
fields significantly differs from the mass of a free MIT bag. A few sets of
model parameters are obtained so that the mass of a dressed MIT bag becomes the
nucleon mass. The results of our calculations imply that the self-energy of the
bag in the quark-meson coupling model is significant and needs to be considered
in doing the nuclear matter calculations.Comment: 3 figure
TARGET: Rapid Capture of Process Knowledge
TARGET (Task Analysis/Rule Generation Tool) represents a new breed of tool that blends graphical process flow modeling capabilities with the function of a top-down reporting facility. Since NASA personnel frequently perform tasks that are primarily procedural in nature, TARGET models mission or task procedures and generates hierarchical reports as part of the process capture and analysis effort. Historically, capturing knowledge has proven to be one of the greatest barriers to the development of intelligent systems. Current practice generally requires lengthy interactions between the expert whose knowledge is to be captured and the knowledge engineer whose responsibility is to acquire and represent the expert's knowledge in a useful form. Although much research has been devoted to the development of methodologies and computer software to aid in the capture and representation of some types of knowledge, procedural knowledge has received relatively little attention. In essence, TARGET is one of the first tools of its kind, commercial or institutional, that is designed to support this type of knowledge capture undertaking. This paper will describe the design and development of TARGET for the acquisition and representation of procedural knowledge. The strategies employed by TARGET to support use by knowledge engineers, subject matter experts, programmers and managers will be discussed. This discussion includes the method by which the tool employs its graphical user interface to generate a task hierarchy report. Next, the approach to generate production rules for incorporation in and development of a CLIPS based expert system will be elaborated. TARGET also permits experts to visually describe procedural tasks as a common medium for knowledge refinement by the expert community and knowledge engineer making knowledge consensus possible. The paper briefly touches on the verification and validation issues facing the CLIPS rule generation aspects of TARGET. A description of efforts to support TARGET's interoperability issues on PCs, Macintoshes and UNIX workstations concludes the paper
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