Página 1 dos resultados de 1522 itens digitais encontrados em 0.031 segundos

Incorporating a feature tree geometry into a matcher for a speech recognizer

Maldonado, Aaron (Aaron Theodore), 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 56 leaves, [52] leaves in various foliations; 4054955 bytes; 4054762 bytes; application/pdf; application/pdf
ENG
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by Aaron Maldonado.; Thesis (M.Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, June 1999.; "May 1999."; Includes bibliographical references (leaf 56).

Measurement and modeling of small-geometry MOS transistor capacitances

Paulos, John James
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 306 p.; 13266498 bytes; 13266254 bytes; application/pdf; application/pdf
ENG
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by John James Paulos.; Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1984.; MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.; Includes bibliographical references.

Micromachining and modeling of focused field emitters for flat panel displays

Dvorson, Leonard, 1974-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 126 p.; 14751170 bytes; 14750927 bytes; application/pdf; application/pdf
ENG
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We present a comprehensive study of field emitter arrays with or without an integrated focus electrode. The former configuration is referred to as field emitter with integrated focus (IFE-FEA) or double-gate FEA (DG-FEA). The main application of IFE-FEA is to improve the resolution of field emission displays (FEDs). We developed the first analytical model of conical field emitters that captures all details of device geometry and produces quantitatively accurate closed-form expressions for the FN coefficients. A novel CMP-based process for making IFE-FEA is presented. We obtained devices with gate and focus apertures of 0.8 and 1.2 gm diameter, respectively, which is 1.5 times smaller than in any previously reported IFE-FEA. Single-gate FEAs whose gate was identical to the lower gate of the IFE-FEA were also fabricated. Their emission current was 100 nA/tip at 45 V; for IFE-FEAs with the gate and focus biased at the same potential (VG=VF) this figure was 100 nA/tip at 42 V, in agreement with the analytical model. It was deduced that the tip radius of curvature (ROC) is 2.4-3.6 nm. Analytical model, numerical simulation, and TEM micrographs all gave tip ROC values in this range. We generalized the FN equation to IFE-FEA and used 4-terminal measurements to determine gate and focus field factors...

Reduced-order modeling of MEMS using modal basis functions; Reduced-order modeling of microelectromechanical systems using modal basis functions

Varghese, Mathew, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 93 leaves; 7295972 bytes; 7295731 bytes; application/pdf; application/pdf
ENG
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The field of MEMS has matured significantly over the last two decades increasing in both complexity and level of integration. To keep up with the demands placed by these changes requires the development of computer-aided design and modeling tools (CAD/CAM) that enable designers to reduce the time and cost it takes to produce working prototypes. An ideal scenario is one in which a designer is able to quickly model and simulate an entire microsystem - sensors, actuators and electronics -- with the certainty that their results will match that of physical prototypes. This vision of design requires the existence of system level models of MEMS devices that can capture the complex non-linear coupling between multiple physical domains, yet be sufficiently fast and compact in form to insert into a system dynamics simulator. In this thesis I explore techniques of automatically constructing such models from meshed representations of device geometry. These dynamical models are known as "reduced-order" models or "macromodels." They are characterized by few degrees of freedom (DOF), and a small set of state equations. Our process for constructing macromodels is built upon two well-established methodologies - normal mode superposition and Lagrangian mechanics. This is referred to as the "CHURN process" and was originally developed by Gabbay et al. to create models of electromechanical devices with two electrodes under conditions satisfying linear mechanics.; (cont.) In this thesis I significantly extend this process to model multi-port magnetostatic devices...

CMOS passive pixel image design techniques; Complementary metal oxide semiconductors passive pixel image design techniques

Fujimori, Iliana L. (Iliana Lucia)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 122 p.; 6749098 bytes; 6748857 bytes; application/pdf; application/pdf
ENG
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CMOS technology provides an attractive alternative to the currently dominant CCD technology for implementing low-power, low-cost imagers with high levels of integration. Two pixel configurations are possible in CMOS technology: active and passive. The active pixel requires a minimum of three transistors to convert light to voltage. The passive pixel, on the other hand, consists of a single transistor, and its output is in the form of charge. Column-parallel opamps are used to amplify the charge to a voltage output. The main advantage of the passive pixel is a higher fill factor in a given pixel geometry. This advantage becomes increasingly important as we scale to smaller pixel sizes. The higher fill factor comes at a high cost as the charge output on the high impedance node of the column line is susceptible to disturbances, namely a parasitic current and temporal noise. The goal of this thesis is to determine the source and effects of the disturbances on the image sensor characteristics and the repercussions for scaling to high-density arrays. A signal-dependent parasitic current composed of optically-generated carrier diffusion, blooming and subthreshold currents contaminates the pixel output. This parasitic current is detrimental to the imager because a few bright pixels can affect the rest of the pixels on the column line...

