57,475 research outputs found
Journal Bearings
A plurality of bearing sectors are mounted in a housing. Each sector functions as a lobed area in the bearing to obtain the required lubricant film geometry
An Uplifting Discussion of T-Duality
It is well known that string theory has a T-duality symmetry relating circle
compactifications of large and small radius. This symmetry plays a foundational
role in string theory. We note here that while T-duality is order two acting on
the moduli space of compactifications, it is order four in its action on the
conformal field theory state space. More generally, involutions in the Weyl
group which act at points of enhanced symmetry have canonical lifts
to order four elements of , a phenomenon first investigated by J. Tits in
the mathematical literature on Lie groups and generalized here to conformal
field theory. This simple fact has a number of interesting consequences. One
consequence is a reevaluation of a mod two condition appearing in asymmetric
orbifold constructions. We also briefly discuss the implications for the idea
that T-duality and its generalizations should be thought of as discrete gauge
symmetries in spacetime.Comment: 47 pages, claims regarding valued cocycles remove
Commercial energy efficiency and the environment
The production and use of energy create serious, extensive environmental affects at every level, in every country, argue the authors. That impact may be more serious in developing than in developed countries as developing countries depend more on natural resources and lack the economic strength to withstand environmental consequences. At the same time, a reliable energy supply is vital to economic growth and development. Energy consumption and economic growth have been somewhat delinked at high income levels, but increased energy consumption (especially of electricity) is inevitable with higher GDP. Greater energy efficiency in developing countries and Eastern Europe is a high-priority way to mitigate the harm to the environment of growing energy consumption, say the authors. They outline four advantages of greater energy efficiency. It requires measures that are in the economic self-interest of those regions. Political obstacles make these measures difficult, but there are well-established techniques for addressing concerns about low-income consumers (such as direct income support or life-line rates). It will help conserve the world supply of nonrenewable (especially fossil) fuels. It will encourage appropriate fuel switching. It addresses every level of concern, up to the global effects of global warming. Any strategy to make energy use and production more efficient must rely more extensively than before on markets that are allowed to function with less government interference. The crucial components of such a straetegy (also crucial to economic development generally) are: more domestic and external competition; the gradual elimination of energy pricing distortions; the reduction of macroeconomic and sectoral distortions (for example, in foreign exchange and credit markets); the reform of energy supply enterprises - reducing state interference, providing more financial autonomy and a greater role for the private sector; consumer incentives to select more efficient lights, space heating, and so on. The authors are not convinced of the need for nonmarket approaches beyond those geared to correct externalities, provide essential information, support basic research and development, and possibly promote pilot projects. They also conclude that a government is far more likely to take action to reduce an environmental externality if it captures benefits within its own national boundaries that exceed the cost of the action. Reducing the large difference between energy prices and economic costs in developing countries and Eastern Europe is a more immediate issue than carbon taxes. The developed countries, say the authors, have an indispensable role to play in improving energy efficiency in the developing countries and Eastern Europe. They can encourage the flow of efficient technology, they can increase conventional aid, and they must accept a greater share of the burden of protecting the global commonalities.Energy and Environment,Environmental Economics&Policies,Energy Demand,Transport and Environment,Power&Energy Conversion
Influence of Dislocations in Thomson's Problem
We investigate Thomson's problem of charges on a sphere as an example of a
system with complex interactions. Assuming certain symmetries we can work with
a larger number of charges than before. We found that, when the number of
charges is large enough, the lowest energy states are not those with the
highest symmetry. As predicted previously by Dodgson and Moore, the complex
patterns in these states involve dislocation defects which screen the strains
of the twelve disclinations required to satisfy Euler's theorem.Comment: 9 pages, 4 figures in gif format. Original PS files can be obtained
in http://fermi.fcu.um.es/thomso
Reinforcement Learning: A Survey
This paper surveys the field of reinforcement learning from a
computer-science perspective. It is written to be accessible to researchers
familiar with machine learning. Both the historical basis of the field and a
broad selection of current work are summarized. Reinforcement learning is the
problem faced by an agent that learns behavior through trial-and-error
interactions with a dynamic environment. The work described here has a
resemblance to work in psychology, but differs considerably in the details and
in the use of the word ``reinforcement.'' The paper discusses central issues of
reinforcement learning, including trading off exploration and exploitation,
establishing the foundations of the field via Markov decision theory, learning
from delayed reinforcement, constructing empirical models to accelerate
learning, making use of generalization and hierarchy, and coping with hidden
state. It concludes with a survey of some implemented systems and an assessment
of the practical utility of current methods for reinforcement learning.Comment: See http://www.jair.org/ for any accompanying file
Interface free energies in p-spin glass models
The replica method has been used to calculate the interface free energy
associated with the change from periodic to anti-periodic boundary conditions
in finite-dimensional p-spin glass models in the phase which at mean-field
level has one-step replica symmetry breaking (1RSB). In any finite dimension
the interface free energy is exponentially small for a large system. This
result implies that in finite dimensions, the 1RSB state does not exist, as it
is destroyed by thermal excitation of arbitrarily large droplets. The
implications of this for the theory of structural glasses are discussed.Comment: 4 page
Confinement-induced Berry phase and helicity-dependent photocurrents
The photocurrent in an optically active metal is known to contain a component
that switches sign with the helicity of the incident radiation. At low
frequencies, this current depends on the orbital Berry phase of the Bloch
electrons via the "anomalous velocity" of Karplus and Luttinger. We consider
quantum wells in which the parent material, such as GaAs, is not optically
active and the relevant Berry phase only arises as a result of quantum
confinement. Using an envelope approximation that is supported by numerical
tight-binding results, it is shown that the Berry phase contribution is
determined for realistic wells by a cubic Berry phase intrinsic to the bulk
material, the well width, and the well direction. These results for the
magnitude of the Berry-phase effect suggest that it may already have been
observed in quantum well experiments.Comment: 4 pages, 2 figure
- …