56,656 research outputs found
A bibliography on formal methods for system specification, design and validation
Literature on the specification, design, verification, testing, and evaluation of avionics systems was surveyed, providing 655 citations. Journal papers, conference papers, and technical reports are included. Manual and computer-based methods were employed. Keywords used in the online search are listed
A formal method for identifying distinct states of variability in time-varying sources: SgrA* as an example
Continuously time variable sources are often characterized by their power
spectral density and flux distribution. These quantities can undergo dramatic
changes over time if the underlying physical processes change. However, some
changes can be subtle and not distinguishable using standard statistical
approaches. Here, we report a methodology that aims to identify distinct but
similar states of time variability. We apply this method to the Galactic
supermassive black hole, where 2.2 um flux is observed from a source associated
with SgrA*, and where two distinct states have recently been suggested. Our
approach is taken from mathematical finance and works with conditional flux
density distributions that depend on the previous flux value. The discrete,
unobserved (hidden) state variable is modeled as a stochastic process and the
transition probabilities are inferred from the flux density time series. Using
the most comprehensive data set to date, in which all Keck and a majority of
the publicly available VLT data have been merged, we show that SgrA* is
sufficiently described by a single intrinsic state. However the observed flux
densities exhibit two states: a noise-dominated and a source-dominated one. Our
methodology reported here will prove extremely useful to assess the effects of
the putative gas cloud G2 that is on its way toward the black hole and might
create a new state of variability.Comment: Submitted to ApJ; 33 pages, 4 figures; comments welcom
Continuum theory of tilted chiral smectic phases
We demonstrate that the sequence of distorted commensurate phases observed in
tilted chiral smectics is explained by the gain in electrostatic energy due to
the lock-in of the unit cell to a number of layers which is the integer closest
to the ratio pitch over thickness of the subjacent Sm-C phase. We
also explain the sign change of the helicity in the middle of the sequence by a
balance between two twist sources one intrinsic and another due to the
distortion of the Sm-C
Properties of Intercalated 2H-NbSe2, 4Hb-TaS2 and 1T-TaS2
The layered compounds 2H-NbSe, 24Hb-TaS, 2and 1T-TaS2 have been intercalated with organic molecules; and the resulting crystal structure, heat capacity, conductivity, and superconductivity have been studied. The coordination in the disulfide layers was found to be unchanged in the product phase. Resistance minima appear and the superconducting transition temperature is reduced in the NbSe2 complex. Conversely, superconductivity is induced in the 4Hb-TaS2 complex. Corresponding evidence of a large change of the density of states, negative for 2H-NbSe2 and positive for 4Hb-TaS2, was also observed upon intercalation. The transport properties of all the intercalation complexes show a pronounced dependence upon the coordination of the transition metal
Recurrence Tracking Microscope
In order to probe nanostructures on a surface we present a microscope based
on the quantum recurrence phenomena. A cloud of atoms bounces off an atomic
mirror connected to a cantilever and exhibits quantum recurrences. The times at
which the recurrences occur depend on the initial height of the bouncing atoms
above the atomic mirror, and vary following the structures on the surface under
investigation. The microscope has inherent advantages over existing techniques
of scanning tunneling microscope and atomic force microscope. Presently
available experimental technology makes it possible to develop the device in
the laboratory
Performability evaluation of the SIFT computer
Performability modeling and evaluation techniques are applied to the SIFT computer as it might operate in the computational evironment of an air transport mission. User-visible performance of the total system (SIFT plus its environment) is modeled as a random variable taking values in a set of levels of accomplishment. These levels are defined in terms of four attributes of total system behavior: safety, no change in mission profile, no operational penalties, and no economic process whose states describe the internal structure of SIFT as well as relavant conditions of the environment. Base model state trajectories are related to accomplishment levels via a capability function which is formulated in terms of a 3-level model hierarchy. Performability evaluation algorithms are then applied to determine the performability of the total system for various choices of computer and environment parameter values. Numerical results of those evaluations are presented and, in conclusion, some implications of this effort are discussed
The Nature of the Giant Outbursts in the Bursting Pulsar GRO J 1744-28
We investigate the possible role of an accretion disk instability in
producing the giant outbursts seen in GRO J1744-28. Specifically, we study the
global, time dependent evolution of the Lightman-Eardley instability which can
develop near the inner edge of an accretion disk when the radiation pressure
becomes comparable to the gas pressure. Broadly speaking, our results are
compatible with earlier works by Taam \& Lin and by Lasota \& Pelat. The
uniqueness of GRO J1744-28 appears to be associated with the constraint that,
in order for outbursts to occur, the rate of accretion at the inner edge must
be within a narrow range just above the critical accretion rate at which
radiation pressure is beginning to become significant.Comment: 11 pages in .tex file, 4 Postscript figures, .tex file uses
aasms.sty; Ap. J. L. 1996, in pres
On a general theory of characteristics and the method of invariant imbedding
Invariant imbedding equations for transport problems derived and applied to boundary value and initial value problem
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