304 research outputs found
One-dimensional asymmetrically coupled maps with defects
In this letter we study chaotic dynamical properties of an asymmetrically
coupled one-dimensional chain of maps. We discuss the existence of coherent
regions in terms of the presence of defects along the chain. We find out that
temporal chaos is instantaneously localized around one single defect and that
the tangent vector jumps from one defect to another in an apparently random
way. We quantitatively measure the localization properties by defining an
entropy-like function in the space of tangent vectors.Comment: 9 pages + 4 figures TeX dialect: Plain TeX + IOP macros (included
Boltzmann entropy and chaos in a large assembly of weakly interacting systems
We introduce a high dimensional symplectic map, modeling a large system
consisting of weakly interacting chaotic subsystems, as a toy model to analyze
the interplay between single-particle chaotic dynamics and particles
interactions in thermodynamic systems. We study the growth with time of the
Boltzmann entropy, S_B, in this system as a function of the coarse graining
resolution. We show that a characteristic scale emerges, and that the behavior
of S_B vs t, at variance with the Gibbs entropy, does not depend on the coarse
graining resolution, as far as it is finer than this scale. The interaction
among particles is crucial to achieve this result, while the rate of entropy
growth depends essentially on the single-particle chaotic dynamics (for t not
too small). It is possible to interpret the basic features of the dynamics in
terms of a suitable Markov approximation.Comment: 21 pages, 11 figures, submitted to Journal of Statistical Physic
Numerical Observation of a Tubular Phase in Anisotropic Membranes
We provide the first numerical evidence for the existence of a tubular phase,
predicted by Radzihovsky and Toner (RT), for anisotropic tethered membranes
without self-avoidance. Incorporating anisotropy into the bending rigidity of a
simple model of a tethered membrane with free boundary conditions, we show that
the model indeed has two phase transitions corresponding to the flat-to-tubular
and tubular-to-crumpled transitions. For the tubular phase we measure the Flory
exponent and the roughness exponent . We find
and , which are in reasonable agreement with the theoretical
predictions of RT --- and .Comment: 8 pages, LaTeX, REVTEX, final published versio
Recurrence quantification analysis of heart rate variability to detect both ventilatory thresholds
Aims of this study were: To verify if Recurrence Quantification Analysis (RQA) of Heart Rate Variability (HRV) time series could determine both ventilatory thresholds in individuals with different fitness levels, and to assess the validity of RQA method compared to gasexchange method (GE). The two thresholds were estimated in thirty young individuals during incremental exercise on cycle-ergometer: Heart rate (HR), Oxygen consumption (VO2) and Workload were measured by the two methods (RQA and GE). Repeated measures ANOVA was used to assess main effects of methods and methods-by-groups interaction effects for HR, VO2 and Workload at aerobic (AerT) and anaerobic (AnT) thresholds. Validity of RQA at both thresholds was assessed for HR, VO2 and Workload by Ordinary Least Products (OLP) regression, Typical Percentage Error (TE), Intraclass Correlation Coefficients (ICC) and the Bland Altman plots. No methods-by-groups interaction effects were detected for HR, VO2 and Workload at AerT and AnT. The OLP analysis showed that at both thresholds RQA and GE methods had very strong correlations (r >0.8) in all variables (HR, VO2 and Workload). Slope and intercept values always included the 1 and the 0, respectively. At AerT the TE ranged from 4.02% (5.48 bpm) to 10.47% (8.53 Watts) (HR and Workload, respectively) and in all variables ICC values were excellent (≥0.85). At AnT the TE ranged from 2.53% (3.98 bpm) to 6.64% (7.81 Watts) (HR and Workload, respectively) and in all variables ICC values were excellent (≥0.90). Therefore, RQA of HRV time series is a new valid approach to determine both ventilatory thresholds in individuals with different physical fitness levels, it can be used when gas analysis is not possible or not convenient
Effect of permethrin plus antioxidants on locomotor activity and striatum in adolescent rats
Pyrethroids are important insecticides used largely because of their high activity as an insecticide and
their low mammalian toxicity. Some studies have demonstrated that these products show neurotoxic
effects on the mammalian central nervous system.
