79 research outputs found
Connectivity measures for internet topologies.
The topology of the Internet has initially been modelled as an undirected graph, where vertices correspond to so-called Autonomous Systems (ASs),and edges correspond to physical links between pairs of ASs. However, in order to capture the impact of routing policies, it has recently become apparent that one needs to classify the edges according to the existing economic relationships (customer-provider, peer-to-peer or siblings) between the ASs. This leads to a directed graph model in which traffic can be sent only along so-called valley-free paths. Four different algorithms have been proposed in the literature for inferring AS relationships using publicly available data from routing tables. We investigate the differences in the graph models produced by these algorithms, focussing on connectivity measures. To this aim, we compute the maximum number of vertex-disjoint valley-free paths between ASs as well as the size of a minimum cut separating a pair of ASs. Although these problems are solvable in polynomial time for ordinary graphs, they are NP-hard in our setting. We formulate the two problems as integer programs, and we propose a number of exact algorithms for solving them. For the problem of finding the maximum number of vertex-disjoint paths, we discuss two algorithms; the first one is a branch-and-price algorithm based on the IP formulation, and the second algorithm is a non LP based branch-and-bound algorithm. For the problem of finding minimum cuts we use a branch-and-cut algo rithm, based on the IP formulation of this problem. Using these algorithms, we obtain exact solutions for both problems in reasonable time. It turns out that there is a large gap in terms of the connectivity measures between the undirected and directed models. This finding supports our conclusion that economic relationships need to be taken into account when building a topology of the Internet.Research; Internet;
Scaling Invariance in Spectra of Complex Networks: A Diffusion Factorial Moment Approach
A new method called diffusion factorial moment (DFM) is used to obtain
scaling features embedded in spectra of complex networks. For an Erdos-Renyi
network with connecting probability , the scaling
parameter is , while for the scaling
parameter deviates from it significantly. For WS small-world networks, in the
special region , typical scale invariance is found. For GRN
networks, in the range of , we have .
And the value of oscillates around abruptly. In the range
of , we have basically . Scale invariance is one
of the common features of the three kinds of networks, which can be employed as
a global measurement of complex networks in a unified way.Comment: 6 pages, 8 figures. to appear in Physical Review
Automated Quantification of Atherosclerosis in CTA of Carotid Arteries
How is the human body built and how does it function? What are the causes of
disease, and where is disease located? Throughout the history of mankind these
questions were answered by the use of invasive methods that included the
“opening” of the human body, mainly cadavers. Thanks to these invasive
techniques the first precise and complete anatomy works started to appear in
the 16th century. The most influential works were published by Leonardo da
Vinci and the anatomist and physician Andreas Vesalius.
The discovery of X-rays in 1895, and their use for medical applications,
introduced a new era, in which non-invasive imaging of the functioning human
body became feasible. Nowadays, medical imaging includes many different
imaging modalities, such as X-ray, computed tomography (CT), magnetic
resonance imaging (MRI), ultrasound (US), nuclear and optical imaging, and
has become an indispensable diagnostic tool for a wide range of applications.
Initially, the application of medical imaging focused on the visualization of
anatomy and on the detection and localization of disease. However, with the
development of different modalities it has evolved into a much more versatile
tool providing important information on e.g. physiology and organ function,
biochemistry and metabolism using nuclear imaging (mainly positron emission
tomography (PET) imaging), molecular and processes on the molecular
and cellular level using molecular imaging techniques
Temporal Series Analysis Approach to Spectra of Complex Networks
The spacing of nearest levels of the spectrum of a complex network can be
regarded as a time series. Joint use of Multi-fractal Detrended Fluctuation
Approach (MF-DFA) and Diffusion Entropy (DE) is employed to extract
characteristics from this time series. For the WS (Watts and Strogatz)
small-world model, there exist a critical point at rewiring probability . For a
network generated in the range, the correlation exponent is in the range of .
Above this critical point, all the networks behave similar with that at . For
the ER model, the time series behaves like FBM (fractional Brownian motion)
noise at . For the GRN (growing random network) model, the values of the
long-range correlation exponent are in the range of . For most of the GRN
networks the PDF of a constructed time series obeys a Gaussian form. In the
joint use of MF-DFA and DE, the shuffling procedure in DE is essential to
obtain a reliable result. PACS number(s): 89.75.-k, 05.45.-a, 02.60.-xComment: 10 pages, 9 figures, to appear in PR
Global attractor and asymptotic dynamics in the Kuramoto model for coupled noisy phase oscillators
We study the dynamics of the large N limit of the Kuramoto model of coupled
phase oscillators, subject to white noise. We introduce the notion of shadow
inertial manifold and we prove their existence for this model, supporting the
fact that the long term dynamics of this model is finite dimensional. Following
this, we prove that the global attractor of this model takes one of two forms.
