5,084 research outputs found
Structural efficiency of percolation landscapes in flow networks
Complex networks characterized by global transport processes rely on the
presence of directed paths from input to output nodes and edges, which organize
in characteristic linked components. The analysis of such network-spanning
structures in the framework of percolation theory, and in particular the key
role of edge interfaces bridging the communication between core and periphery,
allow us to shed light on the structural properties of real and theoretical
flow networks, and to define criteria and quantities to characterize their
efficiency at the interplay between structure and functionality. In particular,
it is possible to assess that an optimal flow network should look like a "hairy
ball", so to minimize bottleneck effects and the sensitivity to failures.
Moreover, the thorough analysis of two real networks, the Internet
customer-provider set of relationships at the autonomous system level and the
nervous system of the worm Caenorhabditis elegans --that have been shaped by
very different dynamics and in very different time-scales--, reveals that
whereas biological evolution has selected a structure close to the optimal
layout, market competition does not necessarily tend toward the most customer
efficient architecture.Comment: 8 pages, 5 figure
Message-Passing Methods for Complex Contagions
Message-passing methods provide a powerful approach for calculating the
expected size of cascades either on random networks (e.g., drawn from a
configuration-model ensemble or its generalizations) asymptotically as the
number of nodes becomes infinite or on specific finite-size networks. We
review the message-passing approach and show how to derive it for
configuration-model networks using the methods of (Dhar et al., 1997) and
(Gleeson, 2008). Using this approach, we explain for such networks how to
determine an analytical expression for a "cascade condition", which determines
whether a global cascade will occur. We extend this approach to the
message-passing methods for specific finite-size networks (Shrestha and Moore,
2014; Lokhov et al., 2015), and we derive a generalized cascade condition.
Throughout this chapter, we illustrate these ideas using the Watts threshold
model.Comment: 14 pages, 3 figure
Inference of hidden structures in complex physical systems by multi-scale clustering
We survey the application of a relatively new branch of statistical
physics--"community detection"-- to data mining. In particular, we focus on the
diagnosis of materials and automated image segmentation. Community detection
describes the quest of partitioning a complex system involving many elements
into optimally decoupled subsets or communities of such elements. We review a
multiresolution variant which is used to ascertain structures at different
spatial and temporal scales. Significant patterns are obtained by examining the
correlations between different independent solvers. Similar to other
combinatorial optimization problems in the NP complexity class, community
detection exhibits several phases. Typically, illuminating orders are revealed
by choosing parameters that lead to extremal information theory correlations.Comment: 25 pages, 16 Figures; a review of earlier work
Recruitment of ethnic minority patients to a cardiac rehabilitation trial: The Birmingham Rehabilitation Uptake Maximisation (BRUM) study [ISRCTN72884263]
Background: Concerns have been raised about low participation rates of people from minority ethnic groups
in clinical trials. However, the evidence is unclear as many studies do not report the ethnicity of participants and
there is insufficient information about the reasons for ineligibility by ethnic group. Where there are data, there
remains the key question as to whether ethnic minorities more likely to be ineligible (e.g. due to language) or
decline to participate. We have addressed these questions in relation to the Birmingham Rehabilitation Uptake
Maximisation (BRUM) study, a randomized controlled trial (RCT) comparing a home-based with a hospital-based
cardiac rehabilitation programme in a multi-ethnic population in the UK.
Methods: Analysis of the ethnicity, age and sex of presenting and recruited subjects for a trial of cardiac
rehabilitation in the West-Midlands, UK.
Participants: 1997 patients presenting post-myocardial infarction, percutaneous transluminal coronary angioplasty
or coronary artery bypass graft surgery.
Data collected: exclusion rates, reasons for exclusion and reasons for declining to participate in the trial by ethnic
group.
