7,701 research outputs found
Information-Based Distance Measures and the Canonical Reflection of View Updates
For the problem of reflecting an update on a database view to the main schema, the constant-complement strategies are precisely those which avoid all update anomalies, and so define the gold standard for well-behaved solutions to the problem. However, the families of view updates which are supported under such strategies are limited, so it is sometimes necessary to go beyond them, albeit in a systematic fashion. In this work, an investigation of such extended strategies is initiated for relational schemata. The approach is to characterize the information content of a database instance, and then require that the optimal reflection of a view update to the main schema embody the least possible change of information. The key property is identified to be strong monotonicity of the view, meaning that view insertions may always be reflected as insertions to the main schema, and likewise for deletions. In that context it is shown that for insertions and deletions, an optimal update, entailing the least change of information, exists and is unique up to isomorphism for wide classes of constraints
Region Operators of Wigner Function: Transformations, Realizations and Bounds
An integral of the Wigner function of a wavefunction |psi >, over some region
S in classical phase space is identified as a (quasi) probability measure (QPM)
of S, and it can be expressed by the |psi > average of an operator referred to
as the region operator (RO). Transformation theory is developed which provides
the RO for various phase space regions such as point, line, segment, disk and
rectangle, and where all those ROs are shown to be interconnected by completely
positive trace increasing maps. The latter are realized by means of unitary
operators in Fock space extended by 2D vector spaces, physically identified
with finite dimensional systems. Bounds on QPMs for regions obtained by tiling
with discs and rectangles are obtained by means of majorization theory.Comment: 16 pages, 4 figures. Hurst Bracken Festschrift, Reports of
Mathematical Physics, Feb 2006, to appea
In defense of the epistemic view of quantum states: a toy theory
We present a toy theory that is based on a simple principle: the number of
questions about the physical state of a system that are answered must always be
equal to the number that are unanswered in a state of maximal knowledge. A wide
variety of quantum phenomena are found to have analogues within this toy
theory. Such phenomena include: the noncommutativity of measurements,
interference, the multiplicity of convex decompositions of a mixed state, the
impossibility of discriminating nonorthogonal states, the impossibility of a
universal state inverter, the distinction between bi-partite and tri-partite
entanglement, the monogamy of pure entanglement, no cloning, no broadcasting,
remote steering, teleportation, dense coding, mutually unbiased bases, and many
others. The diversity and quality of these analogies is taken as evidence for
the view that quantum states are states of incomplete knowledge rather than
states of reality. A consideration of the phenomena that the toy theory fails
to reproduce, notably, violations of Bell inequalities and the existence of a
Kochen-Specker theorem, provides clues for how to proceed with this research
program.Comment: 32 pages, REVTEX, based on a talk given at the Rob Clifton Memorial
Conference, College Park, May 2003; v2: minor modifications throughout,
updated reference
Monte Carlo cluster algorithm for fluid phase transitions in highly size-asymmetrical binary mixtures
Highly size-asymmetrical fluid mixtures arise in a variety of physical
contexts, notably in suspensions of colloidal particles to which much smaller
particles have been added in the form of polymers or nanoparticles.
