2,725 research outputs found
Nuclear electromagnetic dipole response with the Self-Consistent Green's Function formalism
Microscopic calculations of the electromagnetic response of medium-mass
nuclei are now feasible thanks to the availability of realistic nuclear
interactions with accurate saturation and spectroscopic properties, and the
development of large-scale computing methods for many-body physics. The purpose
is to compute isovector dipole electromagnetic (E1) response and related
quantities, i.e. integrated dipole cross section and polarizability, and
compare with data from photoabsorption and Coulomb excitation experiments. The
single-particle propagator is obtained by solving the Dyson equation, where the
self-energy includes correlations non-perturbatively through the Algebraic
Diagrammatic Construction (ADC) method. The particle-hole () polarization
propagator is treated in the Dressed Random Phase Approximation (DRPA), based
on an effective correlated propagator that includes some effects but
keeps the same computation scaling as the standard Hartree-Fock propagator. The
E1 responses for O, Ca and Ni have
been computed: the presence of a soft dipole mode of excitation for
neutron-rich nuclei is found, and there is a fair reproduction of the
low-energy part of the experimental excitation spectrum. This is reflected in a
good agreement with the empirical dipole polarizability values. For a realistic
interaction with an accurate reproduction of masses and radii up to medium-mass
nuclei, the Self-Consistent Green's Function method provides a good description
of the E1 response, especially in the part of the excitation spectrum below the
Giant Dipole Resonance. The dipole polarizability is largely independent from
the strategy of mapping the dressed propagator to a simplified one that is
computationally manageableComment: 14 pages, 12 figure
Using multi-agent systems to go beyond temporal patterns verification
A key step in formal verification is the translation of requirements into logic formulae. Various flavours of temporal logic are commonly used in academia and in industry to capture, among others, liveness and safety requirements. In the past two decades there has been a substantial amount of work in the area of verification of extensions of temporal logic. In this column I will provide a high level overview of some work in this area, focussing in particular on the verification of temporal-epistemic properties, showing how temporal-epistemic logics can be used to capture requirements that are common in many concrete systems, and describing a model checker for multi-agent systems called MCMAS
Effective theory for low-energy nuclear energy density functionals
We introduce a new class of effective interactions to be used within the
energy-density-functional approaches. They are based on regularized zero-range
interactions and constitute a consistent application of the effective-theory
methodology to low-energy phenomena in nuclei. They allow for defining the
order of expansion in terms of the order of derivatives acting on the
finite-range potential. Numerical calculations show a rapid convergence of the
expansion and independence of results of the regularization scale.Comment: 5 RevTex pages, 5 figures, misprints corrected, extended version, see
also http://iopscience.iop.org/0954-3899/labtalk-article/5109
Verification of the TESLA protocol in MCMAS-X
We present MCMAS-X, an extension of the OBDD-based model
checker MCMAS for multi-agent systems, to explicit and deductive knowledge. We use MCMAS-X to verify authentication properties in the TESLA secure stream protocol
Effective pseudopotential for energy density functionals with higher order derivatives
We derive a zero-range pseudopotential that includes all possible terms up to
sixth order in derivatives. Within the Hartree-Fock approximation, it gives the
average energy that corresponds to a quasi-local nuclear Energy Density
Functional (EDF) built of derivatives of the one-body density matrix up to
sixth order. The direct reference of the EDF to the pseudopotential acts as a
constraint that divides the number of independent coupling constants of the EDF
by two. This allows, e.g., for expressing the isovector part of the functional
in terms of the isoscalar part, or vice versa. We also derive the analogous set
of constraints for the coupling constants of the EDF that is restricted by
spherical, space-inversion, and time-reversal symmetries.Comment: 18 LaTeX pages, 2 EPS Figures, 27 Tables, and 18 files of the
supplemental material (LaTeX, Mathematica, and Fortran), introduction
rewritten, table XXVII and figure 2 corrected, in press in Physical Review
Software theory change for resilient near-complete specifications
Software evolution and its laws are essential for antifragile system design and development. In this paper we model early-stage
perfective and corrective changes to software system architecture in terms of logical operations of expansion and safe contraction
on a theory. As a result, we formulate an inference-based notion of property specification resilience for computational systems,
intended as resistance to change. The individuated resilient core of a software system is used to characterize adaptability properties
The secret santa problem.
