18,533 research outputs found
Modal Interface Automata
De Alfaro and Henzinger's Interface Automata (IA) and Nyman et al.'s recent
combination IOMTS of IA and Larsen's Modal Transition Systems (MTS) are
established frameworks for specifying interfaces of system components. However,
neither IA nor IOMTS consider conjunction that is needed in practice when a
component shall satisfy multiple interfaces, while Larsen's MTS-conjunction is
not closed and Bene\v{s} et al.'s conjunction on disjunctive MTS does not treat
internal transitions. In addition, IOMTS-parallel composition exhibits a
compositionality defect. This article defines conjunction (and also
disjunction) on IA and disjunctive MTS and proves the operators to be
'correct', i.e., the greatest lower bounds (least upper bounds) wrt. IA- and
resp. MTS-refinement. As its main contribution, a novel interface theory called
Modal Interface Automata (MIA) is introduced: MIA is a rich subset of IOMTS
featuring explicit output-must-transitions while input-transitions are always
allowed implicitly, is equipped with compositional parallel, conjunction and
disjunction operators, and allows a simpler embedding of IA than Nyman's. Thus,
it fixes the shortcomings of related work, without restricting designers to
deterministic interfaces as Raclet et al.'s modal interface theory does.Comment: 28 page
Need for a subtropical wind profiling system
The purpose is to point out the need for, and the benefit that can be derived from, a national wind profiling facility located in the subtropics. At present no such facility exists. There are several advantages associated with a low-latitude location. The first is that wave motions and large-scale circulations unique to the tropics can be studied. The second is that the relatively steady mean flows in the subtropical belt may provide a cleaner environment for studies of waves common at all latitudes. Researchers suggest the Arecibo Observatory as an ideal site for a wind profiling facility since the land and much of the computing, technical, and scientific support is already available
Exact String Solutions in Nontrivial Backgrounds
We show how the classical string dynamics in -dimensional gravity
background can be reduced to the dynamics of a massless particle constrained on
a certain surface whenever there exists at least one Killing vector for the
background metric. We obtain a number of sufficient conditions, which ensure
the existence of exact solutions to the equations of motion and constraints.
These results are extended to include the Kalb-Ramond background. The
-brane dynamics is also analyzed and exact solutions are found. Finally, we
illustrate our considerations with several examples in different dimensions.
All this also applies to the tensionless strings.Comment: 22 pages, LaTeX, no figures; V2:Comments and references added;
V3:Discussion on the properties of the obtained solutions extended, a
reference and acknowledgment added; V4:The references renumbered, to appear
in Phys Rev.
From the WZWN Model to the Liouville Equation: Exact String Dynamics in Conformally Invariant AdS Background
It has been known for some time that the SL(2,R) WZWN model reduces to
Liouville theory. Here we give a direct and physical derivation of this result
based on the classical string equations of motion and the proper string size.
This allows us to extract precisely the physical effects of the metric and
antisymmetric tensor, respectively, on the {\it exact} string dynamics in the
SL(2,R) background. The general solution to the proper string size is also
found. We show that the antisymmetric tensor (corresponding to conformal
invariance) generally gives rise to repulsion, and it precisely cancels the
dominant attractive term arising from the metric.
Both the sinh-Gordon and the cosh-Gordon sectors of the string dynamics in
non-conformally invariant AdS spacetime reduce here to the Liouville equation
(with different signs of the potential), while the original Liouville sector
reduces to the free wave equation. Only the very large classical string size is
affected by the torsion. Medium and small size string behaviours are unchanged.
We also find illustrative classes of string solutions in the SL(2,R)
background: dynamical closed as well as stationary open spiralling strings, for
which the effect of torsion is somewhat like the effect of rotation in the
metric. Similarly, the string solutions in the 2+1 BH-AdS background with
torsion and angular momentum are fully analyzed.Comment: 24 pages including 4 postscript figures. Enlarged version including a
section on string solutions in 2+1 black hole background. To be published in
Phys. Rev. D., December 199
Installation of insecticide-treated durable wall lining: evaluation of attachment materials and product durability under field conditions.
