4,018 research outputs found
Controlled quantum evolutions and transitions
We study the nonstationary solutions of Fokker-Planck equations associated to
either stationary or nonstationary quantum states. In particular we discuss the
stationary states of quantum systems with singular velocity fields. We
introduce a technique that allows to realize arbitrary evolutions ruled by
these equations, to account for controlled quantum transitions. The method is
illustrated by presenting the detailed treatment of the transition
probabilities and of the controlling time-dependent potentials associated to
the transitions between the stationary, the coherent, and the squeezed states
of the harmonic oscillator. Possible extensions to anharmonic systems and mixed
states are briefly discussed and assessed.Comment: 24 pages, 4 figure
Rotavirus genetic diversity, disease association, and temporal change in hospitalized rural Kenyan children
Background. The effectiveness of rotavirus vaccines will be dependent on the immunity conferred against prevalent and emergent variants causing severe diarrheal disease. Longitudinal surveillance of disease-causing strains is a prerequisite to intervention.
Methods. Molecular characterization was conducted on rotavirus-positive stool samples from children admitted with diarrhea to a rural district hospital during 2002-2004. Extracted viral RNA was separated by polyacrylamide gel electrophoresis, and rotavirus VP4 (P types) and VP7 (G types) specificities were determined.
Results. Among 558 investigated cases, the predominant genotype was P[8]G1 (42%), followed by P[8]G9 (15%), P[4]G8 (7%), P[6]G8 (6%), and P[8]G8 (4%), with 10% mixed strains. Overall, there were 6 different P types and 7 G types. No association was identified between genotype and child age, sex, or severity of diarrhea. The P and G genotypes and polyacrylamide gel electropherotypes showed significant temporal variation in frequency: P[8]G1 decreased from 51% (95% confidence interval [CI], 43%-58%) in 2002 to 30% (95% CI, 24%-37%) in 2004, and P[4]G8 increased from 2% (95% CI, 0%-5%) in 2002 to 13% (95% CI, 9%-19%). Quarterly data revealed seasonally endemic and emergence and/or decay patterns.
Conclusions. Our study of rotavirus strains causing severe diarrhea in rural Kenyan children showed a predominance of P[8]G1 and confirms the importance of G8 and G9 strains in sub-Saharan Africa. Considerable genetic diversity of rotavirus strains was observed, including substantial mixed and unusual types, coupled with significant temporal strain variation and emergence. These results warn of variable vaccine efficacy and the need for long-term surveillance of circulating rotavirus genotypes
Monitoring Networks through Multiparty Session Types
In large-scale distributed infrastructures, applications are realised through communications among distributed components. The need for methods for assuring safe interactions in such environments is recognized, however the existing frameworks, relying on centralised verification or restricted specification methods, have limited applicability. This paper proposes a new theory of monitored π-calculus with dynamic usage of multiparty session types (MPST), offering a rigorous foundation for safety assurance of distributed components which asynchronously communicate through multiparty sessions. Our theory establishes a framework for semantically precise decentralised run-time enforcement and provides reasoning principles over monitored distributed applications, which complement existing static analysis techniques. We introduce asynchrony through the means of explicit routers and global queues, and propose novel equivalences between networks, that capture the notion of interface equivalence, i.e. equating networks offering the same services to a user. We illustrate our static-dynamic analysis system with an ATM protocol as a running example and justify our theory with results: satisfaction equivalence, local/global safety and transparency, and session fidelity
A stochastic-hydrodynamic model of halo formation in charged particle beams
The formation of the beam halo in charged particle accelerators is studied in
the framework of a stochastic-hydrodynamic model for the collective motion of
the particle beam. In such a stochastic-hydrodynamic theory the density and the
phase of the charged beam obey a set of coupled nonlinear hydrodynamic
equations with explicit time-reversal invariance. This leads to a linearized
theory that describes the collective dynamics of the beam in terms of a
classical Schr\"odinger equation. Taking into account space-charge effects, we
derive a set of coupled nonlinear hydrodynamic equations. These equations
define a collective dynamics of self-interacting systems much in the same
spirit as in the Gross-Pitaevskii and Landau-Ginzburg theories of the
collective dynamics for interacting quantum many-body systems. Self-consistent
solutions of the dynamical equations lead to quasi-stationary beam
configurations with enhanced transverse dispersion and transverse emittance
growth. In the limit of a frozen space-charge core it is then possible to
determine and study the properties of stationary, stable core-plus-halo beam
distributions. In this scheme the possible reproduction of the halo after its
elimination is a consequence of the stationarity of the transverse distribution
which plays the role of an attractor for every other distribution.Comment: 18 pages, 20 figures, submitted to Phys. Rev. ST A
Adaptive Importance Sampling Simulation of Queueing Networks
