468 research outputs found
Clustering and information in correlation based financial networks
Networks of companies can be constructed by using return correlations. A
crucial issue in this approach is to select the relevant correlations from the
correlation matrix. In order to study this problem, we start from an empty
graph with no edges where the vertices correspond to stocks. Then, one by one,
we insert edges between the vertices according to the rank of their correlation
strength, resulting in a network called asset graph. We study its properties,
such as topologically different growth types, number and size of clusters and
clustering coefficient. These properties, calculated from empirical data, are
compared against those of a random graph. The growth of the graph can be
classified according to the topological role of the newly inserted edge. We
find that the type of growth which is responsible for creating cycles in the
graph sets in much earlier for the empirical asset graph than for the random
graph, and thus reflects the high degree of networking present in the market.
We also find the number of clusters in the random graph to be one order of
magnitude higher than for the asset graph. At a critical threshold, the random
graph undergoes a radical change in topology related to percolation transition
and forms a single giant cluster, a phenomenon which is not observed for the
asset graph. Differences in mean clustering coefficient lead us to conclude
that most information is contained roughly within 10% of the edges.Comment: 11 pages including 14 figures. Uses REVTeX4. To be published in a
special volume of EPJ on network
Dynamics of polymer ejection from capsid
Polymer ejection from a capsid through a nanoscale pore is an important
biological process with relevance to modern biotechnology. Here, we study
generic capsid ejection using Langevin dynamics. We show that even when the
ejection takes place within the drift-dominated region there is a very high
probability for the ejection process not to be completed. Introducing a small
aligning force at the pore entrance enhances ejection dramatically. Such a pore
asymmetry is a candidate for a mechanism by which a viral ejection is
completed. By detailed high-resolution simulations we show that such capsid
ejection is an out-of-equilibrium process that shares many common features with
the much studied driven polymer translocation through a pore in a wall or a
membrane. We find that the escape times scale with polymer length, . We show that for the pore without the asymmetry the previous
predictions corroborated by Monte Carlo simulations do not hold. For the pore
with the asymmetry the scaling exponent varies with the initial monomer density
(monomers per capsid volume) inside the capsid. For very low densities
the polymer is only weakly confined by the capsid, and we
measure , which is close to obtained for polymer
translocation. At intermediate densities the scaling exponents
and for and , respectively. These scalings are in
accord with a crude derivation for the lower limit . For the
asymmetrical pore precise scaling breaks down, when the density exceeds the
value for complete confinement by the capsid, . The
high-resolution data show that the capsid ejection for both pores, analogously
to polymer translocation, can be characterized as a multiplicative stochastic
process that is dominated by small-scale transitions.Comment: 10 pages, 6 figure
Dynamic asset trees and Black Monday
The minimum spanning tree, based on the concept of ultrametricity, is
constructed from the correlation matrix of stock returns. The dynamics of this
asset tree can be characterised by its normalised length and the mean
occupation layer, as measured from an appropriately chosen centre called the
`central node'. We show how the tree length shrinks during a stock market
crisis, Black Monday in this case, and how a strong reconfiguration takes
place, resulting in topological shrinking of the tree.Comment: 6 pages, 3 eps figues. Elsevier style. Will appear in Physica A as
part of the Bali conference proceedings, in pres
Dynamic asset trees and portfolio analysis
The minimum spanning tree, based on the concept of ultrametricity, is
constructed from the correlation matrix of stock returns and provides a
meaningful economic taxonomy of the stock market. In order to study the
dynamics of this asset tree we characterize it by its normalized length and by
the mean occupation layer, as measured from an appropriately chosen center. We
show how the tree evolves over time, and how it shrinks particularly strongly
during a stock market crisis. We then demonstrate that the assets of the
optimal Markowitz portfolio lie practically at all times on the outskirts of
the tree. We also show that the normalized tree length and the investment
diversification potential are very strongly correlated.Comment: 9 pages, 3 figures (encapsulated postscript
The International Trade Network: weighted network analysis and modelling
Tools of the theory of critical phenomena, namely the scaling analysis and
universality, are argued to be applicable to large complex web-like network
structures. Using a detailed analysis of the real data of the International
Trade Network we argue that the scaled link weight distribution has an
approximate log-normal distribution which remains robust over a period of 53
years. Another universal feature is observed in the power-law growth of the
trade strength with gross domestic product, the exponent being similar for all
countries. Using the 'rich-club' coefficient measure of the weighted networks
it has been shown that the size of the rich-club controlling half of the
world's trade is actually shrinking. While the gravity law is known to describe
well the social interactions in the static networks of population migration,
international trade, etc, here for the first time we studied a non-conservative
dynamical model based on the gravity law which excellently reproduced many
empirical features of the ITN.Comment: 5 pages, 5 figure
