A Simple Particle Action from a Twistor Parametrization of AdS_5

The SO(4,2) isometries of AdS_5 are realized non-linearly on its horospherical coordinates (x^m,\rho). On the other hand, Penrose twistors have long been known to linearly realize these symmetries on 4-dimensional Minkowski space, the boundary of AdS_5, parametrized by x^m. Here we extend the twistor construction and define a pair of twistors, allowing us to include a radial coordinate in the construction. The linear action of SO(4,2) on the twistors induces the correct isometries of AdS_5. We apply this new construction to the study of the dynamics of a massive particle in AdS_5. We show that in terms of the twistor variables the action takes a simple form of a 1-dimensional gauge theory. Our result might open up the possibility to find a simple worldvolume action also for the string propagating on AdS_5.Comment: 11 pages, LaTe

An algebraic basis for specifying and enforcing access control in security systems

Security services in a multi-user environment are often based on access control mechanisms. Static aspects of an access control policy can be formalised using abstract algebraic models. We integrate these static aspects into a dynamic framework considering requesting access to resources as a process aiming at the prevention of access control violations when a program is executed. We use another algebraic technique, monads, as a meta-language to integrate access control operations into a functional programming language. The integration of monads and concepts from a denotational model for process algebras provides a framework for programming of access control in security systems

Semantic model-driven development of web service architectures.

Building service-based architectures has become a major area of interest since the advent of Web services. Modelling these architectures is a central activity. Model-driven development is a recent approach to developing software systems based on the idea of making models the central artefacts for design representation, analysis, and code generation. We propose an ontology-based engineering methodology for semantic model-driven composition and transformation of Web service architectures. Ontology technology as a logic-based knowledge representation and reasoning framework can provide answers to the needs of sharable and reusable semantic models and descriptions needed for service engineering. Based on modelling, composition and code generation techniques for service architectures, our approach provides a methodological framework for ontology-based semantic service architecture

The inverse mean curvature flow in cosmological spacetimes

We prove that the leaves of an inverse mean curvature flow provide a foliation of a future end of a cosmological spacetime $N$ under the necessary and sufficent assumptions that $N$ satisfies a future mean curvature barrier condition and a strong volume decay condition. Moreover, the flow parameter $t$ can be used to define a new physically important time function.Comment: 22 pages, v2: Some minor typos corrected, will be the published versio

Super M-brane actions in adS_4 x S^7 and adS_7 x S^4

The world-volume action of the M2 brane and the M5-brane in an adS_4 x S^7 and a adS_7 x S^4 background is derived to all orders in anticommuting superspace coordinates. Contrary to recent constructions of super p-brane actions relying on supercoset methods, we only use 11 dimensional supergravity torsion and curvature constraints. Complete agreement between the two methods is found. The simplification of the actions by choosing a suitable kappa-gauge is discussed.Comment: 14 pages, LaTeX, discussion of simplification by kappa-gauge corrected, reference adde