256 research outputs found
Homology-based Distributed Coverage Hole Detection in Wireless Sensor Networks
Homology theory provides new and powerful solutions to address the coverage
problems in wireless sensor networks (WSNs). They are based on algebraic
objects, such as Cech complex and Rips complex. Cech complex gives accurate
information about coverage quality but requires a precise knowledge of the
relative locations of nodes. This assumption is rather strong and hard to
implement in practical deployments. Rips complex provides an approximation of
Cech complex. It is easier to build and does not require any knowledge of nodes
location. This simplicity is at the expense of accuracy. Rips complex can not
always detect all coverage holes. It is then necessary to evaluate its
accuracy. This work proposes to use the proportion of the area of undiscovered
coverage holes as performance criteria. Investigations show that it depends on
the ratio between communication and sensing radii of a sensor. Closed-form
expressions for lower and upper bounds of the accuracy are also derived. For
those coverage holes which can be discovered by Rips complex, a homology-based
distributed algorithm is proposed to detect them. Simulation results are
consistent with the proposed analytical lower bound, with a maximum difference
of 0.5%. Upper bound performance depends on the ratio of communication and
sensing radii. Simulations also show that the algorithm can localize about 99%
coverage holes in about 99% cases
Formally Verified Tableau-Based Reasoners for a Description Logic
Description Logics are a family of logics used to represent and reason
about conceptual and terminological knowledge. One of the most basic description
logics is ALC , used as a basis from which to obtain others. Description logics are
particularly important to provide a logical basis for the web ontology languages (such
as OWL) used in the Semantic Web. In order to increase the reliability of the Semantic
Web, formal methods can be applied, and in particular formal verification of its
reasoning services can be carried out. In this paper, we present the formal verification
of a tableau-based satisfiability algorithm for the logic ALC . The verification has
been completed in several stages. First, we develop an abstract formalization of
satisfiability-checking of ALC -concepts. Secondly, we define and formally verify a
tableau-based algorithm in which the order of rule application and branch selection
can be flexibly specified, using a methodology of refinements to transfer the main
properties from the ALC abstract formalization. Finally, we obtain verified and
executable reasoners from the algorithm via a process of instantiation.Ministerio de Ciencia e InnovaciĂłn TIN2009-09492Junta de AndalucĂa TIC-0606
KP solitons in shallow water
The main purpose of the paper is to provide a survey of our recent studies on
soliton solutions of the Kadomtsev-Petviashvili (KP) equation. The
classification is based on the far-field patterns of the solutions which
consist of a finite number of line-solitons. Each soliton solution is then
defined by a point of the totally non-negative Grassmann variety which can be
parametrized by a unique derangement of the symmetric group of permutations.
Our study also includes certain numerical stability problems of those soliton
solutions. Numerical simulations of the initial value problems indicate that
certain class of initial waves asymptotically approach to these exact solutions
of the KP equation. We then discuss an application of our theory to the Mach
reflection problem in shallow water. This problem describes the resonant
interaction of solitary waves appearing in the reflection of an obliquely
incident wave onto a vertical wall, and it predicts an extra-ordinary four-fold
amplification of the wave at the wall. There are several numerical studies
confirming the prediction, but all indicate disagreements with the KP theory.
Contrary to those previous numerical studies, we find that the KP theory
actually provides an excellent model to describe the Mach reflection phenomena
when the higher order corrections are included to the quasi-two dimensional
approximation. We also present laboratory experiments of the Mach reflection
recently carried out by Yeh and his colleagues, and show how precisely the KP
theory predicts this wave behavior.Comment: 50 pages, 25 figure
AN INTERDISCIPLINARY APPROACH FOR THE SEISMIC VULNERABILITY ASSESSMENT OF HISTORICAL CENTRES IN MASONRY BUILDING AGGREGATES: APPLICATION TO THE CITY OF SCARPERIA, ITALY
Abstract. The seismic vulnerability of masonry building aggregates is very difficult to determine, since it is affected by many uncertainties. The most uncertain quantities concern the historical periodization of structural aggregates. Moreover, the studies made at the urban scale can hardly be thorough, and usually the knowledge achieved on the single units is not fully satisfactory, so that the structural designer has to deal with uncompleted architectonical surveys and partial data; one of the most important problems concerns the lack of knowledge about the boundary conditions between adjacent structures. In order to perform mechanical analyses, an extensive knowledge of materials and techniques adopted is required. In this paper, an integrated methodology for the seismic assessment of building aggregate is presented. It concerns a multidisciplinary knowledge-based approach calibrated over the historical centres and the urban aggregates; the procedure joins different aspects, such as the use of modern technologies for an integrated knowledge, plans reconstructions through archival documents, laser scanner digital survey of urban fronts, non-destructive investigations of the materials. GIS and BIM platforms have been used to implement and collect data in order to perform detailed analyses. The information allowed to assess the seismic vulnerability of the building aggregates and the expected damage scenarios through empirical methodologies. The city of Scarperia, founded a few kilometres from Florence during the Medieval Age and characterized by a medium seismicity, has been chosen as a case study for the presented procedure
SUPERSYMMETRIC OBJECTS IN GAUGED SUPERGRAVITIES
The formulation of a unified description of fundamental interactions has always been the most relevant and intriguing challenge in physics. To this end, a microscopic description of gravity and its consequent unification to electromagnetic and nuclear forces is needed. A theory realizing such a framework would be called theory of quantum gravity and, so far, the only consistent setup which seeks to realize this purpose is given by string theory. In this context, the divergences appearing in classical gravity theory are resolved at high energies in terms of contributions coming from the physics of extra dimensions. From this point of view, in order to gain the stability of higher-dimensional objects, like strings and branes, the formulation of quantum theories of gravity requires supersymmetry. These fundamental extended objects give rise, at low energies, to supersymmetric systems that are strongly coupled to gravity like, for example, black holes.
