122 research outputs found
Proving Safety with Trace Automata and Bounded Model Checking
Loop under-approximation is a technique that enriches C programs with
additional branches that represent the effect of a (limited) range of loop
iterations. While this technique can speed up the detection of bugs
significantly, it introduces redundant execution traces which may complicate
the verification of the program. This holds particularly true for verification
tools based on Bounded Model Checking, which incorporate simplistic heuristics
to determine whether all feasible iterations of a loop have been considered.
We present a technique that uses \emph{trace automata} to eliminate redundant
executions after performing loop acceleration. The method reduces the diameter
of the program under analysis, which is in certain cases sufficient to allow a
safety proof using Bounded Model Checking. Our transformation is precise---it
does not introduce false positives, nor does it mask any errors. We have
implemented the analysis as a source-to-source transformation, and present
experimental results showing the applicability of the technique
Checking bounded reachability in asynchronous systems by symbolic event tracing
This report presents a new symbolic technique for checking reachability properties of asynchronous systems by reducing the problem to satisfiability in restrained difference logic. The analysis is bounded by fixing a finite set of potential events, each of which may occur at most once in any order. The events are specified using high-level Petri nets. The logic encoding describes the space of possible causal links between events rather than possible sequences of states as in Bounded Model Checking. Independence between events is exploited intrinsically without partial order reductions, and the handling of data is symbolic. On a family of benchmarks, the proposed approach is consistently faster than Bounded Model Checking. In addition, this report presents a compact encoding of the restrained subset of difference logic in propositional logic
A Comparison of Solar Cycle Variations in the Equatorial Rotation Rates of the Sun's Subsurface, Surface, Corona, and Sunspot Groups
Using the Solar Optical Observing Network (SOON) sunspot-group data for the
period 1985-2010, the variations in the annual mean equatorial-rotation rates
of the sunspot groups are determined and compared with the known variations in
the solar equatorial-rotation rates determined from the following data: i) the
plasma rotation rates at 0.94Rsun, 0.95Rsun,...,1.0Rsun measured by Global
Oscillation Network Group (GONG) during the period 1995-2010, ii) the data on
the soft X-ray corona determined from Yohkoh/SXT full disk images for the years
1992-2001, iii) the data on small bright coronal structures (SBCS) which were
traced in Solar and Heliospheric Observatory (SOHO)/EIT images during the
period 1998-2006, and iv) the Mount Wilson Doppler-velocity measurements during
the period 1986-2007. A large portion (up to approximate 30 deg latitude) of
the mean differential-rotation profile of the sunspot groups lies between those
of the internal differential-rotation rates at 0.94Rsun and 0.98Rsun.The
variation in the yearly mean equatorial-rotation rate of the sunspot groups
seems to be lagging that of the equatorial-rotation rate determined from the
GONG measurements by one to two years.The amplitude of the latter is very
small.The solar-cycle variation in the equatorial-rotation rate of the solar
corona closely matches that determined from the sunspot-group data.The
variation in the equatorial-rotation rate determined from the Mount Wilson
Doppler-velocity data closely resembles the corresponding variation in the
equatorial-rotation rate determined from the sunspot-group data that included
the values of the abnormal angular motions (> 3 deg per day) of the sunspot
groups. Implications of these results are pointed out.Comment: 22 pages, 10 figures, accepted by Solar Physic
Active region formation through the negative effective magnetic pressure instability
The negative effective magnetic pressure instability operates on scales
encompassing many turbulent eddies and is here discussed in connection with the
formation of active regions near the surface layers of the Sun. This
instability is related to the negative contribution of turbulence to the mean
magnetic pressure that causes the formation of large-scale magnetic structures.
For an isothermal layer, direct numerical simulations and mean-field
simulations of this phenomenon are shown to agree in many details in that their
onset occurs at the same depth. This depth increases with increasing field
strength, such that the maximum growth rate of this instability is independent
of the field strength, provided the magnetic structures are fully contained
within the domain. A linear stability analysis is shown to support this
finding. The instability also leads to a redistribution of turbulent intensity
and gas pressure that could provide direct observational signatures.Comment: 19 pages, 10 figures, submitted to Solar Physic
Quenching of Meridional Circulation in Flux Transport Dynamo Models
Guided by the recent observational result that the meridional circulation of
the Sun becomes weaker at the time of the sunspot maximum, we have included a
parametric quenching of the meridional circulation in solar dynamo models such
that the meridional circulation becomes weaker when the magnetic field at the
base of the convection zone is stronger. We find that a flux transport solar
dynamo tends to become unstable on including this quenching of meridional
circulation if the diffusivity in the convection zone is less than about 2 *
10^{11} cm^2/s. The quenching of alpha, however, has a stabilizing effect and
it is possible to stabilize a dynamo with low diffusivity with sufficiently
strong alpha-quenching. For dynamo models with high diffusivity, the quenching
of meridional circulation does not produce a large effect and the dynamo
remains stable. We present a solar-like solution from a dynamo model with
diffusivity 2.8 * 10^{12} cm^2/s in which the quenching of meridional
circulation makes the meridional circulation vary periodically with solar cycle
as observed and does not have any other significant effect on the dynamo.Comment: Consistent with the published version. Solar Physics, in pres
Modeling the Subsurface Structure of Sunspots
While sunspots are easily observed at the solar surface, determining their
subsurface structure is not trivial. There are two main hypotheses for the
subsurface structure of sunspots: the monolithic model and the cluster model.
