6,937 research outputs found
Parallels between the dynamics at the noise-perturbed onset of chaos in logistic maps and the dynamics of glass formation
We develop the characterization of the dynamics at the noise-perturbed edge
of chaos in logistic maps in terms of the quantities normally used to describe
glassy properties in structural glass formers. Following the recognition [Phys.
Lett. \textbf{A 328}, 467 (2004)] that the dynamics at this critical attractor
exhibits analogies with that observed in thermal systems close to
vitrification, we determine the modifications that take place with decreasing
noise amplitude in ensemble and time averaged correlations and in diffusivity.
We corroborate explicitly the occurrence of two-step relaxation, aging with its
characteristic scaling property, and subdiffusion and arrest for this system.
We also discuss features that appear to be specific of the map.Comment: Revised version with substantial improvements. Revtex, 8 pages, 11
figure
Public Channel Cryptography: Chaos Synchronization and Hilbert's Tenth Problem
The synchronization process of two mutually delayed coupled deterministic
chaotic maps is demonstrated both analytically and numerically. The
synchronization is preserved when the mutually transmitted signal is concealed
by two commutative private filters that are placed on each end of the
communication channel. We demonstrate that when the transmitted signal is a
convolution of the truncated time delayed output signals or some powers of the
delayed output signals synchronization is still maintained. The task of a
passive attacker is mapped onto Hilbert's tenth problem, solving a set of
nonlinear Diophantine equations, which was proven to be in the class of
NP-Complete problems. This bridge between two different disciplines,
synchronization in nonlinear dynamical processes and the realm of the NPC
problems, opens a horizon for a new type of secure public-channel protocols
Towards Interpretable Deep Learning Models for Knowledge Tracing
As an important technique for modeling the knowledge states of learners, the
traditional knowledge tracing (KT) models have been widely used to support
intelligent tutoring systems and MOOC platforms. Driven by the fast
advancements of deep learning techniques, deep neural network has been recently
adopted to design new KT models for achieving better prediction performance.
However, the lack of interpretability of these models has painfully impeded
their practical applications, as their outputs and working mechanisms suffer
from the intransparent decision process and complex inner structures. We thus
propose to adopt the post-hoc method to tackle the interpretability issue for
deep learning based knowledge tracing (DLKT) models. Specifically, we focus on
applying the layer-wise relevance propagation (LRP) method to interpret
RNN-based DLKT model by backpropagating the relevance from the model's output
layer to its input layer. The experiment results show the feasibility using the
LRP method for interpreting the DLKT model's predictions, and partially
validate the computed relevance scores from both question level and concept
level. We believe it can be a solid step towards fully interpreting the DLKT
models and promote their practical applications in the education domain
Do columnar defects produce bulk pinning?
From magneto-optical imaging performed on heavy-ion irradiated YBaCuO single
crystals, it is found that at fields and temperatures where strong single
vortex pinning by individual irradiation-induced amorphous columnar defects is
to be expected, vortex motion is limited by the nucleation of vortex kinks at
the specimen surface rather than by half-loop nucleation in the bulk. In the
material bulk, vortex motion occurs through (easy) kink sliding. Depinning in
the bulk determines the screening current only at fields comparable to or
larger than the matching field, at which the majority of moving vortices is not
trapped by an ion track.Comment: 5 pages, 5 figures, submitted to Physical Review Letter
Nonextensive Pesin identity. Exact renormalization group analytical results for the dynamics at the edge of chaos of the logistic map
We show that the dynamical and entropic properties at the chaos threshold of
the logistic map are naturally linked through the nonextensive expressions for
the sensitivity to initial conditions and for the entropy. We corroborate
analytically, with the use of the Feigenbaum renormalization group(RG)
transformation, the equality between the generalized Lyapunov coefficient
and the rate of entropy production given by the
nonextensive statistical mechanics. Our results advocate the validity of the
-generalized Pesin identity at critical points of one-dimensional nonlinear
dissipative maps.Comment: Revtex, 5 pages, 3 figure
Sensitivity to initial conditions at bifurcations in one-dimensional nonlinear maps: rigorous nonextensive solutions
Using the Feigenbaum renormalization group (RG) transformation we work out
exactly the dynamics and the sensitivity to initial conditions for unimodal
maps of nonlinearity at both their pitchfork and tangent
bifurcations. These functions have the form of -exponentials as proposed in
Tsallis' generalization of statistical mechanics. We determine the -indices
that characterize these universality classes and perform for the first time the
calculation of the -generalized Lyapunov coefficient . The
pitchfork and the left-hand side of the tangent bifurcations display weak
insensitivity to initial conditions, while the right-hand side of the tangent
bifurcations presents a `super-strong' (faster than exponential) sensitivity to
initial conditions. We corroborate our analytical results with {\em a priori}
numerical calculations.Comment: latex, 4 figures. Updated references and some general presentation
improvements. To appear published in Europhysics Letter
The large scale dynamics of the outer heliosphere and the long-term modulation of galactic cosmic rays
The network of cosmic ray observatories reaching across the heliosphere has given new insight into the process of solar modulation, establishing that the decreases occur principally in the outer heliosphere and are produced by interplanetary flow systems; that the hysteresis effects appear to be produced by changes in the rigidity dependence of the diffusion coefficient and that the predicted effects on the cosmic ray gradients associated with the reversal of the solar magnetic field polarity are not observed
Rotational modulation of X-ray emission in Orion Nebula young stars
We investigate the spatial distribution of X-ray emitting plasma in a sample
of young Orion Nebula Cluster stars by modulation of their X-ray light-curves
due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project
(COUP), is made possible by the exceptional length of the observation: 10 days
of ACIS integration during a time span of 13 days, yielding a total of 1616
detected sources in the 17x17 arcmin field of view. We here focus on a
subsample of 233 X-ray-bright stars with known rotational periods. We search
for X-ray modulation using the Lomb Normalized Periodogram method.
