pwc-archive/papers-with-abstracts huggingface dataset:
Mapping multiplex hubs in human functional brain network
Date: 2016-03-18 00:00:00

Typical brain networks consist of many peripheral regions and a few highly
central ones, i.e. hubs, playing key functional roles in cerebral
inter-regional interactions. Studies have shown that networks, obtained from
the analysis of specific frequency components of brain activity, present
peculiar architectures with unique profiles of region centrality. However, the
identification of hubs in networks built from different frequency bands
simultaneously is still a challenging problem, remaining largely unexplored.
Here we identify each frequency component with one layer of a multiplex network
and face this challenge by exploiting the recent advances in the analysis of
multiplex topologies. First, we show that each frequency band carries unique
topological information, fundamental to accurately model brain functional
networks. We then demonstrate that hubs in the multiplex network, in general
different from those ones obtained after discarding or aggregating the measured
signals as usual, provide a more accurate map of brain's most important
functional regions, allowing to distinguish between healthy and schizophrenic
populations better than conventional network approaches.
Statistically validated network of portfolio overlaps and systemic risk
Date: 2016-09-27 00:00:00

Common asset holding by financial institutions, namely portfolio overlap, is
nowadays regarded as an important channel for financial contagion with the
potential to trigger fire sales and thus severe losses at the systemic level.
In this paper we propose a method to assess the statistical significance of the
overlap between pairs of heterogeneously diversified portfolios, which then
allows us to build a validated network of financial institutions where links
indicate potential contagion channels due to realized portfolio overlaps. The
method is implemented on a historical database of institutional holdings
ranging from 1999 to the end of 2013, but can be in general applied to any
bipartite network where the presence of similar sets of neighbors is of
interest. We find that the proportion of validated network links (i.e., of
statistically significant overlaps) increased steadily before the 2007-2008
global financial crisis and reached a maximum when the crisis occurred. We
argue that the nature of this measure implies that systemic risk from fire
sales liquidation was maximal at that time. After a sharp drop in 2008,
systemic risk resumed its growth in 2009, with a notable acceleration in 2013,
reaching levels not seen since 2007. We finally show that market trends tend to
be amplified in the portfolios identified by the algorithm, such that it is
possible to have an informative signal about financial institutions that are
about to suffer (enjoy) the most significant losses (gains).
Mathematical modeling of T-cells activation dynamics for CD3 molecules recycling process
Date: 2016-03-15 00:00:00

The down regulation of CD3 protein present on the surface of a T-cell
triggers the process of activation. A rapid decrease of CD3 from the surface is
required to reach the threshold that is necessary for the continuation of the
activation process. By flow cytometry technique, it is well established that an
amount of down regulated CD3 proteins recycle and may come on the surface of
T-cell. This article is dedicated to study the impact of recycled CD3 protein
that is recruited into the immune synapse at different rates. The population
density of T-cells is distributed over surface protein (CD3) concentration
under the framework of population balance models by using coupled system of
PDEs. Such problems need non-conventional population balance modeling so that
classical numerical technique can be applied. A recently developed efficient
numerical method is validated against the analytical and numerical solutions.
How to Combine a Billion Alphas
Date: 2016-06-13 00:00:00
Tasks: Clustering, Position

We give an explicit algorithm and source code for computing optimal weights
for combining a large number N of alphas. This algorithm does not cost O(N^3)
or even O(N^2) operations but is much cheaper, in fact, the number of required
operations scales linearly with N. We discuss how in the absence of binary or
quasi-binary clustering of alphas, which is not observed in practice, the
optimization problem simplifies when N is large. Our algorithm does not require
computing principal components or inverting large matrices, nor does it require
iterations. The number of risk factors it employs, which typically is limited
by the number of historical observations, can be sizably enlarged via using
position data for the underlying tradables.
Solutions with a bounded support promote permanence of a distributed replicator equation
Date: 2016-03-18 00:00:00

The now classical replicator equation describes a wide variety of biological
phenomena, including those in theoretical genetics, evolutionary game theory,
or in the theories of the origin of life. Among other questions, the permanence
of the replicator equation is well studied in the local, well-mixed case.
Inasmuch as the spatial heterogeneities are key to understanding the species
coexistence at least in some cases, it is important to supplement the classical
theory of the non-distributed replicator equation with a spatially explicit
framework. One possible approach, motivated by the porous medium equation, is
introduced. It is shown that the solutions to the spatially heterogeneous
replicator equation may evolve to equilibrium states that have a bounded
support, and, moreover, that these solutions are of paramount importance for
the overall system permanence, which is shown to be a more commonplace
phenomenon for the spatially explicit equation if compared with the local
model.
Solving Society's Big Ills, A Small Step
Date: 2017-03-26 00:00:00

