Graviton Propagation in an Asymmetric Warped Background: Lorentz Violation and the Null Energy Condition

The graviton propagation in an asymmetric background is studied. The background is a configuration in the six-dimensional Salam-Sezgin model, in which a 3-form H-field turned on [JHEP 0910(2009)086]. The compact dimensions form a cylindrical space with branes as boundaries. The background gets asymmetry due to the H-field and violates the Lorentz symmetry. We derive the graviton equation in this background and show that it gets massless mode traveling with superluminal speed. A tower of K-K modes exists with a mass gap. On the other hand, it is known that breaking the Lorentz symmetry on an asymmetric background is constrained by the null energy condition. This no-go theorem doesn't work well in six-dimensional space-times and by this model we provide a counterexample for which the null energy condition is satisfied while the Lorentz symmetry is gravitationally violated.Comment: 20 pages, 1 figure. v2: published version, more phenomenological discussions added, references added, numerical error fixe

Speeding Multicast by Acknowledgment Reduction Technique (SMART)

We present a novel feedback protocol for wireless broadcast networks that utilize linear network coding. We consider transmission of packets from one source to many receivers over a single-hop broadcast erasure channel. Our method utilizes a predictive model to request feedback only when the probability that all receivers have completed decoding is significant. In addition, our proposed NACK-based feedback mechanism enables all receivers to request, within a single time slot, the number of retransmissions needed for successful decoding. We present simulation results as well as analytical results that show the favorable scalability of our technique as the number of receivers, file size, and packet erasure probability increase. We also show the robustness of this scheme to uncertainty in the predictive model, including uncertainty in the number of receiving nodes and the packet erasure probability, as well as to losses of the feedback itself. Our scheme, SMART, is shown to perform nearly as well as an omniscient transmitter that requires no feedback. Furthermore, SMART, is shown to outperform current state of the art methods at any given erasure probability, file size, and numbers of receivers

Parameter selection in particle swarm optimisation: a survey

Nowadays, particle swarm optimisation (PSO) is one of the most commonly used optimisation techniques. However, PSO parameters significantly affect its computational behaviour. That is, while it exposes desirable computational behaviour with some settings, it does not behave so by some other settings, so the way for setting them is of high importance. This paper explains and discusses thoroughly about various existent strategies for setting PSO parameters, provides some hints for its parameter setting and presents some proposals for future research on this area. There exists no other paper in literature that discusses the setting process for all PSO parameters. Using the guidelines of this paper can be strongly useful for researchers in optimisation-related fields

How to regulate a financial market? The impact of the 1893-1898 regulatory reforms on the Paris Bourse

Theoretical and historical experience suggests a financial centre may either include a single, consolidated and loosely regulated stock exchange attracting all intermediaries and actors, or a variety of exchanges going from strictly regulated to completely unregulated and adapted to the needs of different categories of intermediaries, investors and issuers. Choosing between these two solutions is uneasy because few substantial changes occur at this "meta-regulatory" level. The history of the Paris exchanges provides a good example, since two legal changes in opposite directions occurred in the late 19th century, when Paris was the second financial centre in the world. In 1893, a law threatened the existing two-exchanges equilibrium by diminishing the advantages of the more regulated exchange; in 1898, another law brought them back. We analyse the impact of these two changes on the competition between the exchanges in terms of securities listed, traded volumes and spreads. We conclude competition among exchanges is a delicate matter and efficiency is not always where one would think.Paris stock exchange ; microstructure ; monopoly ; regulation