11,602 research outputs found
Single-Photon Observables and Preparation Uncertainty Relations
We propose a procedure for defining all single-photon observables in terms of
Positive-Operator Valued Measures (POVMs), in particular spin and position. We
identify the suppression of -helicity photon states as a projection from an
extended Hilbert space onto the photon Hilbert space. We show that all
single-photon observables are in general described by POVMs, obtained by
applying this projection to opportune Projection-Valued Measures (PVMs),
defined on the extended Hilbert space. The POVMs associated to momentum and
helicity reduce to PVMs, unlike those associated to position and spin, this
fact reflecting the intrinsic unsharpness of these observables. We finally
extensively study the preparation uncertainty relations for position and
momentum and the probability distribution of spin, exploring single photon
Gaussian states for several choices of spin and polarization.Comment: 25 pages (7 Figures); revised and extended version; in submissio
The value function of an asymptotic exit-time optimal control problem
We consider a class of exit--time control problems for nonlinear systems with
a nonnegative vanishing Lagrangian. In general, the associated PDE may have
multiple solutions, and known regularity and stability properties do not hold.
In this paper we obtain such properties and a uniqueness result under some
explicit sufficient conditions. We briefly investigate also the infinite
horizon problem
Market-based Instruments for Environmental Policymaking in Latin America and the Caribbean: Lessons from Eleven Countries
This report is a summary of country studies in Latin America and the Caribbean, addressing the use of market-based instruments (MBIs) and command-and-control (CAC) measures for environmental management in the region. Even though MBIs can significantly add efficiency to existing CAC mechanisms, the scope of MBIs should match the countries institutional capacity to implement them. Gradual and flexible reforms are likely to succeed within the current regional context of continued institutional changes. A key function of MBIs is usually revenue collection, though it does not necessarily lead to successful environmental management. The study suggests that revenues should be channeled to local authorities for an effective MBI's implementation. The report also critiques the regular practice of international donor agencies in recommending the solutions suitable for developed countries, without considering the institutional conditions in developing countries. Further, the study explores both the successes and difficulties experienced in the region regarding regulations, macro-policies, and MBIs; the institutional frameworks of the countries under review; and, the issues considered in the design of MBIs, in order to promote a beneficial dialogue among them
A Higher-order Maximum Principle for Impulsive Optimal Control Problems
We consider a nonlinear system, affine with respect to an unbounded control
which is allowed to range in a closed cone. To this system we associate a
Bolza type minimum problem, with a Lagrangian having sublinear growth with
respect to . This lack of coercivity gives the problem an {\it impulsive}
character, meaning that minimizing sequences of trajectories happen to converge
towards discontinuous paths. As is known, a distributional approach does not
make sense in such a nonlinear setting, where, instead, a suitable embedding in
the graph-space is needed.
We provide higher order necessary optimality conditions for properly defined
impulsive minima, in the form of equalities and inequalities involving iterated
Lie brackets of the dynamical vector fields. These conditions are derived under
very weak regularity assumptions and without any constant rank conditions
Necessary conditions involving Lie brackets for impulsive optimal control problems
We obtain higher order necessary conditions for a minimum of a Mayer optimal
control problem connected with a nonlinear, control-affine system, where the
controls range on an m-dimensional Euclidean space. Since the allowed
velocities are unbounded and the absence of coercivity assumptions makes big
speeds quite likely, minimizing sequences happen to converge toward
"impulsive", namely discontinuous, trajectories. As is known, a distributional
approach does not make sense in such a nonlinear setting, where instead a
suitable embedding in the graph space is needed. We will illustrate how the
chance of using impulse perturbations makes it possible to derive a Higher
Order Maximum Principle which includes both the usual needle variations (in
space-time) and conditions involving iterated Lie brackets. An example, where a
third order necessary condition rules out the optimality of a given extremal,
concludes the paper.Comment: Conference pape
Collaborative semantic web browsing with Magpie
Web browsing is often a collaborative activity. Users involved in a joint information gathering exercise will wish to share knowledge about the web pages visited and the contents found. Magpie is a suite of tools supporting the interpretation of web pages and semantically enriched web browsing. By automatically associating an ontology-based semantic layer to web resources, Magpie allows relevant services to be invoked as well as remotely triggered within a standard web browser. In this paper we describe how Magpie trigger services can provide semantic support to collaborative browsing activities
Minimum Restraint Functions for unbounded dynamics: general and control-polynomial systems
We consider an exit-time minimum problem with a running cost, and
unbounded controls. The occurrence of points where can be regarded as a
transversality loss. Furthermore, since controls range over unbounded sets, the
family of admissible trajectories may lack important compactness properties. In
the first part of the paper we show that the existence of a -minimum
restraint function provides not only global asymptotic controllability (despite
non-transversality) but also a state-dependent upper bound for the value
function (provided ). This extends to unbounded dynamics a former result
which heavily relied on the compactness of the control set.
In the second part of the paper we apply the general result to the case when
the system is polynomial in the control variable. Some elementary, algebraic,
properties of the convex hull of vector-valued polynomials' ranges allow some
simplifications of the main result, in terms of either near-affine-control
systems or reduction to weak subsystems for the original dynamics.Comment: arXiv admin note: text overlap with arXiv:1503.0344
Fast variability as a tracer of accretion regimes in black hole transients
We present the rms-intensity diagram for black hole transients. Using
observations taken with the Rossi X-ray timing explorer we study the relation
between the root mean square (rms) amplitude of the variability and the net
count-rate during the 2002, 2004 and 2007 outbursts of the black hole X-ray
binary GX 339-4. We find that the rms-flux relation previously observed during
the hard state in X-ray binaries does not hold for the other states, when
different relations apply. These relations can be used as a good tracer of the
different accretion regimes. We identify the hard, soft and intermediate states
in the rms-intensity diagram. Transitions between the different states are seen
to produce marked changes in the rms-flux relation. We find that one single
component is required to explain the ~ 40 per cent variability observed at low
count rates, whereas no or very low variability is associated to the
accretion-disc thermal component.Comment: Accepted for publication in MNRAS. 6 pages, 4 figure
Approaches to Semantic Web Services: An Overview and Comparison
Abstract. The next Web generation promises to deliver Semantic Web Services (SWS); services that are self-described and amenable to automated discovery, composition and invocation. A prerequisite to this, however, is the emergence and evolution of the Semantic Web, which provides the infrastructure for the semantic interoperability of Web Services. Web Services will be augmented with rich formal descriptions of their capabilities, such that they can be utilized by applications or other services without human assistance or highly constrained agreements on interfaces or protocols. Thus, Semantic Web Services have the potential to change the way knowledge and business services are consumed and provided on the Web. In this paper, we survey the state of the art of current enabling technologies for Semantic Web Services. In addition, we characterize the infrastructure of Semantic Web Services along three orthogonal dimensions: activities, architecture and service ontology. Further, we examine and contrast three current approaches to SWS according to the proposed dimensions
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