We look at a collection of conjectures with the unifying message that smaller
social systems, tend to be less complex and can be aligned better, towards
fulfilling their intended objectives. We touch upon a framework, referred to as
the four pronged approach that can aid the analysis of social systems. The four
prongs are:
  1. The Uncertainty Principle of the Social Sciences
  2. The Objectives of a Social System or the Responsibilities of the Players
  3. The Need for Smaller Organizations
  4. Redirecting Unintended Outcomes
  Smaller organizations mitigating the disruptive effects of corruption is
discussed and also the need for organizations, whose objective is to foster the
development of other smaller organizations. We consider a way of life, which is
about respect for knowledge and a desire to seek it. Knowledge can help
eradicate ignorance, but the accumulation of knowledge can lead to
overconfidence. Hence it becomes important to instill an attitude that does not
knowledge too seriously, along with the thirst for knowledge. All of this is
important to create an environment that is conducive for smaller organizations
and can be viewed as a natural extension of studies that fall under the wider
category of understanding factors and policies aimed at increasing the welfare
or well-being to society.
The Circle of Investment: Connecting the Dots of the Portfolio Management Cycle...
Date: 2016-03-31 00:00:00
Tasks: Management

We will look at the entire cycle of the investment process relating to all
aspects of, formulating an investment hypothesis, constructing a portfolio
based on that, executing the trades to implement it, on-going risk management,
periodically measuring the performance of the portfolio, and rebalancing the
portfolio either due to an increase in the risk parameters or due to a
deviation from the intended asset allocation. We provide several illustrative
analogies that are meant to intuitively explain the pleasures and the pitfalls
that can arise while managing a portfolio. If we consider the entire investment
management procedure as being akin to connecting the dots of a circle, then the
Circle of Investment can be represented as a dotted circle with many dots
falling approximately on the circumference and with no clue about the exact
location of the centre or the length of the radius. We represent the investment
process as a dotted circle since there is a lot of ambiguity in the various
steps involved. The circle also indicates the repetitive nature of many steps
that are continuously carried out while investing. This work introduces two new
points pertaining to this dotted circle and improves the ability, to understand
how far-off this dotted circle is, from a more well-defined circle and, to
create a well-formed circle. The two innovations we introduce are:
  1. The first, relating to the limitations that apply to any finding in the
social sciences, would be the additional point we introduce that lies near the
centre of the circle. We title this as, 'The Uncertainty Principle of the
Social Sciences'.
  2. The second, relating to establishing confidence levels in a systematic
manner for each view we associate with a security or group of securities as
required by the Black-Litterman framework, would be the new point we present
near the circumference of the circle.
IMPi: An interactive homology modeling pipeline
Date: 2016-03-19 00:00:00

Summary: The development of automated servers to predict the
three-dimensional structure of proteins has seen much progress over the years.
These servers make modeling simpler, but largely exclude users from the
process. We present an Interactive Modeling Pipeline (IMPi) for homology
modeling. The pipeline simplifies the modeling process and reduces the workload
required by the user, while still allowing engagement from the user during
every step. Default parameters are given for each step, which can either be
modified or supplemented with additional external input. As such, it has been
designed for users of varying levels of experience with homology modeling.
  Availability and implementation: The IMPi site is free for non-commercial use
and can be accessed at https://impi.rubi.ru.ac.za.
  Supplementary information: Documentation available at
https://impi.rubi.ru.ac.za/#documentation.
How to assess case-finding in chronic diseases: Comparison of different indices
Date: 2016-03-19 00:00:00

Recently, we have proposed a new illness-death model that comprises a state
of undiagnosed chronic disease preceding the diagnosed disease. Based on this
model, the question arises how case-finding can be assessed in the presence of
mortality from all these states. We simulate two scenarios of different
performance of case-finding and apply several indices to assess case-finding in
both scenarios. One of the prevalence based indices leads to wrong conclusions.
Some indices are partly insensitive to distinguish the quality of case-finding.
The incidence based indices perform well. If possible, incidence based indices
should be preferred.
Risk-Constrained Kelly Gambling
Date: 2016-03-20 00:00:00