Stochastic processes on graphs with cycles : geometric and variational approaches

Wainwright, Martin J. (Martin James), 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 271 leaves; 21824527 bytes; 21824286 bytes; application/pdf; application/pdf
ENG
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Stochastic processes defined on graphs arise in a tremendous variety of fields, including statistical physics, signal processing, computer vision, artificial intelligence, and information theory. The formalism of graphical models provides a useful language with which to formulate fundamental problems common to all of these fields, including estimation, model fitting, and sampling. For graphs without cycles, known as trees, all of these problems are relatively well-understood, and can be solved efficiently with algorithms whose complexity scales in a tractable manner with problem size. In contrast, these same problems present considerable challenges in general graphs with cycles. The focus of this thesis is the development and analysis of methods, both exact and approximate, for problems on graphs with cycles. Our contributions are in developing and analyzing techniques for estimation, as well as methods for computing upper and lower bounds on quantities of interest (e.g., marginal probabilities; partition functions). In order to do so, we make use of exponential representations of distributions, as well as insight from the associated information geometry and Legendre duality. Our results demonstrate the power of exponential representations for graphical models...

FastStokes : a fast 3-D fluid simulation program for micro-electro-mechanical systems; Fast Stokes : a fast three-dimensional fluid simulation program for MEMS

Wang, Xin, 1972 Jan. 8-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 153 p.; 6057849 bytes; 6057656 bytes; application/pdf; application/pdf
ENG
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We have developed boundary integral equation formulas and a corresponding fast 3-D Stokes flow simulation program named FastStokes to accurately simulate viscous drag forces on geometrically complicated MEMS (micro- electro- mechanical systems) devices. Unlike the 3-D finite element or finite difference solvers which often take days to run to completion or fail when geometry gets complicated, the FastStokes 3.0 simulation program is capable of simulating complicated devices such as resonators, accelerometers, and micro-mirrors on PC computers in minutes. The FastStokes 3.0 simulation program is a fast 3-D boundary-element simulation program that uses only surface discretizations. The implementation of the Precorrected-FFT algorithm in combination with the GMRES algorithm substantially improves the speed of this simulation program. An efficient two-step approach that successfully handles the null space of the singular incompressible Stokes BEM operators is developed to avoid numerical errors and solution discontinuities. An analytical flat-panel kernel integration algorithm is implemented in FastStokes and an accurate curved-panel integration algorithm is also developed. Both an incompressible FastStokes solver and a compressible FastStokes solver have been developed and tested. They are not only fast...

Projective minimal analysis of camera geometry; Projective nonlinear minimal analysis of camera geometry

Romano, Raquel Andrea
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 120 p.; 4705947 bytes; 4705755 bytes; application/pdf; application/pdf
ENG
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This thesis addresses the general problem of how to find globally consistent and accurate estimates of multiple-view camera geometry from uncalibrated imagery of an extended scene. After decades of study, the classic problem of recovering camera motion from image correspondences remains an active area of research. This is due to the practical difficulties of estimating many interacting camera parameters under a variety of unknown imaging conditions. Projective geometry offers a useful framework for analyzing uncalibrated imagery. However, the associated multilinear models-the fundamental matrix and trifocal tensorare redundant in that they allow a camera configuration to vary along many more degrees of freedom than are geometrically admissible. This thesis presents a novel, minimal projective model of uncalibrated view triplets in terms of the dependent epipolar geometries among view pairs. By explicitly modeling the trifocal constraints among projective bifocal parameters-the epipoles and epipolar collineations-this model guarantees a solution that lies in the valid space of projective camera configurations. We present a nonlinear incremental algorithm for fitting the trifocally constrained epipolar geometries to observed image point matches. The minimal trifocal model is a practical alternative to the trifocal tensor for commonly found image sequences in which the availability of matched point pairs varies widely among different view pairs. Experimental results on synthetic and real image sequences with typical asymmetries in view overlap demonstrate the improved accuracy of the new trifocally constrained model.; (cont.) We provide an analysis of the objective function surface in the projective parameter space and examine cases in which the projective parameterization is sensitive to the Euclidean camera configuration. Finally...

