1,107 research outputs found
Population Dynamics on Complex Food Webs
In this work we analyse the topological and dynamical properties of a simple
model of complex food webs, namely the niche model. In order to underline
competition among species, we introduce "prey" and "predators" weighted overlap
graphs derived from the niche model and compare synthetic food webs with real
data. Doing so, we find new tests for the goodness of synthetic food web models
and indicate a possible direction of improvement for existing ones. We then
exploit the weighted overlap graphs to define a competition kernel for
Lotka-Volterra population dynamics and find that for such a model the stability
of food webs decreases with its ecological complexity.Comment: 11 Pages, 5 Figures, styles enclosed in the submissio
Power-law distributions in empirical data
Power-law distributions occur in many situations of scientific interest and
have significant consequences for our understanding of natural and man-made
phenomena. Unfortunately, the detection and characterization of power laws is
complicated by the large fluctuations that occur in the tail of the
distribution -- the part of the distribution representing large but rare events
-- and by the difficulty of identifying the range over which power-law behavior
holds. Commonly used methods for analyzing power-law data, such as
least-squares fitting, can produce substantially inaccurate estimates of
parameters for power-law distributions, and even in cases where such methods
return accurate answers they are still unsatisfactory because they give no
indication of whether the data obey a power law at all. Here we present a
principled statistical framework for discerning and quantifying power-law
behavior in empirical data. Our approach combines maximum-likelihood fitting
methods with goodness-of-fit tests based on the Kolmogorov-Smirnov statistic
and likelihood ratios. We evaluate the effectiveness of the approach with tests
on synthetic data and give critical comparisons to previous approaches. We also
apply the proposed methods to twenty-four real-world data sets from a range of
different disciplines, each of which has been conjectured to follow a power-law
distribution. In some cases we find these conjectures to be consistent with the
data while in others the power law is ruled out.Comment: 43 pages, 11 figures, 7 tables, 4 appendices; code available at
http://www.santafe.edu/~aaronc/powerlaws
Visualising the cytoskeletal machinery in neuronal growth cones using cryo-electron tomography
Neurons extend axons to form the complex circuitry of the mature brain. This
depends on the coordinated response and continuous remodelling of the microtubule
and F-actin networks in the axonal growth cone. Growth cone architecture remains
poorly understood at nanoscales. We therefore investigated mouse hippocampal
neuron growth cones using cryo-electron tomography to directly visualise their threedimensional
subcellular architecture with molecular detail. Our data show the
hexagonal arrays of actin bundles that form filopodia penetrate and terminate deep
in the growth cone interior. We directly observe the modulation of these and other
growth cone actin bundles by alteration of individual F-actin helical structures.
Microtubules with blunt, slightly flared, or gently curved ends predominate in the
growth cone, frequently contain lumenal particles and carry lattice defects.
Investigation of the effect of absence of doublecortin, a neurodevelopmental
cytoskeleton regulator, on growth cone cytoskeleton shows no major anomalies in
overall growth cone organisation or in F-actin subpopulations. However, our data
suggest that microtubules sustain more structural defects, highlighting the
importance of microtubule integrity during growth cone migration
Efficient Passive ICS Device Discovery and Identification by MAC Address Correlation
Owing to a growing number of attacks, the assessment of Industrial Control
Systems (ICSs) has gained in importance. An integral part of an assessment is
the creation of a detailed inventory of all connected devices, enabling
vulnerability evaluations. For this purpose, scans of networks are crucial.
Active scanning, which generates irregular traffic, is a method to get an
overview of connected and active devices. Since such additional traffic may
lead to an unexpected behavior of devices, active scanning methods should be
avoided in critical infrastructure networks. In such cases, passive network
monitoring offers an alternative, which is often used in conjunction with
complex deep-packet inspection techniques. There are very few publications on
lightweight passive scanning methodologies for industrial networks. In this
paper, we propose a lightweight passive network monitoring technique using an
efficient Media Access Control (MAC) address-based identification of industrial
devices. Based on an incomplete set of known MAC address to device
associations, the presented method can guess correct device and vendor
information. Proving the feasibility of the method, an implementation is also
introduced and evaluated regarding its efficiency. The feasibility of
predicting a specific device/vendor combination is demonstrated by having
similar devices in the database. In our ICS testbed, we reached a host
discovery rate of 100% at an identification rate of more than 66%,
outperforming the results of existing tools.Comment: http://dx.doi.org/10.14236/ewic/ICS2018.
