743 research outputs found
Scale-free networks in complex systems
In the past few years, several studies have explored the topology of
interactions in different complex systems. Areas of investigation span from
biology to engineering, physics and the social sciences. Although having
different microscopic dynamics, the results demonstrate that most systems under
consideration tend to self-organize into structures that share common features.
In particular, the networks of interaction are characterized by a power law
distribution, , in the number of connections per node,
, over several orders of magnitude. Networks that fulfill this propriety of
scale-invariance are referred to as ``scale-free''. In the present work we
explore the implication of scale-free topologies in the antiferromagnetic (AF)
Ising model and in a stochastic model of opinion formation. In the first case
we show that the implicit disorder and frustration lead to a spin-glass phase
transition not observed for the AF Ising model on standard lattices. We further
illustrate that the opinion formation model produces a coherent, turbulent-like
dynamics for a certain range of parameters. The influence, of random or
targeted exclusion of nodes is studied.Comment: 9 pages, 4 figures. Proceeding to "SPIE International Symposium
Microelectronics, MEMS, and Nanotechnology", 11-15 December 2005, Brisbane,
Australi
Applications of physical methods in high-frequency futures markets
In the present work we demonstrate the application of different physical
methods to high-frequency or tick-by-tick financial time series data. In
particular, we calculate the Hurst exponent and inverse statistics for the
price time series taken from a range of futures indices. Additionally, we show
that in a limit order book the relaxation times of an imbalanced book state
with more demand or supply can be described by stretched exponential laws
analogous to those seen in many physical systems.Comment: 14 Pages and 10 figures. Proceeding to the SPIE conference, 4 - 7
December 2007 Australian National Univ. Canberra, ACT, Australi
Self-Similar Log-Periodic Structures in Western Stock Markets from 2000
The presence of log-periodic structures before and after stock market crashes
is considered to be an imprint of an intrinsic discrete scale invariance (DSI)
in this complex system. The fractal framework of the theory leaves open the
possibility of observing self-similar log-periodic structures at different time
scales. In the present work we analyze the daily closures of three of the most
important indices worldwide since 2000: the DAX for Germany and the Nasdaq100
and the S&P500 for the United States. The qualitative behaviour of these
different markets is similar during the temporal frame studied. Evidence is
found for decelerating log-periodic oscillations of duration about two years
and starting in September 2000. Moreover, a nested sub-structure starting in
May 2002 is revealed, bringing more evidence to support the hypothesis of
self-similar, log-periodic behavior. Ongoing log-periodic oscillations are also
revealed. A Lomb analysis over the aforementioned periods indicates a
preferential scaling factor . Higher order harmonics are also
present. The spectral pattern of the data has been found to be similar to that
of a Weierstrass-type function, used as a prototype of a log-periodic fractal
function.Comment: 17 pages, 14 figures. International Journal of Modern Physics C, in
pres
A Multi Agent Model for the Limit Order Book Dynamics
In the present work we introduce a novel multi-agent model with the aim to
reproduce the dynamics of a double auction market at microscopic time scale
through a faithful simulation of the matching mechanics in the limit order
book. The agents follow a noise decision making process where their actions are
related to a stochastic variable, "the market sentiment", which we define as a
mixture of public and private information. The model, despite making just few
basic assumptions over the trading strategies of the agents, is able to
reproduce several empirical features of the high-frequency dynamics of the
market microstructure not only related to the price movements but also to the
deposition of the orders in the book.Comment: 20 pages, 11 figures, in press European Physical Journal B (EPJB
Neuromorphic decoding of spinal motor neuron behaviour during natural hand movements for a new generation of wearable neural interfaces