We consider the classic Kelly gambling problem with general distribution of
outcomes, and an additional risk constraint that limits the probability of a
drawdown of wealth to a given undesirable level. We develop a bound on the
drawdown probability; using this bound instead of the original risk constraint
yields a convex optimization problem that guarantees the drawdown risk
constraint holds. Numerical experiments show that our bound on drawdown
probability is reasonably close to the actual drawdown risk, as computed by
Monte Carlo simulation. Our method is parametrized by a single parameter that
has a natural interpretation as a risk-aversion parameter, allowing us to
systematically trade off asymptotic growth rate and drawdown risk. Simulations
show that this method yields bets that out perform fractional-Kelly bets for
the same drawdown risk level or growth rate. Finally, we show that a natural
quadratic approximation of our convex problem is closely connected to the
classical mean-variance Markowitz portfolio selection problem.
Bayesian methods for event analysis of intracellular currents
Date: 2016-05-18 00:00:00

Investigation of neural circuit functioning often requires statistical
interpretation of events in subthreshold electrophysiological recordings. This
problem is non-trivial because recordings may have moderate levels of
structured noise and events may have distinct kinetics. In addition, novel
experimental designs that combine optical and electrophysiological methods will
depend upon statistical tools that combine multimodal data. We present a
Bayesian approach for inferring the timing, strength, and kinetics of
postsynaptic currents (PSCs) from voltage-clamp recordings on a per event
basis. The simple generative model for a single voltage-clamp recording
flexibly extends to include network-level structure to enable experiments
designed to probe synaptic connectivity. We validate the approach on simulated
and real data. We also demonstrate that extensions of the basic PSC detection
algorithm can handle recordings contaminated with optically evoked currents,
and we simulate a scenario in which calcium imaging observations, available for
a subset of neurons, can be fused with electrophysiological data to achieve
higher temporal resolution. We apply this approach to simulated and real ground
truth data to demonstrate its higher sensitivity in detecting small
signal-to-noise events and its increased robustness to noise compared to
standard methods for detecting PSCs. The new Bayesian event analysis approach
for electrophysiological recordings should allow for better estimation of
physiological parameters under more variable conditions and help support new
experimental designs for circuit mapping.
Gamma synchronization of the hippocampal spatial map---topological model
Date: 2016-03-20 00:00:00
Tasks: Hippocampus, Rhythm

The mammalian hippocampus plays a principal role in producing a cognitive map
of space---an internalized representation of the animal's environment. The
neuronal mechanisms producing this map depend primarily on the temporal
structure of the hippocampal neurons' spiking activity, which is modulated by
the oscillatory extracellular electrical field potential. In this paper, we
discuss the integrative effect of the gamma rhythm, one of the principal
components of these oscillations, on the ability of the place cell ensembles to
encode a spatial map. Using methods of algebraic topology and statistical
physics, we demonstrate that gamma-modulation of neuronal activity generates a
synchronized spiking of dynamical cell assemblies, which enables learning a
spatial map at faster timescales.
Asymptotics of Stochastic Protein Assembly Models
Date: 2016-03-21 00:00:00

Self-assembly of proteins is a biological phenomenon which gives rise to
spontaneous formation of amyloid fibrils or polymers. The starting point of
this phase, called nucleation exhibits an important variability among
replicated experiments.To analyse the stochastic nature of this phenomenon, one
of the simplest models considers two populations of chemical components:
monomers and polymerised monomers. Initially there are only monomers. There are
two reactions for the polymerization of a monomer: either two monomers collide
to combine into two polymerised monomers or a monomer is polymerised after the
encounter of a polymerised monomer. It turns out that this simple model does
not explain completely the variability observed in the experiments. This paper
investigates extensions of this model to take into account other mechanisms of
the polymerization process that may have impact an impact on fluctuations.The
first variant consists in introducing a preliminary conformation step to take
into account the biological fact that, before being polymerised, a monomer has
two states, regular or misfolded. Only misfolded monomers can be polymerised so
that the fluctuations of the number of misfolded monomers can be also a source
of variability of the number of polymerised monomers. The second variant
represents the reaction rate $\alpha$ of spontaneous formation of a polymer as
of the order of $N^{-\nu}$, with $\nu$ some positive constant. First and second
order results for the starting instant of nucleation are derived from these
limit theorems. The proofs of the results rely on a study of a stochastic
averaging principle for a model related to an Ehrenfest urn model, and also on
a scaling analysis of a population model.
Functional neuroanatomy of meditation: A review and meta-analysis of 78 functional neuroimaging investigations
Date: 2016-03-21 00:00:00

