7,663 research outputs found
A formal support to business and architectural design for service-oriented systems
Architectural Design Rewriting (ADR) is an approach for the design of software architectures developed within Sensoria by reconciling graph transformation and process calculi techniques. The key feature that makes ADR a suitable and expressive framework is the algebraic handling of structured graphs, which improves the support for specification, analysis and verification of service-oriented architectures and applications. We show how ADR is used as a formal ground for high-level modelling languages and approaches developed within Sensoria
Hierarchical models for service-oriented systems
We present our approach to the denotation and representation of hierarchical graphs: a suitable algebra of hierarchical graphs and two domains of interpretations. Each domain of interpretation focuses on a particular perspective of the graph hierarchy: the top view (nested boxes) is based on a notion of embedded graphs while the side view (tree hierarchy) is based on gs-graphs. Our algebra can be understood as a high-level language for describing such graphical models, which are well suited for defining graphical representations of service-oriented systems where nesting (e.g. sessions, transactions, locations) and linking (e.g. shared channels, resources, names) are key aspects
Evaluating the performance of model transformation styles in Maude
Rule-based programming has been shown to be very successful in many application areas. Two prominent examples are the specification of model transformations in model driven development approaches and the definition of structured operational semantics of formal languages. General rewriting frameworks such as Maude are flexible enough to allow the programmer to adopt and mix various rule styles. The choice between styles can be biased by the programmerâs background. For instance, experts in visual formalisms might prefer graph-rewriting styles, while experts in semantics might prefer structurally inductive rules. This paper evaluates the performance of different rule styles on a significant benchmark taken from the literature on model transformation. Depending on the actual transformation being carried out, our results show that different rule styles can offer drastically different performances. We point out the situations from which each rule style benefits to offer a valuable set of hints for choosing one style over the other
An Algebra of Hierarchical Graphs
We define an algebraic theory of hierarchical graphs, whose axioms characterise graph isomorphism: two terms are equated exactly when they represent the same graph. Our algebra can be understood as a high-level language for describing graphs with a node-sharing, embedding structure, and it is then well suited for defining graphical representations of software models where nesting and linking are key aspects
General formulation of general-relativistic higher-order gauge-invariant perturbation theory
Gauge-invariant treatments of general-relativistic higher-order perturbations
on generic background spacetime is proposed. After reviewing the general
framework of the second-order gauge-invariant perturbation theory, we show the
fact that the linear-order metric perturbation is decomposed into
gauge-invariant and gauge-variant parts, which was the important premis of this
general framework. This means that the development the higher-order
gauge-invariant perturbation theory on generic background spacetime is
possible. A remaining issue to be resolve is also disscussed.Comment: 4 pages, no figure. (v3) some explanations are added and a reference
is adde
Two-parameter non-linear spacetime perturbations: gauge transformations and gauge invariance
An implicit fundamental assumption in relativistic perturbation theory is
that there exists a parametric family of spacetimes that can be Taylor expanded
around a background. The choice of the latter is crucial to obtain a manageable
theory, so that it is sometime convenient to construct a perturbative formalism
based on two (or more) parameters. The study of perturbations of rotating stars
is a good example: in this case one can treat the stationary axisymmetric star
using a slow rotation approximation (expansion in the angular velocity Omega),
so that the background is spherical. Generic perturbations of the rotating star
(say parametrized by lambda) are then built on top of the axisymmetric
perturbations in Omega. Clearly, any interesting physics requires non-linear
perturbations, as at least terms lambda Omega need to be considered. In this
paper we analyse the gauge dependence of non-linear perturbations depending on
two parameters, derive explicit higher order gauge transformation rules, and
define gauge invariance. The formalism is completely general and can be used in
different applications of general relativity or any other spacetime theory.Comment: 22 pages, 3 figures. Minor changes to match the version appeared in
Classical and Quantum Gravit
The central structure of Broad Absorption Line QSOs: observational characteristics in the cm-mm wavelength domain
Accounting for ~20% of the total QSO population, Broad Absorption Line QSOs
are still an unsolved problem in the AGN context. They present wide troughs in
the UV spectrum, due to material with velocities up to 0.2 c toward the
observer. The two models proposed in literature try to explain them as a
particular phase of the evolution of QSOs or as normal QSOs, but seen from a
particular line of sight.
