710 research outputs found
First Order Description of D=4 static Black Holes and the Hamilton-Jacobi equation
In this note we discuss the application of the Hamilton-Jacobi formalism to
the first order description of four dimensional spherically symmetric and
static black holes. In particular we show that the prepotential characterizing
the flow coincides with the Hamilton principal function associated with the
one-dimensional effective Lagrangian. This implies that the prepotential can
always be defined, at least locally in the radial variable and in the moduli
space, both in the extremal and non-extremal case and allows us to conclude
that it is duality invariant. We also give, in this framework, a general
definition of the ``Weinhold metric'' in terms of which a necessary condition
for the existence of multiple attractors is given. The Hamilton-Jacobi
formalism can be applied both to the restricted phase space where the
electromagnetic potentials have been integrated out as well as in the case
where the electromagnetic potentials are dualized to scalar fields using the
so-called three-dimensional Euclidean approach. We give some examples of
application of the formalism, both for the BPS and the non-BPS black holes.Comment: A clarifying discussion on the existence of the prepotential and a
comment on multiple attractors are added; typos corrected, references adde
First Order Description of Black Holes in Moduli Space
We show that the second order field equations characterizing extremal
solutions for spherically symmetric, stationary black holes are in fact implied
by a system of first order equations given in terms of a prepotential W. This
confirms and generalizes the results in [14]. Moreover we prove that the
squared prepotential function shares the same properties of a c-function and
that it interpolates between M^2_{ADM} and M^2_{BR}, the parameter of the
near-horizon Bertotti-Robinson geometry. When the black holes are solutions of
extended supergravities we are able to find an explicit expression for the
prepotentials, valid at any radial distance from the horizon, which reproduces
all the attractors of the four dimensional N>2 theories. Far from the horizon,
however, for N-even our ansatz poses a constraint on one of the U-duality
invariants for the non-BPS solutions with Z \neq 0. We discuss a possible
extension of our considerations to the non extremal case.Comment: Some points clarified, a comment on the interpretation of the
prepotential W in terms of c-function added, typos corrected. Version to
appear on JHE
Multi-time delay, multi-point Linear Stochastic Estimation of a cavity shear layer velocity from wall-pressure measurements
Multi-time-delay Linear Stochastic Estimation (MTD-LSE) technique is thoroughly described, focusing on its fundamental properties and potentialities. In the multi-time-delay ap- proach, the estimate of the temporal evolution of the velocity at a given location in the flow field is obtained from multiple past samples of the unconditional sources. The technique is applied to estimate the velocity in a cavity shear layer flow, based on wall-pressure measurements from multiple sensor
Two-Center Black Holes Duality-Invariants for stu Model and its lower-rank Descendants
We classify 2-center extremal black hole charge configurations through
duality-invariant homogeneous polynomials, which are the generalization of the
unique invariant quartic polynomial for single-center black holes based on
homogeneous symmetric cubic special Kaehler geometries. A crucial role is
played by an horizontal SL(p,R) symmetry group, which classifies invariants for
p-center black holes. For p = 2, a (spin 2) quintet of quartic invariants
emerge. We provide the minimal set of independent invariants for the rank-3 N =
2, d = 4 stu model, and for its lower-rank descendants, namely the rank-2 st^2
and rank-1 t^3 models; these models respectively exhibit seven, six and five
independent invariants. We also derive the polynomial relations among these and
other duality invariants. In particular, the symplectic product of two charge
vectors is not independent from the quartic quintet in the t^3 model, but
rather it satisfies a degree-16 relation, corresponding to a quartic equation
for the square of the symplectic product itself.Comment: 1+31 pages; v2: amendments in Sec. 9, App. C added, other minor
refinements, Refs. added; v3: Ref. added, typos fixed. To appear on
J.Math.Phy
Effects of Ti6Al4V mechanical and thermal surface modification on the adhesion of a chitosan-bioactive glass coating
Biomedical implants interact with human tissues introducing significant perturbation into the body. Implant surfaces can be then functionalized enabling better biocompatibility. At the same time, the additional use of a coating provides further functions such as corrosion protection, osteointegration, and drug delivery. In this context, a composite made of chitosan and bioactive glass nanoparticles has been used for coating Ti6Al4V alloy samples processed beforehand using different processes, i.e., polishing, milling, grit blasting, and electrical discharge machining. Experiments have been carried out to correlate substrate surface conditions and coating effectiveness in terms of scratch resistance with the final aim to obtain suitable guidelines to improve substrate-coating performances
Sistemi di trasmissione WiFi per il monitoraggio sismico del Vesuvio
First-year engineering students at the University of Queensland used an interactive webbook to acquire information skills. These helped them search information resources for their projects, which they are required to undertake as part of the subject Introduction to professional engineering. The information skills exercise was an integral part of the project and worth 10% of the overall assessment. The exercises were only available on the Web, allowing the students to enter their answers from home or wherever they had access to the Internet. All answers were marked automatically using a database of all possible answers. Students were able to go back to check their answers. Students were assessed on both their responses to the exercises and also their final bibliography which largely reflected the impact of the webbook. The entire process was evaluated. This paper presents the process and the outcomes of the first-year engineering project involving use of WWW for information skills instruction. The webbooks can be found at http://www.library.uq.edu.au/9e105/
HOW TO EXTRACT USEFUL INFORMATION ABOUT THE DECAY OF BASS RELIEVES IN ARCHAEOLOGICAL AREA
Abstract. Cultural Heritage goods represent the memory and the history of the civilization. Notwithstanding, there are not sufficient public resources to guarantee their preservation and maintenance. Nowadays between several geomatic techniques available, the pillar for the preservation of mankinds heritage is the low cost close photogrammetric acquisition. The advantages of virtual reconstructions based on Multi View Stereo (MVS) and Structure from Motion (SfM) algorithms is extended from the heritage documentation to its virtualization or modelling. The digital preservation of archaeological sites is committed in more agile and friendly procedures that give automatic extraction of information to perform in depth analysis over ancient artefacts. In the field of CH research, the characterization and classification of the conservation state of the materials composing the surface of the artefacts are essential to study their damage. The first step for conservation state of a goods is the study of the changes in different times. The possibility to automatically study this time modification due to different factor represents a key point for the archaeologists' work. With this in mind, the aim of this work is to propose a completely automatic methods for change detection between three data set acquired in different époques. The work flow applied is based on the unsupervised clustering techniques applied on a combination of two type of differences images. The results, unlike the objective, demonstrate that the unsupervised methods are not effectiveness in the CH study, instead of the supervised methods that outperforms in terms of reliability of results.</p
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