A Continuation Method for Nash Equilibria in Structured Games

Structured game representations have recently attracted interest as models for multi-agent artificial intelligence scenarios, with rational behavior most commonly characterized by Nash equilibria. This paper presents efficient, exact algorithms for computing Nash equilibria in structured game representations, including both graphical games and multi-agent influence diagrams (MAIDs). The algorithms are derived from a continuation method for normal-form and extensive-form games due to Govindan and Wilson; they follow a trajectory through a space of perturbed games and their equilibria, exploiting game structure through fast computation of the Jacobian of the payoff function. They are theoretically guaranteed to find at least one equilibrium of the game, and may find more. Our approach provides the first efficient algorithm for computing exact equilibria in graphical games with arbitrary topology, and the first algorithm to exploit fine-grained structural properties of MAIDs. Experimental results are presented demonstrating the effectiveness of the algorithms and comparing them to predecessors. The running time of the graphical game algorithm is similar to, and often better than, the running time of previous approximate algorithms. The algorithm for MAIDs can effectively solve games that are much larger than those solvable by previous methods

Real time plasma equilibrium reconstruction in a Tokamak

The problem of equilibrium of a plasma in a Tokamak is a free boundary problemdescribed by the Grad-Shafranov equation in axisymmetric configurations. The right hand side of this equation is a non linear source, which represents the toroidal component of the plasma current density. This paper deals with the real time identification of this non linear source from experimental measurements. The proposed method is based on a fixed point algorithm, a finite element resolution, a reduced basis method and a least-square optimization formulation

Improved position measurement of nano electromechanical systems using cross correlations

We consider position measurements using the cross-correlated output of two tunnel junction position detectors. Using a fully quantum treatment, we calculate the equation of motion for the density matrix of the coupled detector-detector-mechanical oscillator system. After discussing the presence of a bound on the peak-to-background ratio in a position measurement using a single detector, we show how one can use detector cross correlations to overcome this bound. We analyze two different possible experimental realizations of the cross correlation measurement and show that in both cases the maximum cross-correlated output is obtained when using twin detectors and applying equal bias to each tunnel junction. Furthermore, we show how the double-detector setup can be exploited to drastically reduce the added displacement noise of the oscillator.Comment: 9 pages, 1 figure; v2: new Sec.

The Refractory-to-Ice Mass Ratio in Comets

We review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the Refractory-to-Ice mass ratio inside the nucleus, which constrains the origin of comets. Such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. This makes the Refractory-to-Ice mass ratio inside the nucleus up to ten times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. We review the Refractory-to-Ice mass ratios available for the comet nuclei visited by space missions, and for the Kuiper Belt Objects with well defined bulk density, finding the 1-σ lower limit of 3. Therefore, comets and KBOs may have less water than CI-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc

Dual Liquid Flyback Booster for the Space Shuttle

Liquid Flyback Boosters provide an opportunity to improve shuttle safety, increase performance, and reduce operating costs. The objective of the LFBB study is to establish the viability of a LFBB configuration to integrate into the shuffle vehicle and meet the goals of the Space Shuttle upgrades program. The design of a technically viable LFBB must integrate into the shuffle vehicle with acceptable impacts to the vehicle elements, i.e. orbiter and external tank and the shuttle operations infrastructure. The LFBB must also be capable of autonomous return to the launch site. The smooth integration of the LFBB into the space shuttle vehicle and the ability of the LFBB to fly back to the launch site are not mutually compatible capabilities. LFBB wing configurations optimized for ascent must also provide flight quality during the powered return back to the launch site. This paper will focus on the core booster design and ascent performance. A companion paper 'Conceptual Design for a Space Shuttle Liquid Flyback Booster' will focus on the flyback system design and performance. The LFBB study developed design and aerodynamic data to demonstrate the viability of a dual booster configuration to meet the shuttle upgrade goals, i.e. enhanced safety, improved performance and reduced operations costs

Polarization and angular distribution of the radiation emitted in laser-assisted recombination

The effect of an intense external linear polarized radiation field on the angular distributions and polarization states of the photons emitted during the radiative recombination is investigated. It is predicted, on symmetry grounds, and corroborated by numerical calculations of approximate recombination rates, that emission of elliptically polarized photons occurs when the momentum of the electron beam is not aligned to the direction of the oscillating field. Moreover, strong modifications to the angular distributions of the emitted photons are induced by the external radiation field.Comment: 5 pages, 3 figure

HIV with contact-tracing: a case study in Approximate Bayesian Computation

Missing data is a recurrent issue in epidemiology where the infection process may be partially observed. Approximate Bayesian Computation, an alternative to data imputation methods such as Markov Chain Monte Carlo integration, is proposed for making inference in epidemiological models. It is a likelihood-free method that relies exclusively on numerical simulations. ABC consists in computing a distance between simulated and observed summary statistics and weighting the simulations according to this distance. We propose an original extension of ABC to path-valued summary statistics, corresponding to the cumulated number of detections as a function of time. For a standard compartmental model with Suceptible, Infectious and Recovered individuals (SIR), we show that the posterior distributions obtained with ABC and MCMC are similar. In a refined SIR model well-suited to the HIV contact-tracing data in Cuba, we perform a comparison between ABC with full and binned detection times. For the Cuban data, we evaluate the efficiency of the detection system and predict the evolution of the HIV-AIDS disease. In particular, the percentage of undetected infectious individuals is found to be of the order of 40%

The Functional Derivation of Master Equations

Master equations describe the quantum dynamics of open systems interacting with an environment. They play an increasingly important role in understanding the emergence of semiclassical behavior and the generation of entropy, both being related to quantum decoherence. Presently we derive the exact master equation for a homogeneous scalar Higgs or inflaton like field coupled to an environment field represented by an infinite set of harmonic oscillators. Our aim is to demonstrate a derivation directly from the path integral representation of the density matrix propagator. Applications and generalizations of this result are discussed.Comment: 10 pages; LaTex. - Contribution to the workshop Hadron Physics VI, March 1998, Florianopolis (Brazil); proceedings, E. Ferreira et al., eds. (World Scientific). Replaced by slightly modified published versio