Feature preserving smoothing of 3D surface scans

Jones, Thouis Raymond, 1971-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 70 p.; 4486548 bytes; 4486357 bytes; application/pdf; application/pdf
ENG
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With the increasing use of geometry scanners to create 3D models, there is a rising need for effective denoising of data captured with these devices. This thesis presents new methods for smoothing scanned data, based on extensions of the bilateral filter to 3D. The bilateral filter is a non-linear, edge-preserving image filter; its extension to 3D leads to an efficient, feature preserving filter for a wide class of surface representations, including points and "polygon soups."; by Thouis Raymond Jones.; Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, February 2004.; Includes bibliographical references (p. 63-70).

Spectral partitioning, eigenvalue bounds, and circle packings for graphs of bounded genus

Kelner, Jonathan, 1980-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 39 leaves; 1794456 bytes; 1796720 bytes; application/pdf; application/pdf
ENG
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In this paper, we address two longstanding questions about finding good separators in graphs of bounded genus and degree: 1. It is a classical result of Gilbert, Hutchinson, and Tarjan [12] that one can find asymptotically optimal separators on these graphs if he is given both the graph and an embedding of it onto a low genus surface. Does there exist a simple, efficient algorithm to find these separators given only the graph and not the embedding? 2. In practice, spectral partitioning heuristics work extremely well on these graphs. Is there a theoretical reason why this should be the case? We resolve these two questions by showing that a simple spectral algorithm finds separators of cut ratio O(sqrt(g/n)) and vertex bisectors of size O(sqrt(gn)) in these graphs, both of which are optimal. As our main technical lemma, we prove an O(g/n) bound on the second smallest eigenvalue of the Laplacian of such graphs and show that this is tight, thereby resolving a conjecture of Spielman and Teng. While this lemma is essentially combinatorial in nature, its proof comes from continuous mathematics, drawing on the theory of circle packings and the geometry of compact Riemann surfaces.; by Jonathan Kelner.; Thesis (S.M.)--Massachusetts Institute of Technology...

Enhancement of electromagnetic propagation through complex media for Radio Frequency Identification

Marti, Uttara P
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 94 p.; 4595409 bytes; 4600126 bytes; application/pdf; application/pdf
ENG
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In this thesis, I present and examine the fundamental limitations involved in Radio Frequency Identification (RFID) as well as provide a means to improve reader-tag communication in ultra high frequency RFID systems. The ultimate goal in an RFID system is to maximize the communication link between the reader and the tags while, at the same time, minimizing the effect of product material, geometry and orientation. Reader-tag communication has improved significantly over the past five years, however, tag operations continue to be extremely sensitive to their environment. Ultra high frequencies present unique problems in transmission, generation and circuit design that; are not encountered at lower frequencies. Based on the fundamental constraints on these passive RFID systems, such as electromagnetics, power limitations and government regulations, I analyzed electromagnetic propagation through materials as applied to RFID tagged cases and pallets. Applying the electromagnetic concept of conductive parallel plates to enhance electromagnetic power to RFID tagged cases and pallets, I suggest an alternative to the current pallet structure.; by Uttara P. Marti.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science...

Operational behavior of a double-fed permanent magnet generator for wind turbines

Reddy, Sivananda Kumjula
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 118 leaves; 5260673 bytes; 5266173 bytes; application/pdf; application/pdf
ENG
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Greater efficiency in wind turbine systems is achieved by allowing the rotor to change its rate of rotation as the wind speed changes. The wind turbine system is decoupled from the utility grid and a variable speed operation is implemented. Since wind speed varies, the shaft speed should too. But electrical output frequency should be synchronous and Induction machines on their own do not allow for variable ratio of shaft speed to electrical frequency. Permanent Magnet (PM) machines have major advantages: Efficiency and power density are both high: they are also inherently synchronous. A prototype ([approx]20 kW) of a novel hybrid machine that encompasses the desired features of Permanent Magnet Generators and Doubly-Fed Induction Generators (DFIGs) is being built. This novel geometry machine has to be tested for stability on a test-bed and controlled using an appropriate power electronics and controller circuit. A MATLAB 5th order model of the machine is built, linearized. simulated and examined for stability. The dynamics of the PM rotor is studied. The electrical equivalent behavior of this component of the machine is vital because the frequency of the output power is a direct function of its mechanical speed. As a culmination of this research work...

