Gravitational collapse with tachyon field and barotropic fluid

A particular class of space-time, with a tachyon field, \phi, and a barotropic fluid constituting the matter content, is considered herein as a model for gravitational collapse. For simplicity, the tachyon potential is assumed to be of inverse square form i.e., V(\phi) \sim \phi^{-2}. Our purpose, by making use of the specific kinematical features of the tachyon, which are rather different from a standard scalar field, is to establish the several types of asymptotic behavior that our matter content induces. Employing a dynamical system analysis, complemented by a thorough numerical study, we find classical solutions corresponding to a naked singularity or a black hole formation. In particular, there is a subset where the fluid and tachyon participate in an interesting tracking behaviour, depending sensitively on the initial conditions for the energy densities of the tachyon field and barotropic fluid. Two other classes of solutions are present, corresponding respectively, to either a tachyon or a barotropic fluid regime. Which of these emerges as dominant, will depend on the choice of the barotropic parameter, \gamma. Furthermore, these collapsing scenarios both have as final state the formation of a black hole.Comment: 18 pages, 7 figures. v3: minor changes. Final version to appear in GR

The behaviour of repeat visitors to museums: Review and empirical findings

This study presents a theoretical and operational framework for analysing repeat visit to museums. Starting from the literature on repeat visit in tourism, the specificities of these cultural attractions are made explicit through a review of theoretical and applied works. Consistently with previous contributors, the paper suggests that the analysis of actual past behaviours has to be preferred to the one of attitudes. The application of proper econometric models is also remarked in order to put into account individual profiles. Information coming from three techniques is then used in an integrated way in order to provide a more comprehensive view of the phenomenon. Evidence from an ad hoc survey suggests the necessity to give a greater attention to perceived cultural value during the visit, promoting cultural events during the week and addressed to children, and taking care of those visitors that come from far places also through an integrated tourist supply. © 2013 Springer Science+Business Media Dordrecht

Novel HIV-1 Knockdown Targets Identified by an Enriched Kinases/Phosphatases shRNA Library Using a Long-Term Iterative Screen in Jurkat T-Cells

HIV-1 is a complex retrovirus that uses host machinery to promote its replication. Understanding cellular proteins involved in the multistep process of HIV-1 infection may result in the discovery of more adapted and effective therapeutic targets. Kinases and phosphatases are a druggable class of proteins critically involved in regulation of signal pathways of eukaryotic cells. Here, we focused on the discovery of kinases and phosphatases that are essential for HIV-1 replication but dispensable for cell viability. We performed an iterative screen in Jurkat T-cells with a short-hairpin-RNA (shRNA) library highly enriched for human kinases and phosphatases. We identified 14 new proteins essential for HIV-1 replication that do not affect cell viability. These proteins are described to be involved in MAPK, JNK and ERK pathways, vesicular traffic and DNA repair. Moreover, we show that the proteins under study are important in an early step of HIV-1 infection before viral integration, whereas some of them affect viral transcription/translation. This study brings new insights for the complex interplay of HIV-1/host cell and opens new possibilities for antiviral strategies

SLWV- A Logic Programing Theorem Prover

Abstract: The purpose of this work is to define a theorem prover that retains the procedural aspects of logic programing. The proof system we propose (SLWV 1 resolution) is defined for a set of clauses in the implicational form (keeping to the form of logic programs), not requiring contrapositives, and has an execution method that respects the execution order of literals in a clause, preserving the procedural flavor of logic programming. SLWV resolution can be seen as a combination of SL-resolution [Chan73] and case-analysis, that admits a form of linear derivation. We prove its soundness and completeness, give it an operational semantics by defining a standard derivation, and produce an implementation. Our work can be seen as an extension to logic programs that goes beyond normal programs, as defined in [Lloy87], and thus beyond(positive) definite clause programming, by allowing also definite negative heads. Thus we admit program clauses with both positive and (classically) negated atoms conjoined in the body, and at most one literal as its head (clauses with disjunctions of literals in the head are transformed into a single clause of that form). As this approach does not require alternative clause contrapositives, it provides for better control over the search space. We provide a method of execution keeping to the implicational clausal form of program statements typical of Prolog (without the use of clause contrapositives), adding an increased expressiveness, but at a tolerable computational cost for regular Prolog programs. The implementation relies on the source program being preprocessed into directly executable Prolog. Since preprocessing only involves the addition of three additional variables to each predicate definition while keeping the overall program structure untouched, a directly recognizable execution pattern that mimics Prolog is obtained: this can be useful in debugging

Modelling Agent Interaction in Logic Programming

We present a logic programming framework implemented over Prolog which is able to model an agent's mental state. An agent is modeled by a set of extended logic programming rules representing the agent's behavior, attitudes (beliefs, intentions, and goals), world knowledge, and temporal and reasoning procedures. At each stage the agents's mental state is defined by the well founded model of the extended logic program plus some constraints. Via this modeling an agent is able to interact with other agents, updating and revising its mental state after each event. The revision process includes the ability to remove contradictions in the agent's mental state. It is shown how this framework can handle interactions between agents with different behavior rules, namely, with different levels of cooperativeness and credulity. 1 Introduction In order to interact with other agents, an agent needs the ability to model its mental state. Namely, it is necessary to represent its attitudes (beliefs, in..