Conventional schemes for simulating models of such systems are hamstrung by the
difficulty of relaxing the large species in the presence of the small one. Here
we describe how the rejection-free geometrical cluster algorithm (GCA) of Liu
and Luijten [Phys. Rev. Lett 92, 035504 (2004)] can be embedded within a
restricted Gibbs ensemble to facilitate efficient and accurate studies of fluid
phase behavior of highly size-asymmetrical mixtures. After providing a detailed
description of the algorithm, we summarize the bespoke analysis techniques of
Ashton et al. [J. Chem. Phys. 132, 074111 (2010)] that permit accurate
estimates of coexisting densities and critical-point parameters. We apply our
methods to study the liquid--vapor phase diagram of a particular mixture of
Lennard-Jones particles having a 10:1 size ratio. As the reservoir volume
fraction of small particles is increased in the range 0--5%, the critical
temperature decreases by approximately 50%, while the critical density drops by
some 30%. These trends imply that in our system, adding small particles
decreases the net attraction between large particles, a situation that
contrasts with hard-sphere mixtures where an attractive depletion force occurs.Comment: 11 pages, 10 figure
Building with Drones: Accurate 3D Facade Reconstruction using MAVs
Automatic reconstruction of 3D models from images using multi-view
Structure-from-Motion methods has been one of the most fruitful outcomes of
computer vision. These advances combined with the growing popularity of Micro
Aerial Vehicles as an autonomous imaging platform, have made 3D vision tools
ubiquitous for large number of Architecture, Engineering and Construction
applications among audiences, mostly unskilled in computer vision. However, to
obtain high-resolution and accurate reconstructions from a large-scale object
using SfM, there are many critical constraints on the quality of image data,
which often become sources of inaccuracy as the current 3D reconstruction
pipelines do not facilitate the users to determine the fidelity of input data
during the image acquisition. In this paper, we present and advocate a
closed-loop interactive approach that performs incremental reconstruction in
real-time and gives users an online feedback about the quality parameters like
Ground Sampling Distance (GSD), image redundancy, etc on a surface mesh. We
also propose a novel multi-scale camera network design to prevent scene drift
caused by incremental map building, and release the first multi-scale image
sequence dataset as a benchmark. Further, we evaluate our system on real
outdoor scenes, and show that our interactive pipeline combined with a
multi-scale camera network approach provides compelling accuracy in multi-view
reconstruction tasks when compared against the state-of-the-art methods.Comment: 8 Pages, 2015 IEEE International Conference on Robotics and
Automation (ICRA '15), Seattle, WA, US
X-ray observations of the hot phase in Sgr~A*
We analyze 134 ks Chandra ACIS-I observations of the Galactic Centre (GC)
performed in July 2011. The X-ray image with the field of view
contains the hot plasma surrounding the Sgr~A*. The obtained surface brightness
map allow us to fit Bondi hot accretion flow to the innermost hot plasma around
the GC. We have fitted spectra from region up to from Sgr~A* using a
thermal bremsstrahlung model and four Gaussian profiles responsible for
K emission lines of Fe, S, Ar, and Ca. The X-ray surface brightness
profile up to from Sgr~A* found in our data image, was successfully fitted
with the dynamical model of Bondi spherical accretion. By modelling the surface
brightness profile, we derived the temperature and number density profiles in
the vicinity of the black hole. The best fitted model of spherical Bondi
accretion shows that this type of flow works only up to and implies outer
plasma density and temperature to be:
cm and keV respectively. We show
that the Bondi flow can reproduce observed surface brightness profile up to
from Sgr~A* in the Galactic Center. This result strongly suggests the
position of stagnation radius in the complicated dynamics around GC. The
Faraday rotation computed from our model towards the pulsar PSR J1745-2900 near
the GC agrees with the observed one, recently reported.Comment: 10 pages, 7 figures, accepted for publication in A&
Lattice-switch Monte Carlo
We present a Monte Carlo method for the direct evaluation of the difference
between the free energies of two crystal structures. The method is built on a
lattice-switch transformation that maps a configuration of one structure onto a
candidate configuration of the other by `switching' one set of lattice vectors
for the other, while keeping the displacements with respect to the lattice
sites constant. The sampling of the displacement configurations is biased,
multicanonically, to favor paths leading to `gateway' arrangements for which
the Monte Carlo switch to the candidate configuration will be accepted. The
configurations of both structures can then be efficiently sampled in a single
process, and the difference between their free energies evaluated from their
measured probabilities. We explore and exploit the method in the context of
extensive studies of systems of hard spheres. We show that the efficiency of
the method is controlled by the extent to which the switch conserves correlated
microstructure. We also show how, microscopically, the procedure works: the
system finds gateway arrangements which fulfill the sampling bias
intelligently. We establish, with high precision, the differences between the
free energies of the two close packed structures (fcc and hcp) in both the
constant density and the constant pressure ensembles.Comment: 34 pages, 9 figures, RevTeX. To appear in Phys. Rev.
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