ColorPhylo: A Color Code to Accurately Display Taxonomic Classifications
Color may be very useful to visualise complex data. As far as taxonomy is concerned, color may help observing various speciesâ characteristics in correlation with classification. However, choosing the number of subclasses to display is often a complex task: on the one hand, assigning a limited number of colors to taxa of interest hides the structure imbedded in the subtrees of the taxonomy; on the other hand, differentiating a high number of taxa by giving them specific colors, without considering the underlying taxonomy, may lead to unreadable results since relationships between displayed taxa would not be supported by the color code. In the present paper, an automatic color coding scheme is proposed to visualise the levels of taxonomic relationships displayed as overlay on any kind of data plot. To achieve this goal, a dimensionality reduction method allows displaying taxonomic âdistancesâ onto a Euclidean two-dimensional space. The resulting map is projected onto a 2D color space (the Hue, Saturation, Brightness colorimetric space with brightness set to 1). Proximity in the taxonomic classification corresponds to proximity on the map and is therefore materialised by color proximity. As a result, each species is related to a color code showing its position in the taxonomic tree. The so called ColorPhylo displays taxonomic relationships intuitively and can be combined with any biological result. A Matlab version of ColorPhylo is available at http://sy.lespi.free.fr/ColorPhylo-homepage.html. Meanwhile, an ad-hoc distance in case of taxonomy with unknown edge lengths is proposed
Case Report: Acute Onset Fear of Falling and Treatment With "Cognitive Physical Therapy"
Fear of falling (FoF) is prevalent in older adults, especially those with previous falls, and typically starts insidiously. We present a 78-year-old woman with an abrupt onset FoF and no history of falls, balance problems, vertigo, oscillopsia, psychiatric or psychological issues to account for this. These cognitive changes led to a behavioural alteration of her gait that became slow and wide-based, with her gaze fixed on the floor. She began a tailored program of âCognitive Physical Therapy (CPT)â combining cognitive behavioural therapy (CBT) and physical rehabilitation. 1 month later her 6 m walk time and steps were reduced by a 25 and 35%, respectively, and the stride length increased by 34%, with further improvement 2 months later. We postulate that the abrupt onset of symptoms triggered a central shift toward postural hypervigilance and anxiety, suppression of anticipatory (feed forward) postural adjustments (APA) leading to FoF. CPT improved objective gait parameters related to FoF and reduced postural anxiety suggesting that early diagnosis and prompt treatment may avoid chronic symptoms and social isolation
Effective Field Theory of the Zero-Temperature Triangular-Lattice Antiferromagnet: A Monte Carlo Study
Using a Monte Carlo coarse-graining technique introduced by Binder et al., we
have explicitly constructed the continuum field theory for the zero-temperature
triangular Ising antiferromagnet. We verify the conjecture that this is a
gaussian theory of the height variable in the interface representation of the
spin model. We also measure the height-height correlation function and deduce
the stiffness constant. In addition, we investigate the nature of defect-defect
interactions at finite temperatures, and find that the two-dimensional Coulomb
gas scenario applies at low temperatures.Comment: 26 pages, 9 figure
Close relationships: A study of mobile communication records
Mobile phone communication as digital service generates ever-increasing
datasets of human communication actions, which in turn allow us to investigate
the structure and evolution of social interactions and their networks. These
datasets can be used to study the structuring of such egocentric networks with
respect to the strength of the relationships by assuming direct dependence of
the communication intensity on the strength of the social tie. Recently we have
discovered that there are significant differences between the first and further
"best friends" from the point of view of age and gender preferences. Here we
introduce a control parameter based on the statistics of
communication with the first and second "best friend" and use it to filter the
data. We find that when is decreased the identification of the
"best friend" becomes less ambiguous and the earlier observed effects get
stronger, thus corroborating them.Comment: 11 pages, 7 figure
Multiscale Analysis of Spreading in a Large Communication Network
In temporal networks, both the topology of the underlying network and the
timings of interaction events can be crucial in determining how some dynamic
process mediated by the network unfolds. We have explored the limiting case of
the speed of spreading in the SI model, set up such that an event between an
infectious and susceptible individual always transmits the infection. The speed
of this process sets an upper bound for the speed of any dynamic process that
is mediated through the interaction events of the network. With the help of
temporal networks derived from large scale time-stamped data on mobile phone
calls, we extend earlier results that point out the slowing-down effects of
burstiness and temporal inhomogeneities. In such networks, links are not
permanently active, but dynamic processes are mediated by recurrent events
taking place on the links at specific points in time. We perform a multi-scale
analysis and pinpoint the importance of the timings of event sequences on
individual links, their correlations with neighboring sequences, and the
temporal pathways taken by the network-scale spreading process. This is
achieved by studying empirically and analytically different characteristic
relay times of links, relevant to the respective scales, and a set of temporal
reference models that allow for removing selected time-domain correlations one
by one
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