A Neural Networks Committee for the Contextual Bandit Problem

This paper presents a new contextual bandit algorithm, NeuralBandit, which does not need hypothesis on stationarity of contexts and rewards. Several neural networks are trained to modelize the value of rewards knowing the context. Two variants, based on multi-experts approach, are proposed to choose online the parameters of multi-layer perceptrons. The proposed algorithms are successfully tested on a large dataset with and without stationarity of rewards.Comment: 21st International Conference on Neural Information Processin

On the accuracy of the ALI method for solving the radiative transfer equation

We solve the integral equation describing the propagation of light in an isothermal plane-parallel atmosphere of optical thickness $\tau^*$, adopting a uniform thermalization parameter $\epsilon$. The solution given by the ALI method, widely used in the field of stellar atmospheres modelling, is compared to the exact solution. Graphs are given that illustrate the accuracy of the ALI solution as a function of the parameters $\epsilon$, $\tau^*$ and optical depth variable $\tau$.Comment: 7 pages, 11 figures, A&A, accepted 30 July 2003, minor correction

Preconditioned Bi-Conjugate Gradient Method for Radiative Transfer in Spherical Media

A robust numerical method called the Preconditioned Bi-Conjugate Gradient (Pre-BiCG)method is proposed for the solution of radiative transfer equation in spherical geometry.A variant of this method called Stabilized Preconditioned Bi-Conjugate Gradient (Pre-BiCG-STAB) is also presented. These are iterative methods based on the construction of a set of bi-orthogonal vectors. The application of Pre-BiCG method in some benchmark tests show that the method is quite versatile, and can handle hard problems that may arise in astrophysical radiative transfer theory.Comment: 19 pages, 12 figure

Spinodal fractionation in a polydisperse square well fluid

Using Kinetic Monte Carlo simulation, we model gas-liquid spinodal decomposition in a size-polydisperse square well fluid, representing a 'near-monodisperse' colloidal dispersion. We find that fractionation (demixing) of particle sizes between the phases begins asserting itself shortly after the onset of phase ordering. Strikingly, the direction of size fractionation can be reversed by a seemingly trivial choice between two inter-particle potentials which, in the monodisperse case, are identical -- we rationalise this in terms of a perturbative, equilibrium theory of polydispersity. Furthermore, our quantitative results show that Kinetic Monte Carlo simulation can provide detailed insight into the role of fractionation in real colloidal systems.Comment: 7 pages, 7 figures, to be published in Phys. Rev.

Concurrent bandits and cognitive radio networks

We consider the problem of multiple users targeting the arms of a single multi-armed stochastic bandit. The motivation for this problem comes from cognitive radio networks, where selfish users need to coexist without any side communication between them, implicit cooperation or common control. Even the number of users may be unknown and can vary as users join or leave the network. We propose an algorithm that combines an $\epsilon$-greedy learning rule with a collision avoidance mechanism. We analyze its regret with respect to the system-wide optimum and show that sub-linear regret can be obtained in this setting. Experiments show dramatic improvement compared to other algorithms for this setting

Bootstrapping Monte Carlo Tree Search with an Imperfect Heuristic

We consider the problem of using a heuristic policy to improve the value approximation by the Upper Confidence Bound applied in Trees (UCT) algorithm in non-adversarial settings such as planning with large-state space Markov Decision Processes. Current improvements to UCT focus on either changing the action selection formula at the internal nodes or the rollout policy at the leaf nodes of the search tree. In this work, we propose to add an auxiliary arm to each of the internal nodes, and always use the heuristic policy to roll out simulations at the auxiliary arms. The method aims to get fast convergence to optimal values at states where the heuristic policy is optimal, while retaining similar approximation as the original UCT in other states. We show that bootstrapping with the proposed method in the new algorithm, UCT-Aux, performs better compared to the original UCT algorithm and its variants in two benchmark experiment settings. We also examine conditions under which UCT-Aux works well.Comment: 16 pages, accepted for presentation at ECML'1

Oblivion: Mitigating Privacy Leaks by Controlling the Discoverability of Online Information

Search engines are the prevalently used tools to collect information about individuals on the Internet. Search results typically comprise a variety of sources that contain personal information -- either intentionally released by the person herself, or unintentionally leaked or published by third parties, often with detrimental effects on the individual's privacy. To grant individuals the ability to regain control over their disseminated personal information, the European Court of Justice recently ruled that EU citizens have a right to be forgotten in the sense that indexing systems, must offer them technical means to request removal of links from search results that point to sources violating their data protection rights. As of now, these technical means consist of a web form that requires a user to manually identify all relevant links upfront and to insert them into the web form, followed by a manual evaluation by employees of the indexing system to assess if the request is eligible and lawful. We propose a universal framework Oblivion to support the automation of the right to be forgotten in a scalable, provable and privacy-preserving manner. First, Oblivion enables a user to automatically find and tag her disseminated personal information using natural language processing and image recognition techniques and file a request in a privacy-preserving manner. Second, Oblivion provides indexing systems with an automated and provable eligibility mechanism, asserting that the author of a request is indeed affected by an online resource. The automated ligibility proof ensures censorship-resistance so that only legitimately affected individuals can request the removal of corresponding links from search results. We have conducted comprehensive evaluations, showing that Oblivion is capable of handling 278 removal requests per second, and is hence suitable for large-scale deployment

Synthetic spectra of H Balmer and HeI absorption lines. I: Stellar library

We present a grid of synthetic profiles of stellar H Balmer and HeI lines at optical wavelengths with a sampling of 0.3 A. The grid spans a range of effective temperature 4000 K < Teff < 50000 K, and gravity 0.0 < log g < 5.0 at solar metallicity. For Teff > 25000 K, NLTE stellar atmosphere models are computed using the code TLUSTY (Hubeny 1988). For cooler stars, Kurucz (1993) LTE models are used to compute thesynthetic spectra. The grid includes the profiles of the high-order hydrogen Balmer series and HeI lines for effective temperatures and gravities that have not been previously synthesized. The behavior of H8 to H13 and HeI 3819 with effective temperature and gravity is very similar to that of the lower terms of the series (e.g. Hb) and the other HeI lines at longer wavelengths; therefore, they are suited for the determination of the atmospheric parameters of stars. These lines are potentially important to make predictions for these stellar absorption features in galaxies with active star formation. Evolutionary synthesis models of these lines for starburst and post-starburst galaxies are presented in a companion paper. The full set of the synthetic stellar spectra is available for retrieval at our website http://www.iaa.es/ae/e2.html and http://www.stsci.edu/science/starburst/ or on request from the authors at [email protected]: To be published in ApJS. 28 pages and 12 figure