10,079 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
A solution to the anisotropy problem in bouncing cosmologies
Bouncing cosmologies are often proposed as alternatives to standard inflation
for the explanation of the homogeneity and flatness of the universe. In such
scenarios, the present cosmological expansion is preceded by a contraction
phase. However, during the contraction, in general the anisotropy of the
universe grows and eventually leads to a chaotic mixmaster behavior. This would
either be hard to reconcile with observations or even lead to a singularity
instead of the bounce. In order to preserve a smooth and isotropic bounce, the
source for the contraction must have a super-stiff equation of state with
. In this letter we propose a new mechanism to solve the anisotropy
problem for any low-energy value of by arguing that high energy physics
leads to a modification of the equation of state, with the introduction of
non-linear terms. In such a scenario, the anisotropy is strongly suppressed
during the high energy phase, allowing for a graceful isotropic bounce, even
when the low-energy value of is smaller than unity.Comment: 9 pages, accepted for publication in JCA
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
Dynamics of Silent Universes
We investigate the local non--linear dynamics of irrotational dust with
vanishing magnetic part of the Weyl tensor, . Once coded in the initial
conditions, this dynamical restriction is respected by the relativistic
evolution equations. Thus, the outcome of the latter are {\it exact solutions}
for special initial conditions with , but with no symmetries: they
describe inhomogeneous triaxial dynamics generalizing that of a fluid element
in a Tolman--Bondi, Kantowski--Sachs or Szekeres geometry. A subset of these
solutions may be seen as (special) perturbations of Friedmann models, in the
sense that there are trajectories in phase--space that pass arbitrarily close
to the isotropic ones. We find that the final fate of ever--expanding
configurations is a spherical void, locally corresponding to a Milne universe.
For collapsing configurations we find a whole family of triaxial attractors,
with vanishing local density parameter . These attractors locally
correspond to Kasner vacuum solutions: there is a single physical configuration
collapsing to a degenerate {\it pancake}, while the generic configuration
collapses to a triaxial {\it spindle} singularity. These {\it silent universe}
models may provide a fair representation of the universe on super horizon
scales. Moreover, one might conjecture that the non--local information carried
by becomes negligible during the late highly non--linear stages of
collapse, so that the attractors we find may give all of the relevant expansion
or collapse configurations of irrotational dust.Comment: 40 pages with 4 figures, compressed and uuencoded PostScript file,
submitted to ApJ, SISSA preprint Ref. 85/94/
A molecular dynamics simulation of water confined in a cylindrical SiO2 pore
A molecular dynamics simulation of water confined in a silica pore is
performed in order to compare it with recent experimental results on water
confined in porous Vycor glass at room temperature. A cylindrical pore of 40 A
is created inside a vitreous SiO2 cell, obtained by computer simulation. The
resulting cavity offers to water a rough hydrophilic surface and its geometry
and size are similar to those of a typical pore in porous Vycor glass. The
site-site distribution functions of water inside the pore are evaluated and
compared with bulk water results. We find that the modifications of the
site-site distribution functions, induced by confinement, are in qualitative
agreement with the recent neutron diffraction experiment, confirming that the
disturbance to the microscopic structure of water mainly concerns orientational
arrangement of neighbouring molecules. A layer analysis of MD results indicates
that, while the geometrical constraint gives an almost constant density profile
up to the layers closest to the interface, with an uniform average number of
hydrogen bonds (HB), the hydrophilic interaction produces the wetting of the
pore surface at the expenses of the adjacent water layers. Moreover the
orientational disorder togheter with a reduction of the average number of HB
persists in the layers close to the interface, while water molecules cluster in
the middle of the pore at a density and with a coordination similar to bulk
water.Comment: RevTex, 11 pages, 12 figures; to appear in June 15 issue of J. Chem.
Phy
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
On Relativistic Perturbations of Second and Higher Order
We present the results of a study of the gauge dependence of spacetime
perturbations. In particular, we consider gauge invariance in general, we give
a generating formula for gauge transformations to an arbitrary order n, and
explicit transformation rules at second order.Comment: 6 pages, latex, with special style included, Proceedings of the 12th
Italian Conference on General Relativity and Gravitational Physic
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 programmer’s 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
Towards a novel wave-extraction method for numerical relativity
We present the recent results of a research project aimed at constructing a
robust wave extraction technique for numerical relativity. Our procedure makes
use of Weyl scalars to achieve wave extraction. It is well known that, with a
correct choice of null tetrad, Weyl scalars are directly associated to physical
properties of the space-time under analysis in some well understood way. In
particular it is possible to associate with the outgoing gravitational
radiation degrees of freedom, thus making it a promising tool for numerical
wave--extraction. The right choice of the tetrad is, however, the problem to be
addressed. We have made progress towards identifying a general procedure for
choosing this tetrad, by looking at transverse tetrads where .
As a direct application of these concepts, we present a numerical study of
the evolution of a non-linearly disturbed black hole described by the
Bondi--Sachs metric. This particular scenario allows us to compare the results
coming from Weyl scalars with the results coming from the news function which,
in this particular case, is directly associated with the radiative degrees of
freedom. We show that, if we did not take particular care in choosing the right
tetrad, we would end up with incorrect results.Comment: 6 pages, 1 figure, to appear in the Proceedings of the Albert
Einstein Century International Conference, Paris, France, 200
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