Meditation is a family of mental practices that encompasses a wide array of
techniques employing distinctive mental strategies. We systematically reviewed
78 functional neuroimaging (fMRI and PET) studies of meditation, and used
activation likelihood estimation to meta-analyze 257 peak foci from 31
experiments involving 527 participants. We found reliably dissociable patterns
of brain activation and deactivation for four common styles of meditation
(focused attention, mantra recitation, open monitoring, and
compassion/loving-kindness), and suggestive differences for three others
(visualization, sense-withdrawal, and non-dual awareness practices). Overall,
dissociable activation patterns are congruent with the psychological and
behavioral aims of each practice. Some brain areas are recruited consistently
across multiple techniques - including insula, pre/supplementary motor
cortices, dorsal anterior cingulate cortex, and frontopolar cortex - but
convergence is the exception rather than the rule. A preliminary effect-size
meta-analysis found medium effects for both activations (d = .59) and
deactivations (d = -.74), suggesting potential practical significance. Our
meta-analysis supports the neurophysiological dissociability of meditation
practices, but also raises many methodological concerns and suggests avenues
for future research.
Non-classicality in mental states : an experimental study with ambiguous audio (music) stimuli
Date: 2016-03-18 00:00:00

This paper attempts to address the question that whether the present physical
or mathematical theories are sufficient to understand the complexities of human
brain when it interacts with the external environment in the form of an
auditory stimulus.There have been efforts reporting that the introduction of
ambiguity in visual stimuli causes effects which can't be explained
classically.In this paper,it is investigated whether ambiguity in auditory
stimuli can introduce any non-classical effects in human brain.Simple
experiments were performed on normal subjects where they listened to an
ambiguous auditory signal and responded to a question with 'yes' or 'no'.The
outcome of the test showed that the classical formula of total probability does
not hold true in this case.Results were interesting and indicate that there is
a definite non-classicality in mental states in perception of ambiguous audio
stimuli.
The mathematics of non-linear metrics for nested networks
Date: 2016-03-21 00:00:00

Numerical analysis of data from international trade and ecological networks
has shown that the non-linear fitness-complexity metric is the best candidate
to rank nodes by importance in bipartite networks that exhibit a nested
structure. Despite its relevance for real networks, the mathematical properties
of the metric and its variants remain largely unexplored. Here, we perform an
analytic and numeric study of the fitness-complexity metric and a new variant,
called minimal extremal metric. We rigorously derive exact expressions for node
scores for perfectly nested networks and show that these expressions explain
the non-trivial convergence properties of the metrics. A comparison between the
fitness-complexity metric and the minimal extremal metric on real data reveals
that the latter can produce improved rankings if the input data are reliable.
The movement of a forager: Strategies for the efficient use of resources
Date: 2016-03-21 00:00:00

We study a simple model of a foraging animal that modifies the substrate on
which it moves. This substrate provides its only resource, and the forager
manage it by taking a limited portion at each visited site. The resource
recovers its value after the visit following a relaxation law. We study
different scenarios to analyze the efficiency of the managing strategy,
corresponding to control the bite size. We observe the non trivial emergence of
a home range, that is visited in a periodic way. The duration of the
corresponding cycles and the transient until it emerges is affected by the bite
size. Our results show that the most efficient use of the resource, measured as
the balance between gathering and travelled distance, corresponds to foragers
that take larger portions but without exhausting the resource. We also analyze
the use of space determining the number of attractors of the dynamics, and we
observe that it depends on the bite size and the recovery time of the resource.
Enriching Ontologies with Encyclopedic Background Knowledge for Document Indexing
Authors: Posch Lisa
Date: 2016-03-21 00:00:00
Tasks: BIG-bench Machine Learning

The rapidly increasing number of scientific documents available publicly on
the Internet creates the challenge of efficiently organizing and indexing these
documents. Due to the time consuming and tedious nature of manual
classification and indexing, there is a need for better methods to automate
this process. This thesis proposes an approach which leverages encyclopedic
background knowledge for enriching domain-specific ontologies with textual and
structural information about the semantic vicinity of the ontologies' concepts.
The proposed approach aims to exploit this information for improving both
ontology-based methods for classifying and indexing documents and methods based
on supervised machine learning.
Natural site-directed mutagenesis might exist in eukaryotic cells
Date: 2016-04-18 00:00:00

Site-directed mutagenesis refers to a man-made molecular biology method that
is used to make genetic alterations in the DNA sequence of a gene of interest.
But based on our recently published experimental findings, we propose that
natural site-directed mutagenesis might exist in the eukaryotic cells, which is
triggered by harmful agents and co-directed by special transcription hotspots
and mutation-contained intranuclear primers.
Universal trading under proportional transaction costs
Date: 2016-03-21 00:00:00

The theory of optimal trading under proportional transaction costs has been
considered from a variety of perspectives. In this paper, we show that all the
results can be interpreted using a universal law, illustrating the results in
trading algorithm design.