65,804 research outputs found
Independence in constraint logic programs
Studying independence of literals, variables, and substitutions has proven very useful in the context of logic programming (LP). Here we study independence in the broader context of constraint logic programming (CLP). We show that a naive extrapolation of the LP definitions of independence to CLP is unsatisfactory (in fact, wrong) for two reasons. First, because interaction between variables through constraints is more complex than in the case of logic programming. Second, in order to ensure the efUciency of several optimizations not only must independence of the search space be considered, but also an orthogonal issue - "independence of constraint solving." We clarify these issues by proposing various types of search independence
and constraint solver independence, and show how they can be combined to allow different independence-related optimizations, from parallelism to intelligent backtracking. Sufficient conditions for independence which can be evaluated "a-priori" at run-time are also proposed. Our results suggest that independence, provided a suitable definition is chosen, is even more useful in CLP than in LP
Analyzing logic programs with dynamic scheduling
Traditional logic programming languages, such as Prolog, use a fixed left-to-right atom scheduling rule. Recent logic programming languages, however, usually provide more flexible scheduling in which computation generally proceeds leftto- right but in which some calis are dynamically
"delayed" until their arguments are sufRciently instantiated
to allow the cali to run efficiently. Such dynamic scheduling has a significant cost. We give a framework for the global analysis of logic programming languages with dynamic scheduling and show that program analysis based on this framework supports optimizations which remove much
of the overhead of dynamic scheduling
Magnetic Color Flavor Locking Phase in High Density QCD
We investigate the effects of an external magnetic field in the gap structure
of a color superconductor with three massless quark flavors. Using an effective
theory with four-fermion interactions, inspired by one-gluon exchange, we show
that the long-range component of the external magnetic field
that penetrates the color-flavor locked (CFL) phase modifies its gap structure,
producing a new phase of lower symmetry. A main outcome of our study is that
the field tends to strengthen the gaps formed by
-charged and -neutral quarks that coupled among
themselves through tree-level vertices. These gaps are enhanced by the
field-dependent density of states of the -charged quarks on the
Fermi surface. Our considerations are relevant for the study of highly
magnetized compact stars.Comment: version to be published in PR
The loss of anisotropy in MgB2 with Sc substitution and its relationship with the critical temperature
The electrical conductivity anisotropy of the sigma-bands is calculated for
the (Mg,Sc)B2 system using a virtual crystal model. Our results reveal that
anisotropy drops with relatively little scandium content (< 30%); this
behaviour coincides with the lowering of Tc and the reduction of the Kohn
anomaly. This anisotropy loss is also found in the Al and C doped systems. In
this work it is argued that the anisotropy, or 2D character, of the sigma-bands
is an important parameter for the understanding of the high Tc found in MgB2
The Relation between the Radial Temperature Profile in the Chromosphere and the Solar Spectrum at Centimeter, Millimeter, Sub-millimeter, and Infrared Wavelengths
Solar observations from millimeter to ultraviolet wavelengths show that there
is a temperature minimum between photosphere and chromosphere. Analysis based
on semi-empirical models locate this point at about 500 km over the
photosphere. The consistency of these models has been tested by means of
millimeter to infrared observations.
In the present work, we show that variations of the theoretical radial
temperature profile near the temperature minimum impacts the brightness
temperature at centimeter, submillimeter, and infrared wavelengths, but the
millimeter wavelength emission remains unchanged. We found a region between 500
and 1000 km over the photosphere that remains hidden to observations at the
frequencies under study in this work.Comment: Accepted in Solar Physic
Nanoscale periodicity in stripe-forming systems at high temperature: Au/W(110)
We observe using low-energy electron microscopy the self-assembly of
monolayer-thick stripes of Au on W(110) near the transition temperature between
stripes and the non-patterned (homogeneous) phase. We demonstrate that the
amplitude of this Au stripe phase decreases with increasing temperature and
vanishes at the order-disorder transition (ODT). The wavelength varies much
more slowly with temperature and coverage than theories of stress-domain
patterns with sharp phase boundaries would predict, and maintains a finite
value of about 100 nm at the ODT. We argue that such nanometer-scale stripes
should often appear near the ODT.Comment: 5 page
Equation of state for the MCFL phase and its implications for compact star models
Using the solutions of the gap equations of the magnetic-color-flavor-locked
(MCFL) phase of paired quark matter in a magnetic field, and taking into
consideration the separation between the longitudinal and transverse pressures
due to the field-induced breaking of the spatial rotational symmetry, the
equation of state (EoS) of the MCFL phase is self-consistently determined. This
result is then used to investigate the possibility of absolute stability, which
turns out to require a field-dependent bag constant to hold. That is, only if
the bag constant varies with the magnetic field, there exists a window in the
magnetic field vs. bag constant plane for absolute stability of strange matter.
Implications for stellar models of magnetized (self-bound) strange stars and
hybrid (MCFL core) stars are calculated and discussed.Comment: 11 pp. 11 figure
NuSTAR reveals that the heavily obscured nucleus of NGC 2785 was the contaminant of IRAS 09104+4109 in the BeppoSAX/PDS hard X-rays
The search for heavily obscured active galactic nuclei (AGNs) has been
revitalized in the last five years by NuSTAR, which has provided a good census
and spectral characterization of a population of such objects, mostly at low
redshift, thanks to its enhanced sensitivity above 10 keV compared to previous
X-ray facilities, and its hard X-ray imaging capabilities. We aim at
demonstrating how NGC2785, a local (z=0.009) star-forming galaxy, is
responsible, in virtue of its heavily obscured active nucleus, for significant
contamination in the non-imaging BeppoSAX/PDS data of the relatively nearby
(~17 arcmin) quasar IRAS 09104+4109 (z=0.44), which was originally
mis-classified as Compton thick. We analyzed ~71 ks NuSTAR data of NGC2785
using the MYTorus model and provided a physical description of the X-ray
properties of the source for the first time. We found that NGC2785 hosts a
heavily obscured (NH~3*10^{24} cm^{-2}) nucleus. The intrinsic X-ray luminosity
of the source, once corrected for the measured obscuration (L(2-10 keV)~10^{42}
erg/s), is consistent within a factor of a few with predictions based on the
source mid-infrared flux using widely adopted correlations from the literature.
Based on NuSTAR data and previous indications from the Neil Gehrels Swift
Observatory (BAT instrument), we confirm that NGC2785, because of its hard
X-ray emission and spectral shape, was responsible for at least one third of
the 20-100 keV emission observed using the PDS instrument onboard BeppoSAX,
originally completely associated with IRAS 09104+4109. Such emission led to the
erroneous classification of this source as a Compton-thick quasar, while it is
now recognized as Compton thin.Comment: Six pages, 3 figures, A&A, in pres
Fronts and interfaces in bistable extended mappings
We study the interfaces' time evolution in one-dimensional bistable extended
dynamical systems with discrete time. The dynamics is governed by the
competition between a local piece-wise affine bistable mapping and any
couplings given by the convolution with a function of bounded variation. We
prove the existence of travelling wave interfaces, namely fronts, and the
uniqueness of the corresponding selected velocity and shape. This selected
velocity is shown to be the propagating velocity for any interface, to depend
continuously on the couplings and to increase with the symmetry parameter of
the local nonlinearity. We apply the results to several examples including
discrete and continuous couplings, and the planar fronts' dynamics in
multi-dimensional Coupled Map Lattices. We eventually emphasize on the
extension to other kinds of fronts and to a more general class of bistable
extended mappings for which the couplings are allowed to be nonlinear and the
local map to be smooth.Comment: 27 pages, 3 figures, submitted to Nonlinearit
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