Consider a digraph where the vertices represent people and
an arc (i, j) represents the possibility of i giving a gift to j. The basic question we pose is whether there is an anonymity-preserving “gift assignment” such that each person makes and receives exactly one gift, and such that no person i can infer the remaining gift assignments from the fact that i is assigned to give a gift to j. We formalize this problem as a graph property involving vertex disjoint circuit covers, give a polynomial algorithm to decide this property for any given graph and provide a computational validation of the algorithm
A typed natural deduction calculus to reason about secure trust
System integrity can be put at risk by unintentional transitivity of resource access. We present a natural deduction calculus for an access control model with an explicit trust function on resources. Its inference relation is designed to limit unintentionally transitive access from untrusted parties. We also offer results for ordered cut and normalization related to security and hint at a prototype implementation
Model checking multi-agent systems
A multi-agent system (MAS) is usually understood as a system composed of interacting
autonomous agents. In this sense, MAS have been employed successfully as a modelling
paradigm in a number of scenarios, especially in Computer Science. However, the process
of modelling complex and heterogeneous systems is intrinsically prone to errors: for this
reason, computer scientists are typically concerned with the issue of verifying that a system
actually behaves as it is supposed to, especially when a system is complex.
Techniques have been developed to perform this task: testing is the most common technique,
but in many circumstances a formal proof of correctness is needed. Techniques
for formal verification include theorem proving and model checking. Model checking
techniques, in particular, have been successfully employed in the formal verification of
distributed systems, including hardware components, communication protocols, security
protocols.
In contrast to traditional distributed systems, formal verification techniques for MAS are
still in their infancy, due to the more complex nature of agents, their autonomy, and
the richer language used in the specification of properties. This thesis aims at making
a contribution in the formal verification of properties of MAS via model checking. In
particular, the following points are addressed:
• Theoretical results about model checking methodologies for MAS, obtained by
extending traditional methodologies based on Ordered Binary Decision Diagrams (OBDDS) for temporal logics to multi-modal logics for time, knowledge, correct behaviour, and strategies of agents. Complexity results for model checking these logics
(and their symbolic representations).
• Development of a software tool (MCMAS) that permits the specification and verification
of MAS described in the formalism of interpreted systems.
• Examples of application of MCMAS to various MAS scenarios (communication, anonymity, games, hardware diagnosability), including experimental results, and comparison with other tools available
Combinatorial optimization based recommender systems.
Recommender systems exploit a set of established user preferences to predict topics or products that a new user might like [2]. Recommender systems have become an important research area in the field of information retrieval. Many approaches have been developed in recent years and the interest is very high. However, despite all the efforts, recommender systems are still in need of further development and more advanced recommendation modelling methods, as these systems must take into account additional requirements on user preferences, such as geographic search and social networking. This fact, in particular, implies that the recommendation must be much more “personalized” than it used to be.
In this paper, we describe the recommender system used in the “DisMoiOu”(“TellMeWhere” in French) on-line service (http://dismoiou.fr), which provides the user with advice on places that may be of interest to him/her; the definition of “interest” in this context is personalized taking into account the geographical position of the user (for example when the service is used with portable phones such as the Apple iPhone), his/her past ratings, and the
ratings of his/her neighbourhood in a known social network.
Using the accepted terminology [6], DisMoiOu is mainly a Collaborative Filtering System (CFS): it employs opinions collected from similar users to suggest likely places. By contrast with existing recommender systems, ours puts
together the use of a graph theoretical model [4] and that of combinatorial optimization methods [1]. Broadly speaking, we encode known relations between users and places and users and other users by means of weighted graphs. We then define essential components of the system by means of combinatorial optimization problems on a reformulation of these graphs, which are finally used
to derive a ranking on the recommendations associated to pairs (user,place). Preliminary computational results on the three classical evaluation parameters for recommender systems (accuracy, recall, precision [3]) show that our system performs well with respect to accuracy and recall, but precision results need to be improved
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