The local geometry of multiattribute tradeoff preferences

McGeachie, Michael J
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 129 p.
ENG
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Existing preference reasoning systems have been successful in simple domains. Broader success requires more natural and more expressive preference representations. This thesis develops a representation of logical preferences that combines numerical tradeoff ratios between partial outcome descriptions with qualitative preference information. We argue our system is unique among preference reasoning systems; previous work has focused on qualitative or quantitative preferences, tradeoffs, exceptions and generalizations, or utility independence, but none have combined all of these expressions under a unified methodology. We present new techniques for representing and giving meaning to quantitative tradeoff statements between different outcomes. The tradeoffs we consider can be multi-attribute tradeoffs relating more than one attribute at a time, they can refer to discrete or continuous domains, be conditional or unconditional, and quantified or qualitative. We present related methods of representing judgments of attribute importance. We then build upon a methodology for representing arbitrary qualitative ceteris paribus preference, or preferences "other things being equal," as presented in [MD04].; (cont.) Tradeoff preferences in our representation are interpreted as constraints on the partial derivatives of the utility function. For example...

Electrical degradation mechanisms of RF power GaAs PHEMTs

Villanueva, Anita A. (Anita Ariel), 1978-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 163 p.
ENG
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GaAs Pseudomorphic High-Electron Mobility Transistors (PHEMTs) are widely used in RF power applications. Since these devices typically operate at high power levels and under high voltage biasing, their electrical reliability is of serious concern. Previous studies have identified several distinct degradation phenomena in these devices, but a complete picture has yet to be formed. In this study, we have carried out a comprehensive study of the mechanisms of electrical degradation on a set of experimental RF power GaAs PHEMTs (non-commercial devices provided by our sponsor, Mitsubishi Electric). A wide variety of electrical stressing experiments employing different conditions (varying temperature, bias, environment) were performed on these devices in order to monitor their degradation with stressing. Our general observations showed several forms of degradation, the most concerning being an increase in the drain resistance RD and a reduction in maximum drain current Imax. Contrary to what is often claimed in the literature, our experiments indicated that these forms of degradation were not driven by impact-ionization or hot-electron effects. Instead, we found the degradation to be strongly correlated with temperature, stressing environment...

Real-time spatial-phase-locked electron-beam lithography; Real-time SPLEBL

Zhang, Feng, 1973-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 139 p.
ENG
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The ability of electron-beam lithography (EBL) to create sub-10-nm features with arbitrary geometry makes it a critical tool in many important applications in nanoscale science and technology. The conventional EBL system is limited by its poor absolute-placement accuracy, often worse than its resolution. Spatial-phase-locked electron-Beam lithography (SPLEBL) improves the placement accuracy of EBL tools to the nanometer level by directly referencing the beam position via a global-fiducial grid placed on the substrate, and providing feedback corrections to the beam position. SPLEBL has several different modes of operation, and it can be applied to both scanning electron-beam lithography (SEBL) and variable-shaped-beam lithography. This research focuses primarily on implementing real-time SPLEBL in SEBL systems. Real-time SPLEBL consists of three major components: a fiducial-reference grid, a beam-position detection algorithm and a partial-beam blanker. Several types of fiducial grids and their fabrication processes were developed and evaluated for their signal-to-noise ratio and ease of usage. An algorithm for detecting the beam position based on Fourier techniques was implemented, and -1 nm placement accuracy achieved. Finally, various approaches to partial-beam blanking were examined...

Integrated optics for holographic video

Smalley, Daniel E
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 85 leaves
ENG
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The goal of this research is to fabricate a guided-wave, two-axis scanner and to modify the design of the MIT holovideo system to take full advantage of the scanner's high bandwidth and two-dimensional deflection. The new display geometry will be designed to use the guided-wave scanner coupled with a holographic optical element to perform a solid-state horizontal de-scan. The development of the guided-wave scanner and the improvements made to the holovideo geometry, will enable the construction of a third generation holovideo display that is higher bandwidth, more solid-state and at least an order of magnitude less expensive than previous generations.; by Daniel E. Smalley.; Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.; Includes bibliographical references (leaves 83-85).

