3,207 research outputs found
Optimal search strategies of space-time coupled random walkers with finite lifetimes
We present a simple paradigm for detection of an immobile target by a
space-time coupled random walker with a finite lifetime. The motion of the
walker is characterized by linear displacements at a fixed speed and
exponentially distributed duration, interrupted by random changes in the
direction of motion and resumption of motion in the new direction with the same
speed. We call these walkers "mortal creepers". A mortal creeper may die at any
time during its motion according to an exponential decay law characterized by a
finite mean death rate . While still alive, the creeper has a finite
mean frequency of change of the direction of motion. In particular, we
consider the efficiency of the target search process, characterized by the
probability that the creeper will eventually detect the target. Analytic
results confirmed by numerical results show that there is an
-dependent optimal frequency that maximizes the
probability of eventual target detection. We work primarily in one-dimensional
() domains and examine the role of initial conditions and of finite domain
sizes. Numerical results in domains confirm the existence of an optimal
frequency of change of direction, thereby suggesting that the observed effects
are robust to changes in dimensionality. In the case, explicit
expressions for the probability of target detection in the long time limit are
given. In the case of an infinite domain, we compute the detection probability
for arbitrary times and study its early- and late-time behavior. We further
consider the survival probability of the target in the presence of many
independent creepers beginning their motion at the same location and at the
same time. We also consider a version of the standard "target problem" in which
many creepers start at random locations at the same time.Comment: 18 pages, 7 figures. The title has been changed with respect to the
one in the previous versio
Preventing Incomplete/Hidden Requirements: Reflections on Survey Data from Austria and Brazil
Many software projects fail due to problems in requirements engineering (RE).
The goal of this paper is analyzing a specific and relevant RE problem in
detail: incomplete/hidden requirements. We replicated a global family of RE
surveys with representatives of software organizations in Austria and Brazil.
We used the data to (a) characterize the criticality of the selected RE
problem, and to (b) analyze the reported main causes and mitigation actions.
Based on the analysis, we discuss how to prevent the problem. The survey
includes 14 different organizations in Austria and 74 in Brazil, including
small, medium and large sized companies, conducting both, plan-driven and agile
development processes. Respondents from both countries cited the
incomplete/hidden requirements problem as one of the most critical RE problems.
We identified and graphically represented the main causes and documented
solution options to address these causes. Further, we compiled a list of
reported mitigation actions. From a practical point of view, this paper
provides further insights into common causes of incomplete/hidden requirements
and on how to prevent this problem.Comment: in Proceedings of the Software Quality Days, 201
Mesoscopic non-equilibrium thermodynamics approach to non-Debye dielectric relaxation
Mesoscopic non-equilibrium thermodynamics is used to formulate a model
describing non-homogeneous and non-Debye dielectric relaxation. The model is
presented in terms of a Fokker-Planck equation for the probability distribution
of non-interacting polar molecules in contact with a heat bath and in the
presence of an external time-dependent electric field. Memory effects are
introduced in the Fokker-Planck description through integral relations
containing memory kernels, which in turn are used to establish a connection
with fractional Fokker-Planck descriptions. The model is developed in terms of
the evolution equations for the first two moments of the distribution function.
These equations are solved by following a perturbative method from which the
expressions for the complex susceptibilities are obtained as a functions of the
frequency and the wave number. Different memory kernels are considered and used
to compare with experiments of dielectric relaxation in glassy systems. For the
case of Cole-Cole relaxation, we infer the distribution of relaxation times and
its relation with an effective distribution of dipolar moments that can be
attributed to different segmental motions of the polymer chains in a melt.Comment: 33 pages, 6 figure
XMM-Newton observations of the neutron star X-ray transient KS 1731-260 in quiescence
We report on XMM-Newton observations performed on 2001 September 13-14 of the
neutron star X-ray transient KS 1731-260 in quiescence. The source was detected
at an unabsorbed 0.5-10 keV flux of only 4 - 8 x 10^{-14} erg/s, depending on
the model used to fit the data, which for a distance of 7 kpc implies a 0.5-10
keV X-ray luminosity of approximately 2 - 5 x 10^{32} erg/s. The September 2001
quiescent flux of KS 1731-260 is lower than that observed during the Chandra
observation in March 2001. In the cooling neutron star model for the quiescent
X-ray emission of neutron star X-ray transients, this decrease in the quiescent
flux implies that the crust of the neutron star in KS 1731-260 cooled down
rapidly between the two epochs, indicating that the crust has a high
conductivity. Furthermore, enhanced cooling in the neutron star core is also
favored by our results.Comment: Accepter for publication in ApJ Letters, 22 May 200
A Retrograde Spin of the Black Hole in MAXI J1659–152
We present the results of spectral analysis of the galactic black-hole binary
