Search results for "Computer Science::Databases"
showing 10 items of 183 documents
Some topological properties of the Inverse Lens Mapping
2011
Away from critical curves, lens mapping can be seen as a linear invertible transformation of the plane even for regions (cells) of relatively large size. However, close to critical curves the departures from linearity can be very strong. We discuss the topological problems induced by the mapping of regions of the image plane that include critical curves (critical cells).
An Ehrenfeucht-Fraïssé Approach to Collapse Results for First-Order Queries over Embedded Databases
2001
We present a new proof technique for collapse results for first-order queries on databases which are embedded in N or R>o. Our proofs are by means of an explicitly constructed winning strategy for Duplicator in an Ehrenfeucht-FraissE game, and can deal with certain infinite databases where previous, highly involved methods fail. Our main result is that first-order logic has the natural-generic collapse over {N,≤ ,+} for arbitrary (i.e., possibly infinite) databases. Furthermore, a first application of this result shows the natural-generic collapse of first-order logic over {R>o,≤,+} for a certain kind of databases over R>o which consist of a possibly infinite number of regions.
Inflation might be caused by the right
2009
15 pages, 4 figures.-- ISI article identifier: 000265600800102.-- ArXiv pre-print avaible at: http://arxiv.org/abs/0811.2998
Instability of relativistic sheared jets and distinction between FRI and FRII sources
2002
We investigate the shear-driven instability of nonmagnetic relativistic jets with the bulk velocity, V , dependent on the cylindric radius, r. It is shown that instability can arise for any dependence of the velocity (or the Lorentz factor that is the same) on r. The shear-driven instability can e ectively operate in the whole volume of a jet. The growth time can be shorter than that of the Kelvin-Helmholtz instability. The considered instability leads to a turbulization of jets and can account for a distiction between the jets in the FRI and FRII sources. Urpin, V., Vadim.Urpin@uv.es
Spatial mapping of price competition using logit-type market share models and store-level scanner-data
2009
This paper proposes a methodology to obtain reliable spatial maps of price competition using store-level scanner data. Specifically, a procedure to obtain a symmetric matrix of similarities between brands considering their substitutability depending on price variations is proposed. The matrix is derived from a market response model where price cross-effects are split into two components. The first component accounts for the fact that price variation in one brand can have different effects to price variation in other brands (ie j → j′≠j′ → j). The second component accounts for the fact that the price of each brand can have different effects across competing brands (ie j → j′≠j → j ″). The ma…
A Generic Method of Line Matching for Central Imaging Systems under Short-Baseline Motion
2009
Line matching across images taken by a central imaging system (perspective or catadioptric) with focus on short baseline motion of the system is proposed. The relationship between images of lines on unitary sphere is studied and a simple algorithm for matching lines are proposed assuming the rotation of the system is known apriori or it can be estimated from some correspondences in two views. Two methods are discussed for retrieving R in the case it is not known apriori. Experimental results on both synthetic and real images are also presented.
The ultrafast dynamics and conductivity of photoexcited graphene at different Fermi energies
2017
The ultrafast dynamics and conductivity of photoexcited graphene can be explained using solely electronic effects.
Photonic processing of microwave signals
2005
In this paper, recent advances in the implementation of photonic tuneable transversal filters for RF signal processing developed by the Optical Communications Group are described. After a brief introduction to the field and the basic concepts, limiting factors and application of these filters are described in the field. This is based on a distinction between filters based on wavelength tuneable optical taps and others based on the tuneability of the dispersive elements that provide the time delay between samples. Recently developed approaches have also been discussed.
The Engineering of Hot Carbon Nanotubes with a Focused Electron Beam
2004
Single-wall and multiwall carbon nanotubes at high temperature are irradiated with the focused electron beam in an electron microscope. Nanotubes can be tailored with monolayer precision, and new morphologies of nanotubes are created. Atoms from layers of multiwall tubes can be removed and the tubes can be bent by a predefined angle. Bundles of single-wall tubes are transformed locally to multiwall tubes with coherent transition between the two modifications.
A novel configuration for the optical characterization of waveguiding layers
1991
When a transparent layer of sufficient thickness is suitably coated on both sides with metal, the structure supports one-dimensionally bound optical modes which propagate in two dimensions but which can be detected externally in a very simple manner. We show that this configuration can be used to determine the optical constants of the layer.