The theories describing the low-energy regime of objects living at higher dimensions are called supergravity theories and constitute the general setup of this thesis. These are classical interacting theories of gravity characterized by local supersymmetry. In this context, the description of supersymmetric objects is realized by classical solutions of the equations of motion and the link with high energies is controlled by the AdS/CFT correspondence and by string compactifications.
Starting from this general framework, in this thesis the results published during the three years of doctoral studies are presented. Three particular supergravity realizations in four- five- and seven dimensions are considered and discussed in relation to the physics of the supersymmetric objects arising from them. The analysis of the results in terms of the underlying microscopic theories is also developed with particular emphasis on string compactifications, giving rise to the three supergravities considered, and on the holographic interpretation, explaining the origin of the supersymmetric solutions presented in terms of non-perturbative states of string theory
A mathematical framework for critical transitions: normal forms, variance and applications
Critical transitions occur in a wide variety of applications including
mathematical biology, climate change, human physiology and economics. Therefore
it is highly desirable to find early-warning signs. We show that it is possible
to classify critical transitions by using bifurcation theory and normal forms
in the singular limit. Based on this elementary classification, we analyze
stochastic fluctuations and calculate scaling laws of the variance of
stochastic sample paths near critical transitions for fast subsystem
bifurcations up to codimension two. The theory is applied to several models:
the Stommel-Cessi box model for the thermohaline circulation from geoscience,
an epidemic-spreading model on an adaptive network, an activator-inhibitor
switch from systems biology, a predator-prey system from ecology and to the
Euler buckling problem from classical mechanics. For the Stommel-Cessi model we
compare different detrending techniques to calculate early-warning signs. In
the epidemics model we show that link densities could be better variables for
prediction than population densities. The activator-inhibitor switch
demonstrates effects in three time-scale systems and points out that excitable
cells and molecular units have information for subthreshold prediction. In the
predator-prey model explosive population growth near a codimension two
bifurcation is investigated and we show that early-warnings from normal forms
can be misleading in this context. In the biomechanical model we demonstrate
that early-warning signs for buckling depend crucially on the control strategy
near the instability which illustrates the effect of multiplicative noise.Comment: minor corrections to previous versio
Remote sensing in Michigan for land resource management: Highway impact assessment
An existing section of M-14 freeway constructed in 1964 and a potential extension from Ann Arbor to Plymouth, Michigan provided an opportunity for investigating the potential uses of remote sensing techniques in providing projective information needed for assessing the impact of highway construction. Remote sensing data included multispectral scanner imagery and aerial photography. Only minor effects on vegetation, soils, and land use were found to have occurred in the existing corridor. Adverse changes expected to take place in the corridor proposed for extension of the freeway can be minimized by proper design of drainage ditches and attention to good construction practices. Remote sensing can be used to collect and present many types of data useful for highway impact assessment on land use, vegetation categories and species, soil properties and hydrologic characteristics
Faster and better: a machine learning approach to corner detection
The repeatability and efficiency of a corner detector determines how likely
it is to be useful in a real-world application. The repeatability is importand
because the same scene viewed from different positions should yield features
which correspond to the same real-world 3D locations [Schmid et al 2000]. The
efficiency is important because this determines whether the detector combined
with further processing can operate at frame rate.
Three advances are described in this paper. First, we present a new heuristic
for feature detection, and using machine learning we derive a feature detector
from this which can fully process live PAL video using less than 5% of the
available processing time. By comparison, most other detectors cannot even
operate at frame rate (Harris detector 115%, SIFT 195%). Second, we generalize
the detector, allowing it to be optimized for repeatability, with little loss
of efficiency. Third, we carry out a rigorous comparison of corner detectors
based on the above repeatability criterion applied to 3D scenes. We show that
despite being principally constructed for speed, on these stringent tests, our
heuristic detector significantly outperforms existing feature detectors.
Finally, the comparison demonstrates that using machine learning produces
significant improvements in repeatability, yielding a detector that is both
very fast and very high quality.Comment: 35 pages, 11 figure
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