BACKGROUND: Insecticide-treated durable wall lining (DL) is a new method of vector control designed to supplement LLINs and overcome two inherent limitations of LLINs and IRS: nightly behavioural compliance and short residual activity, respectively. DL is a deltamethrin-treated polyethylene material, which when used to cover interior house walls, functions as long-lasting IRS. Because the DL concept anticipates minimal upkeep, a primary challenge is how to guarantee correct household installation and in situ longevity for several years. Field trials were undertaken on various wall surfaces in Ghana to identify a logistically feasible, durable and re-usable method for DL wall attachment and to pilot new methods for assessing DL durability. METHODS: Over fifty-five candidate attachment or fixing products, including mechanical fasteners, material anchors and adhesives, were evaluated for their ability to tolerate static loads (simulating long-term installation) and short-term heavy weights (imitating shock damage). Attachment products were also scored using qualitative logistical and feasibility criteria, including ease of preparation, grip of fixing to DL and possibility of re-use. RESULTS: The stress tests provided a standardised, reproducible and reliable system for assessing fixing effectiveness and DL durability, with 64% (14/22) of adhesives and 15% (2/13) of mechanical fasteners failing to meet the minimum requirements of attaching DL to mud walls for set time periods. For most fixings, less outward load (0.2-8.0 kg) was required to detach DL from the wall, compared to downward load (0.2-19.2 kg). Fixings were better able to grip DL onto concrete than clay surfaces. Using a plastic nail cap to increase DL attachment area greatly improved grip and outward load tolerance, more so than varying nail size, length or texture. CONCLUSIONS: Based on a series of systematic stress tests, optimized fixing products for polyethylene DL wall attachment were identified. In parallel, a detailed and adaptable method of DL household installation was developed for routine deployment in malaria endemic areas. These standardized stress tests will form the basis for comparative evaluations of new types of DL textile, which incorporate non-pyrethroid insecticides to control malaria transmitted by resistant mosquito populations
Constraining star cluster disruption mechanisms
Star clusters are found in all sorts of environments and their formation and
evolution is inextricably linked to the star formation process. Their eventual
destruction can result from a number of factors at different times, but the
process can be investigated as a whole through the study of the cluster age
distribution. Observations of populous cluster samples reveal a distribution
following a power law of index approximately -1. In this work we use M33 as a
test case to examine the age distribution of an archetypal cluster population
and show that it is in fact the evolving shape of the mass detection limit that
defines this trend. That is to say, any magnitude-limited sample will appear to
follow a dN/dt=1/t, while cutting the sample according to mass gives rise to a
composite structure, perhaps implying a dependence of the cluster disruption
process on mass. In the context of this framework, we examine different models
of cluster disruption from both theoretical and observational standpoints.Comment: To appear in the proceedings of IAU Symposium 266: "Star Clusters:
Basic Galactic Building Blocks Throughout Time And Space", eds. R. de Grijs
and J. Lepin
UHF and VHF radar observations of thunderstorms
A study of thunderstorms was made in the Summer of 1985 with the 430-MHz and 50-MHz radars at the Arecibo Observatory in Puerto Rico. Both radars use the 300-meter dish, which gives a beam width of less than 2 degrees even at these long wavelengths. Though the radars are steerable, only vertical beams were used in this experiment. The height resolution was 300 and 150 meters for the UHF and VHF, respectively. Lightning echoes, as well as returns from precipitation and clear-air turbulence were detected with both wavelengths. Large increases in the returned power were found to be coincident with increasing downward vertical velocities at UHF, whereas at VHF the total power returned was relatively constant during the life of a storm. This was attributed to the fact that the VHF is more sensitive to scattering from the turbulence-induced inhomogeneities in the refractive index and less sensitive to scatter from precipitation particles. On occasion, the shape of the Doppler spectra was observed to change with the occurrence of a lightning discharge in the pulse volume. Though the total power and mean reflectivity weighted Doppler velocity changed little during these events, the power is Doppler frequency bins near that corresponding to the updraft did increase substantially within a fraction of a second after a discharge was detected in the beam. This suggests some interaction between precipitation and lightning
Circular String-Instabilities in Curved Spacetime
We investigate the connection between curved spacetime and the emergence of
string-instabilities, following the approach developed by Loust\'{o} and
S\'{a}nchez for de Sitter and black hole spacetimes. We analyse the linearised
equations determining the comoving physical (transverse) perturbations on
circular strings embedded in Schwarzschild, Reissner-Nordstr\"{o}m and de
Sitter backgrounds. In all 3 cases we find that the "radial" perturbations grow
infinitely for (ring-collapse), while the "angular"
perturbations are bounded in this limit. For we find that
the perturbations in both physical directions (perpendicular to the string
world-sheet in 4 dimensions) blow up in the case of de Sitter space. This
confirms results recently obtained by Loust\'{o} and S\'{a}nchez who considered
perturbations around the string center of mass.Comment: 24 pages Latex + 2 figures (not included). Observatoire de Paris,
Meudon No. 9305
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