We propose a neuromorphic framework to process the activity of human spinal motor neurons for movement intention recognition. This framework is integrated into a non-invasive interface that decodes the activity of motor neurons innervating intrinsic and extrinsic hand muscles. One of the main limitations of current neural interfaces is that machine learning models cannot exploit the efficiency of the spike encoding operated by the nervous system. Spiking-based pattern recognition would detect the spatio-temporal sparse activity of a neuronal pool and lead to adaptive and compact implementations, eventually running locally in embedded systems. Emergent Spiking Neural Networks (SNN) have not yet been used for processing the activity of in-vivo human neurons. Here we developed a convolutional SNN to process a total of 467 spinal motor neurons whose activity was identified in 5 participants while executing 10 hand movements. The classification accuracy approached 0.95 ±0.14 for both isometric and non-isometric contractions. These results show for the first time the potential of highly accurate motion intent detection by combining non-invasive neural interfaces and SNN
Self-reported adherence supports patient preference for the single tablet regimen (STR) in the current cART era
Objective: To analyze self-reported adherence to antiretroviral regimens containing ritonavir-boosted protease inhibitors, non-nucleoside reverse transcriptase inhibitors (NNRTI), raltegravir, and maraviroc. Methods: Overall, 372 consecutive subjects attending a reference center for HIV treatment in Florence, Italy, were enrolled in the study, from December 2010 to January 2012 (mean age 48 years). A self-report questionnaire was filled in. Patients were defined as “non-adherent” if reporting one of the following criteria:<90% of pills taken in the last month, ≥1 missed dose in the last week, spontaneous treatment interruptions reported, or refill problems in the last 3 months. Gender, age, CD4, HIV-RNA, years of therapy, and type of antiretroviral regimen were analyzed with respect to adherence. Results: At the time of the questionnaire, 89.8% of patients had <50 copies/mL HIV-RNA and 14.2% were on their first combined antiretroviral therapy. 57% of patients were prescribed a regimen containing ritonavir boosted protease inhibitors (boosted PI), 41.7% NNRTI, 17.2% raltegravir, and 4.8% maraviroc; 49.5% of the subjects were on bis-in-die regimens, while 50.5% were on once-daily regimens, with 23.1% of these on the single tablet regimen (STR): tenofovir/emtricitabine/efavirenz. The non-adherence proportion was lower in NNRTI than in boosted-PI treatments (19.4% vs 30.2%), and even lower in STR patients (17.4%). In multivariable logistic regression, patients with the NNRTI regimen (OR: 0.56, 95% CI: 0.34–0.94) and the STR (OR: 0.45, 95% CI: 0.22–0.92) reported lower non-adherence. Efavirenz regimens were also associated with lower non-adherence (OR: 0.42, 95% CI: 0.21–0.83), while atazanavir/ritonavir regimens were associated with higher non-adherence. No other relation to specific antiretroviral drugs was found. A higher CD4 count, lower HIV-RNA, and older age were also found to be associated with lower non-adherence, while a longer time on combined antiretroviral therapy was related to higher non-adherence. Conclusion: In conclusion, older age, higher CD4 cell counts, lower HIV-RNA viral loads, and the use of STR are all related to lower non-adherence. In particular, the use of STR maintains an advantage in improving adherence with respect to other cARTs, even with the availability of new, well-tolerated antiretroviral drugs and drug classes in recent years
Stochastic Opinion Formation in Scale-Free Networks
The dynamics of opinion formation in large groups of people is a complex
non-linear phenomenon whose investigation is just at the beginning. Both
collective behaviour and personal view play an important role in this
mechanism. In the present work we mimic the dynamics of opinion formation of a
group of agents, represented by two state , as a stochastic response of
each of them to the opinion of his/her neighbours in the social network and to
feedback from the average opinion of the whole. In the light of recent studies,
a scale-free Barab\'asi-Albert network has been selected to simulate the
topology of the interactions. A turbulent-like dynamics, characterized by an
intermittent behaviour, is observed for a certain range of the model
parameters. The problem of uncertainty in decision taking is also addressed
both from a topological point of view, using random and targeted removal of
agents from the network, and by implementing a three state model, where the
third state, zero, is related to the information available to each agent.
Finally, the results of the model are tested against the best known network of
social interactions: the stock market. A time series of daily closures of the
Dow Jones index has been used as an indicator of the possible applicability of
our model in the financial context. Good qualitative agreement is found.Comment: 24 pages and 13 figures, Physical Review E, in pres
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