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Advanced in-duct sorbent injection for SOâ control. Topical report No. 5, Task 4: Data analysis and computer modeling
The objective of this research project is to develop a second generation in-duct sorbent injection technology as a cost-effective compliance option for the 1990 Clean Air Act Amendments. Specific performance targets are 90% SO{sub 2} removal and 60% sorbent utilization efficiency. After initial results indicated that the Advanced Coolside process had a potential of exceeding these targets, research focused on the Advanced Coolside process. For Task 4, Data Analysis and Computer Modeling, the objective was to develop two computer models. The first computer model would correlate sorbent properties with hydration parameters, while the second would correlate desulfurization performance with the sorbent properties. A two-level factorial program was undertaken to examine the effects of selected hydration process variables on some of the physical and chemical properties of the hydrates produced and on their SO{sub 2} reactivity. A bench-scale hydrator was used to convert quicklime samples to hydrated limes under controlled processing conditions. Two quicklimes were hydrated -- Mississippi Lime and Black River Lime. Significant differences in physical properties of these hydrates were observed. However, no relationship between the measured physical properties and the SO{sub 2} reactivity was observed. Within the scope of this work, SO{sub 2} reactivity is not a function of quicklime source. When compared with commercial hydrates prepared from the same quicklime, the hydrates produced in the bench-scale unit showed significantly lower surface areas and SO{sub 2} reactivities. As a result, the correlations developed in this study do not apply to commercial hydrates
Activation of Protease-Activated Receptors 3 and 4 Accelerates Tissue Factor-Induced Thrombin Generation on the Surface of Vascular Smooth Muscle Cells
To determine factors regulating human aortic smooth muscle cells (HASMC) supported tissue factor-induced thrombin generation
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Advanced in-duct sorbent injection for SO{sub 2} control. Topical report No. 4, Task 3, Optimized advanced process evaluation
The objective of this research project is to develop second- generation duct injection technology as a cost-effective compliance option for the 1990 Clean Air Act Amendments. Specific performance targets are 90% SO{sup 2} removal and 60% sorbent utilization efficiency. Research focused on the Advanced Coolside process, which showed the potential for exceeding these targets. The objective of Subtask 3.1, Performance Testing, was to develop process performance and operability data for design and scale-up of the optimized Advance Coolside process. Results of long-term pilot plant testing with 24 hour/day operation provided a positive indication of process operability. The objective of Subtask 3.2, Waste Characterization, was to determine the chemical and physical properties of the waste materials for designing the waste handling and disposal systems for the process. Test results show that the combined spent sorbent and fly ash waste is suitable for landfilling. Further, the waste management study indicated a potential for by-product utilization for synthetic aggregate production; a more thorough investigation of this potential is required
Destabilization of the thermohaline circulation by transient perturbations to the hydrological cycle
We reconsider the problem of the stability of the thermohaline circulation as
described by a two-dimensional Boussinesq model with mixed boundary conditions.
We determine how the stability properties of the system depend on the intensity
of the hydrological cycle. We define a two-dimensional parameters' space
descriptive of the hydrology of the system and determine, by considering
suitable quasi-static perturbations, a bounded region where multiple equilibria
of the system are realized. We then focus on how the response of the system to
finite-amplitude surface freshwater forcings depends on their rate of increase.
We show that it is possible to define a robust separation between slow and fast
regimes of forcing. Such separation is obtained by singling out an estimate of
the critical growth rate for the anomalous forcing, which can be related to the
characteristic advective time scale of the system.Comment: 37 pages, 8 figures, submitted to Clim. Dy
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
A Semantic Framework for Enabling Radio Spectrum Policy Management and Evaluation
Because radio spectrum is a finite resource, its usage and sharing is
regulated by government agencies. These agencies define policies to manage
spectrum allocation and assignment across multiple organizations, systems, and
devices. With more portions of the radio spectrum being licensed for commercial
use, the importance of providing an increased level of automation when
evaluating such policies becomes crucial for the efficiency and efficacy of
spectrum management. We introduce our Dynamic Spectrum Access Policy Framework
for supporting the United States government's mission to enable both federal
and non-federal entities to compatibly utilize available spectrum. The DSA
Policy Framework acts as a machine-readable policy repository providing policy
management features and spectrum access request evaluation. The framework
utilizes a novel policy representation using OWL and PROV-O along with a
domain-specific reasoning implementation that mixes GeoSPARQL, OWL reasoning,
and knowledge graph traversal to evaluate incoming spectrum access requests and
explain how applicable policies were used. The framework is currently being
used to support live, over-the-air field exercises involving a diverse set of
federal and commercial radios, as a component of a prototype spectrum
management system
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