158 research outputs found
The Gray-code filter kernels
Abstract In this paper we introduce a family of filter kernels -the Gray-Code Kernels (GCK) and demonstrate their use in image analysis. Filtering an image with a sequence of Gray-Code Kernels is highly efficient and requires only 2 operations per pixel for each filter kernel, independent of the size or dimension of the kernel. We show that the family of kernels is large and includes the Walsh-Hadamard kernels amongst others. The GCK can be used to approximate any desired kernel and as such forms a complete representation. The efficiency of computation using a sequence of GCK filters can be exploited for various real-time applications, such as, pattern detection, feature extraction, texture analysis, texture synthesis, and more
Global well-posedness of the KP-I initial-value problem in the energy space
We prove that the KP-I initial value problem is globally well-posed in the
natural energy space of the equation
Asymptotic Lower Bounds for a class of Schroedinger Equations
We shall study the following initial value problem: \begin{equation}{\bf
i}\partial_t u - \Delta u + V(x) u=0, \hbox{} (t, x) \in {\mathbf R} \times
{\mathbf R}^n, \end{equation} where is a real short--range
potential, whose radial derivative satisfies some supplementary assumptions.
More precisely we shall present a family of identities satisfied by the
solutions to the previous Cauchy problem. As a by--product of these identities
we deduce some uniqueness results and a lower bound for the so called local
smoothing which becomes an identity in a precise asymptotic sense.Comment: 24 pages. to appear on Comm. Math. Phy
Structural resolvent estimates and derivative nonlinear Schrodinger equations
A refinement of uniform resolvent estimate is given and several smoothing
estimates for Schrodinger equations in the critical case are induced from it.
The relation between this resolvent estimate and radiation condition is
discussed. As an application of critical smoothing estimates, we show a global
existence results for derivative nonlinear Schrodinger equations.Comment: 21 page
A class of Schrodinger operators with decaying oscillatory potentials
We discuss Schr\"odinger operators on a half-line with decaying oscillatory
potentials, such as products of an almost periodic function and a decaying
function. We provide sufficient conditions for preservation of absolutely
continuous spectrum and give bounds on the Hausdorff dimension of the singular
part of the spectral measure. We also discuss the analogs for orthogonal
polynomials on the real line and unit circle.Comment: 18 page
Quantum Tomography under Prior Information
We provide a detailed analysis of the question: how many measurement settings
or outcomes are needed in order to identify a quantum system which is
constrained by prior information? We show that if the prior information
restricts the system to a set of lower dimensionality, then topological
obstructions can increase the required number of outcomes by a factor of two
over the number of real parameters needed to characterize the system.
Conversely, we show that almost every measurement becomes informationally
complete with respect to the constrained set if the number of outcomes exceeds
twice the Minkowski dimension of the set. We apply the obtained results to
determine the minimal number of outcomes of measurements which are
informationally complete with respect to states with rank constraints. In
particular, we show that 4d-4 measurement outcomes (POVM elements) is enough in
order to identify all pure states in a d-dimensional Hilbert space, and that
the minimal number is at most 2 log_2(d) smaller than this upper bound.Comment: v3: There was a mistake in the derived finer upper bound in Theorem
3. The corrected upper bound is +1 to the earlier versio
Self-similar extinction for a diffusive Hamilton-Jacobi equation with critical absorption
International audienceThe behavior near the extinction time is identified for non-negative solutions to the diffusive Hamilton-Jacobi equation with critical gradient absorption â_t u â â_p u + |âu|^{pâ1} = 0 in (0, â) Ă R^N , and fast diffusion 2N/(N + 1) < p < 2. Given a non-negative and radially symmetric initial condition with a non-increasing profile which decays sufficiently fast as |x| â â, it is shown that the corresponding solution u to the above equation approaches a uniquely determined separate variable solution of the form U (t, x) = (T_e â t)^{1/(2âp)} f_* (|x|), (t, x) â (0, T_e) Ă R^N , as t â T_e , where T_e denotes the finite extinction time of u. A cornerstone of the convergence proof is an underlying variational structure of the equation. Also, the selected profile f_* is the unique non-negative solution to a second order ordinary differential equation which decays exponentially at infinity. A complete classification of solutions to this equation is provided, thereby describing all separate variable solutions of the original equation. One important difficulty in the uniqueness proof is that no monotonicity argument seems to be available and it is overcome by the construction of an appropriate Pohozaev functional
Quasi-Linear Parabolic Reaction-Diffusion Systems: A User's Guide to Well-Posedness, Spectra, and Stability of Travelling Waves
Encoding order and developmental dyslexia:a family of skills predicting different orthographic components
We investigated order encoding in developmental dyslexia using a task that presented nonalphanumeric visual characters either simultaneously or sequentiallyâto tap spatial and temporal order encoding, respectivelyâand asked participants to reproduce their order. Dyslexic participants performed poorly in the sequential condition, but normally in the simultaneous condition, except for positions most susceptible to interference. These results are novel in demonstrating a selective difficulty with temporal order encoding in a dyslexic group. We also tested the associations between our order reconstruction tasks and: (a) lexical learning and phonological tasks; and (b) different reading and spelling tasks. Correlations were extensive when the whole group of participants was considered together. When dyslexics and controls were considered separately, different patterns of association emerged between orthographic tasks on the one side and tasks tapping order encoding, phonological processing, and written learning on the other. These results indicate that different skills support different aspects of orthographic processing and are impaired to different degrees in individuals with dyslexia. Therefore, developmental dyslexia is not caused by a single impairment, but by a family of deficits loosely related to difficulties with order. Understanding the contribution of these different deficits will be crucial to deepen our understanding of this disorder
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