The aim of the present study was to investigate the propensity of permethrin to induce oxidative stress
in adolescent rats and its possible attenuation by Vitamin E alone or + Coenzyme Q10. Data indicated
that adolescent rats exposed to permethrin exhibited alteration in the locomotor activity and plasma
membrane fluidity of striatum. Vitamin E +Q10 and Vitamin E alone supplementation reversed the negative
effect on central nervous system. Permethrin alteration of striatum plasma membrane fluidity was
restored by Vitamin E +Q10. Data obtained from red blood cells showed that permethrin did not induce
any modification of plasma membrane fluidity in adolescent rats, whereas antioxidants supplementation
induced pro-oxidant effect.
In summary some differences between antioxidant treatments were observed at striatum level: Coenzyme
Q10 + Vitamin E maintains plasma membrane fluidity, while Vitamin E is more effective to preserve
GSH level
Fluctuation-response relation in turbulent systems
We address the problem of measuring time-properties of Response Functions
(Green functions) in Gaussian models (Orszag-McLaughin) and strongly
non-Gaussian models (shell models for turbulence). We introduce the concept of
{\it halving time statistics} to have a statistically stable tool to quantify
the time decay of Response Functions and Generalized Response Functions of high
order. We show numerically that in shell models for three dimensional
turbulence Response Functions are inertial range quantities. This is a strong
indication that the invariant measure describing the shell-velocity
fluctuations is characterized by short range interactions between neighboring
shells
Linear instability and statistical laws of physics
We show that a meaningful statistical description is possible in conservative
and mixing systems with zero Lyapunov exponent in which the dynamical
instability is only linear in time. More specifically, (i) the sensitivity to
initial conditions is given by with
; (ii) the statistical entropy in the infinitely fine graining limit (i.e., {\it
number of cells into which the phase space has been partitioned} ),
increases linearly with time only for ; (iii) a nontrivial,
-generalized, Pesin-like identity is satisfied, namely the . These facts (which are
in analogy to the usual behaviour of strongly chaotic systems with ), seem
to open the door for a statistical description of conservative many-body
nonlinear systems whose Lyapunov spectrum vanishes.Comment: 7 pages including 2 figures. The present version is accepted for
publication in Europhysics Letter
The prediction of future from the past: an old problem from a modern perspective
The idea of predicting the future from the knowledge of the past is quite
natural when dealing with systems whose equations of motion are not known. Such
a long-standing issue is revisited in the light of modern ergodic theory of
dynamical systems and becomes particularly interesting from a pedagogical
perspective due to its close link with Poincar\'e's recurrence. Using such a
connection, a very general result of ergodic theory - Kac's lemma - can be used
to establish the intrinsic limitations to the possibility of predicting the
future from the past. In spite of a naive expectation, predictability results
to be hindered rather by the effective number of degrees of freedom of a system
than by the presence of chaos. If the effective number of degrees of freedom
becomes large enough, regardless the regular or chaotic nature of the system,
predictions turn out to be practically impossible. The discussion of these
issues is illustrated with the help of the numerical study of simple models.Comment: 9 pages, 4 figure
Critical properties of the four-state Commutative Random Permutation Glassy Potts model in three and four dimensions
We investigate the critical properties of the four-state commutative random
permutation glassy Potts model in three and four dimensions by means of Monte
Carlo simulation and of a finite size scaling analysis. Thanks to the use of a
field programmable gate array we have been able to thermalize a large number of
samples of systems with large volume. This has allowed us to observe a
spin-glass ordered phase in d=4 and to study the critical properties of the
transition. In d=3, our results are consistent with the presence of a
Kosterlitz-Thouless transition, but we cannot exclude transient effects due to
a value of the lower critical dimension slightly below 3.Comment: 9 pages, 8 Postscript figure
Library Design in Combinatorial Chemistry by Monte Carlo Methods
Strategies for searching the space of variables in combinatorial chemistry
experiments are presented, and a random energy model of combinatorial chemistry
experiments is introduced. The search strategies, derived by analogy with the
computer modeling technique of Monte Carlo, effectively search the variable
space even in combinatorial chemistry experiments of modest size. Efficient
implementations of the library design and redesign strategies are feasible with
current experimental capabilities.Comment: 5 pages, 3 figure
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