X-ray modulation related to the rotation period is detected in at least 23
stars with periods between 2 and 12 days and relative amplitudes ranging from
20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar
rotation period while in seven cases it is about half that value, possibly due
to the presence of X-ray emitting structures at opposite stellar longitudes.
These results constitute the largest sample of low mass stars in which X-ray
rotational modulation has been observed. The detection of rotational modulation
indicates that the X-ray emitting regions are distributed inhomogeneneously in
longitude and do not extend to distances significantly larger than the stellar
radius. Modulation is observed in stars with saturated activity levels
(L_X/L_bol ~ 10^(-3)) showing that saturation is not due to the filling of the
stellar surface with X-ray emitting regions.Comment: 41 pages, 15 figures, ApJS in press. Figure 15 (34 panels) is an
on-line only figure and is not included. A pdf file which includes figure 15
as well as full resolution versions of figure 10 and 11 is available at:
http://www.astropa.unipa.it/~ettoref/COUP_RotMod.pd
Caby Photometry of the Hyades: Comparisons to the Field Stars
Intermediate-band photometry of the Hyades cluster on the Caby system is
presented for dwarf stars ranging from spectral type A through late K. A mean
hk, b-y relation is constructed using only single stars without anomalous
atmospheres and compared to the field stars of the solar neighborhood. For the
F dwarfs, the Hyades relation defines an approximate LOWER bound in the
two-color diagram, consistent with an [Fe/H] between +0.10 and +0.15. These
index-color diagrams follow the common convention of presenting stars with
highest abundance at the bottom of the plot although the index values for the
metal-rich stars are numerically larger. For field F dwarfs in the range [Fe/H]
between +0.4 and -1.0, [Fe/H] = -5.6 delta-hk + 0.125, with no evidence for a
color dependence in the slope. For the G and K dwarfs, the Hyades mean relation
crosses the field star distribution in the two-color diagram, defining an
approximate UPPER bound for the local disk stars. Stars found above the Hyades
stars fall in at least one of three categories: [Fe/H] below -0.7, [Fe/H] above
that of the Hyades, or chromospherically active. It is concluded that, contrary
to the predictions of model atmospheres, the hk index for cool dwarfs at a
given color hits a maximum value for stars below solar composition and, with
increasing [Fe/H] above some critical value, declines. This trend is
consistent, however, with the predictions from synthetic indices based upon
much narrower Ca filters where the crossover is caused by the metallicity
sensitivity of b-y.Comment: 13 pages, 9 eps figures, 1 tex table, 1 ascii tabl
Anomalous diffusion in the dynamics of complex processes
Anomalous diffusion, process in which the mean-squared displacement of system
states is a non-linear function of time, is usually identified in real
stochastic processes by comparing experimental and theoretical displacements at
relatively small time intervals. This paper proposes an interpolation
expression for the identification of anomalous diffusion in complex signals for
the cases when the dynamics of the system under study reaches a steady state
(large time intervals). This interpolation expression uses the chaotic
difference moment (transient structural function) of the second order as an
average characteristic of displacements. A general procedure for identifying
anomalous diffusion and calculating its parameters in real stochastic signals,
which includes the removal of the regular (low-frequency) components from the
source signal and the fitting of the chaotic part of the experimental
difference moment of the second order to the interpolation expression, is
presented. The procedure was applied to the analysis of the dynamics of
magnetoencephalograms, blinking fluorescence of quantum dots, and X-ray
emission from accreting objects. For all three applications, the interpolation
was able to adequately describe the chaotic part of the experimental difference
moment, which implies that anomalous diffusion manifests itself in these
natural signals. The results of this study make it possible to broaden the
range of complex natural processes in which anomalous diffusion can be
identified. The relation between the interpolation expression and a diffusion
model, which is derived in the paper, allows one to simulate the chaotic
processes in the open complex systems with anomalous diffusion.Comment: 47 pages, 15 figures; Submitted to Physical Review
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