Results: Significantly more patients of South Asian ethnicity were excluded (52% of 'South Asian' v 36% 'White
European' and 36% 'Other', p < 0.001). This difference in eligibility was primarily due to exclusion on the basis of
language (i.e. the inability to speak English or Punjabi). Of those eligible, similar proportions were recruited from
the different ethnic groups (white, South Asian and other). There was a marked difference in eligibility between
people of Indian, Pakistani or Bangladeshi origin
Random Walks on Stochastic Temporal Networks
In the study of dynamical processes on networks, there has been intense focus
on network structure -- i.e., the arrangement of edges and their associated
weights -- but the effects of the temporal patterns of edges remains poorly
understood. In this chapter, we develop a mathematical framework for random
walks on temporal networks using an approach that provides a compromise between
abstract but unrealistic models and data-driven but non-mathematical
approaches. To do this, we introduce a stochastic model for temporal networks
in which we summarize the temporal and structural organization of a system
using a matrix of waiting-time distributions. We show that random walks on
stochastic temporal networks can be described exactly by an
integro-differential master equation and derive an analytical expression for
its asymptotic steady state. We also discuss how our work might be useful to
help build centrality measures for temporal networks.Comment: Chapter in Temporal Networks (Petter Holme and Jari Saramaki
editors). Springer. Berlin, Heidelberg 2013. The book chapter contains minor
corrections and modifications. This chapter is based on arXiv:1112.3324,
which contains additional calculations and numerical simulation
Some remarks on a new exotic spacetime for time travel by free fall
This work is essentially a review of a new spacetime model with closed causal
curves, recently presented in another paper (Class. Quantum Grav.
\textbf{35}(16) (2018), 165003). The spacetime at issue is topologically
trivial, free of curvature singularities, and even time and space orientable.
Besides summarizing previous results on causal geodesics, tidal accelerations
and violations of the energy conditions, here redshift/blueshift effects and
the Hawking-Ellis classification of the stress-energy tensor are examined.Comment: 17 pages, 9 figures. Submitted as a contribution to the proceedings
of "DOMOSCHOOL - International Alpine School of Mathematics and Physics,
Domodossola 2018". Possible text overlaps with my previous work
arXiv:1803.08214, of which this is essentially a review. Additional results
concerning redshift/blueshift effects and the classification of the
stress-energy tensor are presented her
Science Models as Value-Added Services for Scholarly Information Systems
The paper introduces scholarly Information Retrieval (IR) as a further
dimension that should be considered in the science modeling debate. The IR use
case is seen as a validation model of the adequacy of science models in
representing and predicting structure and dynamics in science. Particular
conceptualizations of scholarly activity and structures in science are used as
value-added search services to improve retrieval quality: a co-word model
depicting the cognitive structure of a field (used for query expansion), the
Bradford law of information concentration, and a model of co-authorship
networks (both used for re-ranking search results). An evaluation of the
retrieval quality when science model driven services are used turned out that
the models proposed actually provide beneficial effects to retrieval quality.
From an IR perspective, the models studied are therefore verified as expressive
conceptualizations of central phenomena in science. Thus, it could be shown
that the IR perspective can significantly contribute to a better understanding
of scholarly structures and activities.Comment: 26 pages, to appear in Scientometric
Kerr-Newman Black Hole Thermodynamical State Space: Blockwise Coordinates
A coordinate system that blockwise-simplifies the Kerr-Newman black hole's
thermodynamical state space Ruppeiner metric geometry is constructed, with
discussion of the limiting cases corresponding to simpler black holes. It is
deduced that one of the three conformal Killing vectors of the
Reissner-Nordstrom and Kerr cases (whose thermodynamical state space metrics
are 2 by 2 and conformally flat) survives generalization to the Kerr-Newman
case's 3 by 3 thermodynamical state space metric.Comment: 4 pages incl 2 figs. Accepted by Gen. Rel. Grav. Replaced with
Accepted version (minor corrections
Multiple Imputation Ensembles (MIE) for dealing with missing data
Missing data is a significant issue in many real-world datasets, yet there are no robust methods for dealing with it appropriately. In this paper, we propose a robust approach to dealing with missing data in classification problems: Multiple Imputation Ensembles (MIE). Our method integrates two approaches: multiple imputation and ensemble methods and compares two types of ensembles: bagging and stacking. We also propose a robust experimental set-up using 20 benchmark datasets from the UCI machine learning repository. For each dataset, we introduce increasing amounts of data Missing Completely at Random. Firstly, we use a number of single/multiple imputation methods to recover the missing values and then ensemble a number of different classifiers built on the imputed data. We assess the quality of the imputation by using dissimilarity measures. We also evaluate the MIE performance by comparing classification accuracy on the complete and imputed data. Furthermore, we use the accuracy of simple imputation as a benchmark for comparison. We find that our proposed approach combining multiple imputation with ensemble techniques outperform others, particularly as missing data increases
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