2,460 research outputs found
Semiclassical States in Quantum Cosmology: Bianchi I Coherent States
We study coherent states for Bianchi type I cosmological models, as examples
of semiclassical states for time-reparametrization invariant systems. This
simple model allows us to study explicitly the relationship between exact
semiclassical states in the kinematical Hilbert space and corresponding ones in
the physical Hilbert space, which we construct here using the group averaging
technique. We find that it is possible to construct good semiclassical physical
states by such a procedure in this model; we also discuss the sense in which
the original kinematical states may be a good approximation to the physical
ones, and the situations in which this is the case. In addition, these models
can be deparametrized in a natural way, and we study the effect of time
evolution on an "intrinsic" coherent state in the reduced phase space, in order
to estimate the time for this state to spread significantly.Comment: 21 pages, 1 figure; Version to be published in CQG; The discussion
has been slightly reorganized, two references added, and some typos correcte
Enhanced LFR-toolbox for MATLAB and LFT-based gain scheduling
We describe recent developments and enhancements of the LFR-Toolbox for MATLAB for building LFT-based uncertainty models and for LFT-based gain scheduling. A major development is the new LFT-object definition supporting a large class of uncertainty descriptions: continuous- and discrete-time uncertain models, regular and singular parametric expressions, more general uncertainty blocks (nonlinear, time-varying, etc.). By associating names to uncertainty blocks the reusability of generated LFT-models and the user friendliness of manipulation of LFR-descriptions have been highly increased. Significant enhancements of the computational efficiency and of numerical accuracy have been achieved by employing efficient and numerically robust Fortran implementations of order reduction tools via mex-function interfaces. The new enhancements in conjunction with improved symbolical preprocessing lead generally to a faster generation of LFT-models with significantly lower orders. Scheduled gains can be viewed as LFT-objects. Two techniques for designing such gains are presented. Analysis tools are also considered
Model-Based Edge Detector for Spectral Imagery Using Sparse Spatiospectral Masks
Two model-based algorithms for edge detection in spectral imagery are developed that specifically target capturing intrinsic features such as isoluminant edges that are characterized by a jump in color but not in intensity. Given prior knowledge of the classes of reflectance or emittance spectra associated with candidate objects in a scene, a small set of spectral-band ratios, which most profoundly identify the edge between each pair of materials, are selected to define a edge signature. The bands that form the edge signature are fed into a spatial mask, producing a sparse joint spatiospectral nonlinear operator. The first algorithm achieves edge detection for every material pair by matching the response of the operator at every pixel with the edge signature for the pair of materials. The second algorithm is a classifier-enhanced extension of the first algorithm that adaptively accentuates distinctive features before applying the spatiospectral operator. Both algorithms are extensively verified using spectral imagery from the airborne hyperspectral imager and from a dots-in-a-well midinfrared imager. In both cases, the multicolor gradient (MCG) and the hyperspectral/spatial detection of edges (HySPADE) edge detectors are used as a benchmark for comparison. The results demonstrate that the proposed algorithms outperform the MCG and HySPADE edge detectors in accuracy, especially when isoluminant edges are present. By requiring only a few bands as input to the spatiospectral operator, the algorithms enable significant levels of data compression in band selection. In the presented examples, the required operations per pixel are reduced by a factor of 71 with respect to those required by the MCG edge detector
Stochastic Attribute-Value Grammars
Probabilistic analogues of regular and context-free grammars are well-known
in computational linguistics, and currently the subject of intensive research.
To date, however, no satisfactory probabilistic analogue of attribute-value
grammars has been proposed: previous attempts have failed to define a correct
parameter-estimation algorithm.
In the present paper, I define stochastic attribute-value grammars and give a
correct algorithm for estimating their parameters. The estimation algorithm is
adapted from Della Pietra, Della Pietra, and Lafferty (1995). To estimate model
parameters, it is necessary to compute the expectations of certain functions
under random fields. In the application discussed by Della Pietra, Della
Pietra, and Lafferty (representing English orthographic constraints), Gibbs
sampling can be used to estimate the needed expectations. The fact that
attribute-value grammars generate constrained languages makes Gibbs sampling
inapplicable, but I show how a variant of Gibbs sampling, the
Metropolis-Hastings algorithm, can be used instead.Comment: 23 pages, 21 Postscript figures, uses rotate.st
Robust Mechanism synthesis with random and interval variables
Robust mechanism synthesis ensures that the performance of a mechanism is not sensitive to uncertainties in the mechanism and its environment. The uncertainties include the dimension variations, installation errors, random input motion, and various external forces. Robust mechanism synthesis is used to minimize the impact of these uncertainties on the mechanism performance. Robust mechanism synthesis has been performed by either a probabilistic approach or a worst case approach. The former approach describes uncertain parameters as random variables while the latter approach treats uncertain parameters as interval variables. In this work, methods are developed for robustness assessment and robust mechanism synthesis when both random and interval variables exists --Abstract, page iii
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