Local helioseismology is the only means by which we can investigate
subphotospheric structure. However, as current linear inversion techniques do
not yet allow helioseismology to probe the internal structure with sufficient
confidence to distinguish between the monolith and cluster models, the
development of physically realistic sunspot models are a priority for
helioseismologists. This is because they are not only important indicators of
the variety of physical effects that may influence helioseismic inferences in
active regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In this paper,
we provide a critical review of the existing sunspot models and an overview of
numerical methods employed to model wave propagation through model sunspots. We
then carry out an helioseismic analysis of the sunspot in Active Region 9787
and address the serious inconsistencies uncovered by
\citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find
that this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model) and that
travel-time measurements are consistent with a horizontal outflow in the
surrounding moat.Comment: 73 pages, 19 figures, accepted by Solar Physic
Advances in Global and Local Helioseismology: an Introductory Review
Helioseismology studies the structure and dynamics of the Sun's interior by
observing oscillations on the surface. These studies provide information about
the physical processes that control the evolution and magnetic activity of the
Sun. In recent years, helioseismology has made substantial progress towards the
understanding of the physics of solar oscillations and the physical processes
inside the Sun, thanks to observational, theoretical and modeling efforts. In
addition to the global seismology of the Sun based on measurements of global
oscillation modes, a new field of local helioseismology, which studies
oscillation travel times and local frequency shifts, has been developed. It is
capable of providing 3D images of the subsurface structures and flows. The
basic principles, recent advances and perspectives of global and local
helioseismology are reviewed in this article.Comment: 86 pages, 46 figures; "Pulsation of the Sun and Stars", Lecture Notes
in Physics, Vol. 832, Rozelot, Jean-Pierre; Neiner, Coralie (Eds.), 201
Platelet reactivity influences clot structure as assessed by fractal analysis of viscoelastic properties
<p>Despite the interwoven nature of platelet activation and the coagulation system in thrombosis, few studies relate both analysis of protein and cellular parts of coagulation in the same population. In the present study, we use matched ex vivo samples to determine the influences of standard antiplatelet therapies on platelet function and use advanced rheological analyses to assess clot formation. Healthy volunteers were recruited following fully informed consent then treated for 7 days with single antiplatelet therapy of aspirin (75 mg) or prasugrel (10 mg) or with dual antiplatelet therapy (DAPT) using aspirin (75 mg) plus prasugrel (10 mg) or aspirin (75 mg) plus ticagrelor (90 mg). Blood samples were taken at day 0 before treatment and at day 7 following treatment. We found that aspirin plus prasugrel or aspirin plus ticagrelor inhibited platelet responses to multiple agonists and reduced P-selectin expression. Significant platelet inhibition was coupled with a reduction in fractal dimension corresponding to reductions in mean relative mass both for aspirin plus prasugrel (−35 ± 16% change, p = 0.04) and for aspirin plus ticagrelor (−45 ± 14% change, p = 0.04). Aspirin alone had no effect upon measures of clot structure, whereas prasugrel reduced fractal dimension and mean relative mass. These data demonstrate that platelets are important determinants of clot structure as assessed by fractal dimension (d<sub><i>f</i></sub>) and that effective platelet inhibition is associated with a weaker, more permeable fibrin network. This indicates a strong association between the therapeutic benefits of antiplatelet therapies and their abilities to reduce thrombus density that may be useful in individual patients to determine the functional relationship between platelet reactivity, eventual clot quality, and clinical outcome. d<sub><i>f</i></sub> could represent a novel risk stratification biomarker useful in individualizing antiplatelet therapies.</p
Meat and Nicotinamide:A Causal Role in Human Evolution, History, and Demographics
Hunting for meat was a critical step in all animal and human evolution. A key brain-trophic element in meat is vitamin B 3 /nicotinamide. The supply of meat and nicotinamide steadily increased from the Cambrian origin of animal predators ratcheting ever larger brains. This culminated in the 3-million-year evolution of Homo sapiens and our overall demographic success. We view human evolution, recent history, and agricultural and demographic transitions in the light of meat and nicotinamide intake. A biochemical and immunological switch is highlighted that affects fertility in the ‘de novo’ tryptophan-to-kynurenine-nicotinamide ‘immune tolerance’ pathway. Longevity relates to nicotinamide adenine dinucleotide consumer pathways. High meat intake correlates with moderate fertility, high intelligence, good health, and longevity with consequent population stability, whereas low meat/high cereal intake (short of starvation) correlates with high fertility, disease, and population booms and busts. Too high a meat intake and fertility falls below replacement levels. Reducing variances in meat consumption might help stabilise population growth and improve human capital
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