MAXI J1659--152 in the rising phase of the outburst that lasted for about 65
days starting on 2010 September 25. The presence of a broad Fe line, verified
by Monte-Carlo simulations, and coverage of a wide energy band by utilizing the
combined spectral capabilities of XMM-Newton/EPIC-pn and RXTE/PCA allowed us to
use a combination of reflection spectroscopy and continuum fitting methods to
estimate the spin of the black hole. We explored the entire parameter range
allowed by the present uncertainties on black-hole mass, inclination, and
distance as well as the accretion rate. We show that for about 95 percentage of
parameter space and very reasonable upper limits on mass accretion rate, the
spin of the black hole has to be negative. This is the first clear detection of
negative spin in a galactic black-hole binary.Comment: Published in ApJL. 9 pages, 4 figure
Combinatorial approach to generalized Bell and Stirling numbers and boson normal ordering problem
We consider the numbers arising in the problem of normal ordering of
expressions in canonical boson creation and annihilation operators. We treat a
general form of a boson string which is shown to be associated with
generalizations of Stirling and Bell numbers. The recurrence relations and
closed-form expressions (Dobiski-type formulas) are obtained for these
quantities by both algebraic and combinatorial methods. By extensive use of
methods of combinatorial analysis we prove the equivalence of the
aforementioned problem to the enumeration of special families of graphs. This
link provides a combinatorial interpretation of the numbers arising in this
normal ordering problem.Comment: 10 pages, 5 figure
Combinatorial Assortment Optimization
Assortment optimization refers to the problem of designing a slate of
products to offer potential customers, such as stocking the shelves in a
convenience store. The price of each product is fixed in advance, and a
probabilistic choice function describes which product a customer will choose
from any given subset. We introduce the combinatorial assortment problem, where
each customer may select a bundle of products. We consider a model of consumer
choice where the relative value of different bundles is described by a
valuation function, while individual customers may differ in their absolute
willingness to pay, and study the complexity of the resulting optimization
problem. We show that any sub-polynomial approximation to the problem requires
exponentially many demand queries when the valuation function is XOS, and that
no FPTAS exists even for succinctly-representable submodular valuations. On the
positive side, we show how to obtain constant approximations under a
"well-priced" condition, where each product's price is sufficiently high. We
also provide an exact algorithm for -additive valuations, and show how to
extend our results to a learning setting where the seller must infer the
customers' preferences from their purchasing behavior
A large local rotational speed for the Galaxy found from proper-motions: Implications for the mass of the Milky-Way
Predictions from a Galactic Structure and Kinematic model are compared to the
absolute proper-motions of about 30,000 randomly selected stars with derived from the Southern Proper-Motion Program (SPM) toward
the South Galactic Pole. The absolute nature of the SPM proper-motions allow us
to measure not only the relative motion of the Sun with respect to the local
disk, but also, and most importantly, the overall state of rotation of the
local disk with respect to galaxies. The SPM data are best fit by models having
a solar peculiar motion of +5 km~s in the V-component (pointing in the
direction of Galactic rotation), a large LSR speed of 270 km~s, and a
disk velocity ellipsoid that points towards the Galactic center. We stress,
however, that these results rest crucially on the assumptions of both
axisymmetry and equilibrium dynamics.
The absolute proper-motions in the U-component indicate a solar peculiar
motion of km~s, with no need for a local expansion or
contraction term.
The implications of the large LSR speed are discussed in terms of
gravitational mass of the Galaxy inferred from the most recent and accurate
determination for the proper-motion of the LMC. We find that our derived value
for the LSR is consistent both with the mass of the Galaxy inferred from the
motion of the Clouds ( to kpc), as well
as the timing argument, based on the binary motion of M31 and the Milky Way,
and Leo I and the Milky Way ( to
kpc).Comment: 7 pages (AAS Latex macro v4.0), 2 B&W postscript figures, accepted
for publication on ApJ, Letters sectio
The missing link in early emotional processing
Initial evaluation structures (IESs) currently proposed as the earliest detectors of affective stimuli (e.g., amygdala, orbitofrontal
cortex, or insula) are high-order structures (a) whose response latency cannot account for the first visual cortex emotion-related
response (~80 ms), and (b) lack the necessary infrastructure to locally analyze the visual features that define emotional stimuli.
Several thalamic structures accomplish both criteria. The lateral geniculate nucleus (LGN), a first-order thalamic nucleus that
actively processes visual information, with the complement of the thalamic reticular nucleus (TRN) are proposed as core IESs. This
LGN–TRN tandem could be supported by the pulvinar, a second-order thalamic structure, and by other extrathalamic nuclei. The
visual thalamus, scarcely explored in affective neurosciences, seems crucial in early emotional evaluation.This research was supported by the
Ministerio de Ciencia e Innovación (MICINN) (Grant no. PGC2018-093570-
B-I00) and the Comunidad de Madrid (Grant no. HUM19-HUM5705)
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