Dielectrometry measurements of moisture diffusion and temperature dynamics in oil impregnated paper insulated electric power cables

Thomas, Zachary M. (Zachary Michael)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 452 p.
ENG
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Paper insulated lead covered (PILC) cables have played an important role in underground power distribution for a hundred years. Replacing aged PILC before failure is critical to managing power distribution. Three prominent failure mechanisms accelerate cable aging: temperature stresses, moisture ingress, and partial discharge. The research focuses on the effect of temperature and moisture on the effective (complex) permittivity of the cable insulation. Measurements are performed using cylindrical dielectrometry sensors designed to be wrapped around the cable. The lead sheath of the cable is removed so that the sensors can be placed directly in contact with the insulation. From the measurements, the electrical properties of the material as a function of temperature and transient moisture diffusion are found. A theoretical treatment of the interdigital dielectrometry sensors with a cylindrical geometry is presented. Two classes of the geometry are studied. The periodic sensor geometry has electrodes aligned with the cylindrical axis and periodic around the circumference. The z periodic sensor geometry has the electrodes forming rings around the cylinder that are periodic along the cylindrical axis. The material is modeled by concentric rings of homogeneous materials. The electric field solution consists of an infinite summation of Fourier series terms. In finding the field solution and as a consequence of it...

High speed linear induction motor efficiency optimization

Johnson, Andrew P. (Andrew Peter)
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 189 p.; 6915839 bytes; 6923757 bytes; application/pdf; application/pdf
ENG
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One of the reasons linear motors, a technology nearly a century old, have not been adopted for a large number of linear motion applications is that they have historically had poor efficiencies. This has restricted the progress of linear motor development. The concept of a linear motor as a rotary motor cut and laid out flat with a conventional rotary motor control scheme as a design basis may not be the best way to design and control a high-speed linear motor. End effects and other geometry subtleties of a linear motor make it unique, and a means of optimizing efficiency with both the motor geometry and the motor control scheme will be analyzed to create a High-Speed Linear Induction Motor (LIM) with a higher efficiency than what is possible with conventional motors and controls. This thesis pursues the modeling of a short secondary type Double-Sided Linear Induction Motor (DSLIM) that is proposed for use as an Electromagnetic Aircraft Launch System (EMALS) aboard the CVN-2 1. Mathematical models for the prediction of effects that are peculiar to DSLIM are formulated, and their overall effects on the performance of the proposed machine are analyzed.; (cont.) These effects are used to generate a transient motor model, which is then driven by a motor controller that is specifically designed to the characteristics of the proposed DSLIM. Due to this DSLIM's role as a linear accelerator...

Graphical models for visual object recognition and tracking

Sudderth, Erik B. (Erik Blaine), 1977-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 301 p.; 19955509 bytes; 19954958 bytes; application/pdf; application/pdf
ENG
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We develop statistical methods which allow effective visual detection, categorization, and tracking of objects in complex scenes. Such computer vision systems must be robust to wide variations in object appearance, the often small size of training databases, and ambiguities induced by articulated or partially occluded objects. Graphical models provide a powerful framework for encoding the statistical structure of visual scenes, and developing corresponding learning and inference algorithms. In this thesis, we describe several models which integrate graphical representations with nonparametric statistical methods. This approach leads to inference algorithms which tractably recover high-dimensional, continuous object pose variations, and learning procedures which transfer knowledge among related recognition tasks. Motivated by visual tracking problems, we first develop a nonparametric extension of the belief propagation (BP) algorithm. Using Monte Carlo methods, we provide general procedures for recursively updating particle-based approximations of continuous sufficient statistics. Efficient multiscale sampling methods then allow this nonparametric BP algorithm to be flexibly adapted to many different applications.; (cont.) As a particular example...

Tarski's influence on computer science

Feferman, Solomon
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
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The influence of Alfred Tarski on computer science was indirect but significant in a number of directions and was in certain respects fundamental. Here surveyed is the work of Tarski on the decision procedure for algebra and geometry, the method of elimination of quantifiers, the semantics of formal languages, modeltheoretic preservation theorems, and algebraic logic; various connections of each with computer science are taken up.