In this paper, a method is presented for the efficient estimation of rare-event (overflow) probabilities in Jackson queueing networks using importance sampling. The method differs in two ways from methods discussed in most earlier literature: the change of measure is state-dependent, i.e., it is a function of the content of the buffers, and the change of measure is determined using a cross-entropy-based adaptive procedure. This method yields asymptotically efficient estimation of overflow probabilities of queueing models for which it has been shown that methods using a stateindependent change of measure are not asymptotically efficient. Numerical results demonstrating the effectiveness of the method are presented as well
Tibial tuberosity derotation: a surgical procedure for realignment of the patellofemoral mechanism
We retrospectively reviewed the clinical outcomes of 22 patients (9 men and 13 women) aged 17–42 years, and affected with anterior knee pain. These patients underwent surgical derotation of the tibial tuberosity in the period between September 1992 and December 1993. We describe the details of this new surgical technique to correct a torsional abnormality that has perhaps been underestimated in the past, as a possible cause of anterior knee pain. Follow-up clinical and radiographic controls (average follow-up, 78 months; range 72–87 months) allowed us to document the efficacy of this new procedure as a treatment for anterior knee pain resistant to conservative therapy, in young patients with external hypertorsion of the proximal tibial metaphysis and without significant chondropathology
Orchestrating Tuple-based Languages
The World Wide Web can be thought of as a global computing architecture supporting the deployment of distributed networked applications. Currently, such applications can be programmed by resorting mainly to two distinct paradigms: one devised for orchestrating distributed services, and the other designed for coordinating distributed (possibly mobile) agents. In this paper, the issue of designing a pro-
gramming language aiming at reconciling orchestration and coordination is investigated. Taking as starting point the orchestration calculus Orc and the tuple-based coordination language Klaim, a new formalism is introduced combining concepts and primitives of the original calculi.
To demonstrate feasibility and effectiveness of the proposed approach, a prototype implementation of the new formalism is described and it is then used to tackle a case study dealing with a simplified but realistic electronic marketplace, where a number of on-line stores allow client
applications to access information about their goods and to place orders
Rate-Based Transition Systems for Stochastic Process Calculi
A variant of Rate Transition Systems (RTS), proposed by Klin and Sassone, is introduced and used as the basic model for defining stochastic behaviour of processes. The transition relation used in our variant associates to each process, for each action, the set of possible futures paired with a measure indicating their rates. We show how RTS can be used for providing the operational semantics of stochastic extensions of classical formalisms, namely CSP and CCS. We also show that our semantics for stochastic CCS guarantees associativity of parallel composition. Similarly, in contrast with the original definition by Priami, we argue that a semantics for stochastic π-calculus can be provided that guarantees associativity of parallel composition
Levy-Student Distributions for Halos in Accelerator Beams
We describe the transverse beam distribution in particle accelerators within
the controlled, stochastic dynamical scheme of the Stochastic Mechanics (SM)
which produces time reversal invariant diffusion processes. This leads to a
linearized theory summarized in a Shchr\"odinger--like (\Sl) equation. The
space charge effects have been introduced in a recent paper~\cite{prstab} by
coupling this \Sl equation with the Maxwell equations. We analyze the space
charge effects to understand how the dynamics produces the actual beam
distributions, and in particular we show how the stationary, self--consistent
solutions are related to the (external, and space--charge) potentials both when
we suppose that the external field is harmonic (\emph{constant focusing}), and
when we \emph{a priori} prescribe the shape of the stationary solution. We then
proceed to discuss a few new ideas~\cite{epac04} by introducing the generalized
Student distributions, namely non--Gaussian, L\'evy \emph{infinitely divisible}
(but not \emph{stable}) distributions. We will discuss this idea from two
different standpoints: (a) first by supposing that the stationary distribution
of our (Wiener powered) SM model is a Student distribution; (b) by supposing
that our model is based on a (non--Gaussian) L\'evy process whose increments
are Student distributed. We show that in the case (a) the longer tails of the
power decay of the Student laws, and in the case (b) the discontinuities of the
L\'evy--Student process can well account for the rare escape of particles from
the beam core, and hence for the formation of a halo in intense beams.Comment: revtex4, 18 pages, 12 figure
Low-energy excitations of a linearly Jahn-Teller coupled orbital quintet
The low-energy spectra of the single-mode h x (G+H) linear Jahn-Teller model
is studied by means of exact diagonalization. Both eigenenergies and
photoemission spectral intensities are computed. These spectra are useful to
understand the vibronic dynamics of icosahedral clusters with partly filled
orbital quintet molecular shells, for example C60 positive ions.Comment: 14 pages revte
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