We built a statistically complete sample of Radio-Loud BAL QSOs, and carried
out an observing campaign to piece together the whole spectrum in the cm
wavelength domain, and highlight all the possible differences with respect to a
comparison sample of Radio-Loud non-BAL QSOs. VLBI observations at high angular
resolution have been performed, to study the pc-scale morphology of these
objects. Finally, we tried to detect a possible dust component with
observations at mm-wavelengths.
Results do not seem to indicate a young age for all BAL QSOs. Instead a
variety of orientations and morphologies have been found, constraining the
outflows foreseen by the orientation model to have different possible angles
with respect to the jet axis
A new CAE procedure for railway wheel tribological design
New demands are being imposed on railway wheel wear and reliability to increase the time between wheel reprofiling, improve safety and reduce total wheelset lifecycle costs. In parallel with these requirements, changes in railway vehicle missions are also occurring. These have led to the need to operate rolling stock on track with low as well as high radius curves; increase speeds and axle loads; and contend with a decrease in track quality due to a reduction in maintenance. These changes are leading to an increase in the severity of the wheel/rail contact conditions, which may increase the likelihood of wear or damage occurring.
The aim of this work was to develop a new CAE design methodology to deal with these demands. The model should integrate advanced numerical tools for modelling of railway vehicle dynamics and suitable models to predict wheelset durability under typical operating conditions. This will help in designing wheels for minimum wheel and rail wear; optimising railway vehicle suspensions and wheel profiles; maintenance scheduling and the evaluation of new wheel materials. This work was carried out as part of the project HIPERWheel, funded by the European Community within the Vth Framework Programme
Algebras for Tree Decomposable Graphs
Complex problems can be sometimes solved efficiently via recursive decomposition strategies. In this line, the tree decomposition approach equips problems modelled as graphs with tree-like parsing structures. Following Milnerâs flowgraph algebra, in a previous paper two of the authors introduced a strong network algebra to represent open graphs (up to isomorphism), so that homomorphic properties of open graphs can be computed via structural recursion. This paper extends this graphical-algebraic foundation to tree decomposable graphs. The correspondence is shown: (i) on the algebraic side by a loose network algebra, which relaxes the restriction reordering and scope extension axioms of the strong one; and (ii) on the graphical side by Milnerâs binding bigraphs, and elementary tree decompositions. Conveniently, an interpreted loose algebra gives the evaluation complexity of each graph decomposition. As a key contribution, we apply our results to dynamic programming (DP). The initial statement of the problem is transformed into a term (this is the secondary optimisation problem of DP). Noting that when the scope extension axiom is applied to reduce the scope of the restriction, then also the complexity is reduced (or not changed), only so-called canonical terms (in the loose algebra) are considered. Then, the canonical term is evaluated obtaining a solution which is locally optimal for complexity. Finding a global optimum remains an NP-hard problem
Linearisation instability of gravity waves?
Gravity waves in irrotational dust spacetimes are characterised by nonzero
magnetic Weyl tensor . In the linearised theory, the divergence of
is set to zero. Recently Lesame et al. [Phys. Rev. D {\bf 53}, 738
(1996)] presented an argument to show that, in the exact nonlinear theory, forces , thus implying a linearisation instability for gravity
waves interacting with matter. However a sign error in the equations
invalidates their conclusion. Bianchi type V spacetimes are shown to include
examples with . An improved covariant formalism is used to
show that in a generic irrotational dust spacetime, the covariant constraint
equations are preserved under evolution. It is shown elsewhere that \mbox{div}
H=0 does not generate further conditions.Comment: 8 pages Revtex; to appear Phys. Rev.
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