When coupling strength is below a critical value, the global attractor is a
single equilibrium point corresponding to an incoherent state. Conversely, when
coupling strength is beyond this critical value, the global attractor is a
two-dimensional disk composed of radial trajectories connecting a saddle
equilibrium (the incoherent state) to an invariant closed curve of locally
stable equilibria (partially synchronized state). Our analysis hinges, on the
one hand, upon sharp existence and uniqueness results and their consequence for
the existence of a global attractor, and, on the other hand, on the study of
the dynamics in the vicinity of the incoherent and synchronized equilibria. We
prove in particular non-linear stability of each synchronized equilibrium, and
normal hyperbolicity of the set of such equilibria. We explore mathematically
and numerically several properties of the global attractor, in particular we
discuss the limit of this attractor as noise intensity decreases to zero.Comment: revised version, 28 pages, 4 figure
Edge Detection by Adaptive Splitting II. The Three-Dimensional Case
In Llanas and Lantarón, J. Sci. Comput. 46, 485–518 (2011) we proposed an algorithm (EDAS-d) to approximate the jump discontinuity set of functions defined on subsets of ℝ d . This procedure is based on adaptive splitting of the domain of the function guided by the value of an average integral. The above study was limited to the 1D and 2D versions of the algorithm. In this paper we address the three-dimensional problem. We prove an integral inequality (in the case d=3) which constitutes the basis of EDAS-3. We have performed detailed computational experiments demonstrating effective edge detection in 3D function models with different interface topologies. EDAS-1 and EDAS-2 appealing properties are extensible to the 3D cas
Leaf segmentation in plant phenotyping: a collation study
Image-based plant phenotyping is a growing application area of computer vision in agriculture. A key task is the segmentation of all individual leaves in images. Here we focus on the most common rosette model plants, Arabidopsis and young tobacco. Although leaves do share appearance and shape characteristics, the presence of occlusions and variability in leaf shape and pose, as well as imaging conditions, render this problem challenging. The aim of this paper is to compare several leaf segmentation solutions on a unique and first-of-its-kind dataset containing images from typical phenotyping experiments. In particular, we report and discuss methods and findings of a collection of submissions for the first Leaf Segmentation Challenge of the Computer Vision Problems in Plant Phenotyping workshop in 2014. Four methods are presented: three segment leaves by processing the distance transform in an unsupervised fashion, and the other via optimal template selection and Chamfer matching. Overall, we find that although separating plant from background can be accomplished with satisfactory accuracy (>>90 % Dice score), individual leaf segmentation and counting remain challenging when leaves overlap. Additionally, accuracy is lower for younger leaves. We find also that variability in datasets does affect outcomes. Our findings motivate further investigations and development of specialized algorithms for this particular application, and that challenges of this form are ideally suited for advancing the state of the art. Data are publicly available (online at http://www.plant-phenotyping.org/datasets) to support future challenges beyond segmentation within this application domain
Myalgic encephalomyelitis/chronic fatigue Syndrome (ME/CFS) : Investigating care practices pointed out to disparities in diagnosis and treatment across European Union
ME/CFS is a chronic, complex, multisystem disease that often limits the health and functioning of the affected patients. Diagnosing patients with ME/CFS is a challenge, and many different case definitions exist and are used in clinical practice and research. Even after diagnosis, medical treatment is very challenging. Symptom relief and coping may affect how patients live with their disease and their quality of life. There is no consensus on which diagnostic criteria should be used and which treatment strategies can be recommended for patients. The purpose of the current project was to map the landscape of the Euromene countries in respect of national guidelines and recommendations for case definition, diagnosis and clinical approaches for ME/CFS patients. A 23 items questionnaire was sent out by email to the members of Euromene. The form contained questions on existing guidelines for case definitions, treatment/management of the disease, tests and questionnaires applied, and the prioritization of information for data sampling in research. We obtained information from 17 countries. Five countries reported having national guidelines for diagnosis, and five countries reported having guidelines for clinical approaches. For diagnostic purposes, the Fukuda criteria were most often recommended, and also the Canadian Consensus criteria, the International Consensus Criteria and the Oxford criteria were used. A mix of diagnostic criteria was applied within those countries having no guidelines. Many different questionnaires and tests were used for symptom registration and diagnostic investigation. For symptom relief, pain and anti-depressive medication were most often recommended. Cognitive Behavioral Therapy and Graded Exercise treatment were often recommended as disease management and rehabilitative/palliative strategies. The lack of consistency in recommendations across European countries urges the development of regulations, guidance and standards. The results of this study will contribute to the harmonization of diagnostic criteria and treatment for ME/CFS in Europe
Sleep-spindle detection: crowdsourcing and evaluating performance of experts, non-experts and automated methods
Sleep spindles are discrete, intermittent patterns of brain activity observed in human electroencephalographic data. Increasingly, these oscillations are of biological and clinical interest because of their role in development, learning and neurological disorders. We used an Internet interface to crowdsource spindle identification by human experts and non-experts, and we compared their performance with that of automated detection algorithms in data from middle- to older-aged subjects from the general population. We also refined methods for forming group consensus and evaluating the performance of event detectors in physiological data such as electroencephalographic recordings from polysomnography. Compared to the expert group consensus gold standard, the highest performance was by individual experts and the non-expert group consensus, followed by automated spindle detectors. This analysis showed that crowdsourcing the scoring of sleep data is an efficient method to collect large data sets, even for difficult tasks such as spindle identification. Further refinements to spindle detection algorithms are needed for middle- to older-aged subjects
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