18,747 research outputs found
Adaptive content mapping for internet navigation
The Internet as the biggest human library ever assembled keeps on growing. Although all kinds of information carriers (e.g. audio/video/hybrid file formats) are available, text based documents dominate. It is estimated that about 80% of all information worldwide stored electronically exists in (or can be converted into) text form. More and more, all kinds of documents are generated by means of a text processing system and are therefore available electronically. Nowadays, many printed journals are also published online and may even discontinue to appear in print form tomorrow. This development has many convincing advantages: the documents are both available faster (cf. prepress services) and cheaper, they can be searched more easily, the physical storage only needs a fraction of the space previously necessary and the medium will not age. For most people, fast and easy access is the most interesting feature of the new age; computer-aided search for specific documents or Web pages becomes the basic tool for information-oriented work. But this tool has problems. The current keyword based search machines available on the Internet are not really appropriate for such a task; either there are (way) too many documents matching the specified keywords are presented or none at all. The problem lies in the fact that it is often very difficult to choose appropriate terms describing the desired topic in the first place. This contribution discusses the current state-of-the-art techniques in content-based searching (along with common visualization/browsing approaches) and proposes a particular adaptive solution for intuitive Internet document navigation, which not only enables the user to provide full texts instead of manually selected keywords (if available), but also allows him/her to explore the whole database
Commutators of flows and fields
The well known formula [X,Y]=\tfrac12\tfrac{\partial^2}{\partial t^2}|_0
(\Fl^Y_{-t}\o\Fl^X_{-t}\o\Fl^Y_t\o\Fl^X_t) for vector fields , is
generalized to arbitrary bracket expressions and arbitrary curves of local
diffeomorphisms
Nonlinear optics with full three-dimensional illumination
We investigate the nonlinear optical process of third-harmonic generation in
the thus far unexplored regime of focusing the pump light from a full solid
angle, where the nonlinear process is dominantly driven by a standing
dipole-wave. We elucidate the influence of the focal volume and the pump
intensity on the number of frequency-tripled photons by varying the solid angle
from which the pump light is focused, finding good agreement between the
experiments and numerical calculations. As a consequence of focusing the pump
light to volumes much smaller than a wavelength cubed the Gouy phase does not
limit the yield of frequency-converted photons. This is in stark contrast to
the paraxial regime. We believe that our findings are generic to many other
nonlinear optical processes when the pump light is focused from a full solid
angle.Comment: 6 pages main text + 4 pages appendix, modified abstract and
introduction + some other minor change
Spontaneous breaking of rotational symmetry in the presence of defects
We prove a strong form of spontaneous breaking of rotational symmetry for a
simple model of two-dimensional crystals with random defects in thermal
equilibrium at low temperature. The defects consist of isolated missing atoms.Comment: 18 page
Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy
A combination of optical feedback self-locking of a continuous-wave
distributed feedback diode laser to a V-shaped high finesse cavity, laser phase
modulation at a frequency equal to the free spectral range of the V-cavity and
detection of the transmitted laser beam at this high modulation frequency is
described for possible application in cavity-enhanced absorption
spectroscopy.In order to estimate an absorbance baseline noise of laser
intensity and frequency modulated light triplet passed through the V-cavity in
open air, a 1.5-cm long optical cell filled by C2H2 at low pressure was placed
behind the cavity output mirror. The performance of the setup was evaluated
from the experimental bandwidth normalised relative intensity noise on the
cavity output and the frequency modulation absorption signals induced by C2H2
absorption in the 1.5-cm cell. From these data we estimate that the
noise-equivalent absorption sensitivity of 2.1*10^-11 cm^-1 Hz^-1/2 by a factor
of 11.7 above a shot-noise limit can be achieved for C2H2 absorption spectra
extracted from the heterodyne beat signals recorded at the transmission maxima
intensity peaks of the successive TEM00 resonances.Comment: 18 pages, 8 figures, submitted to Appl. Phys. B 28.07.201
Cyclic Quantum Error-Correcting Codes and Quantum Shift Registers
We transfer the concept of linear feed-back shift registers to quantum
circuits. It is shown how to use these quantum linear shift registers for
encoding and decoding cyclic quantum error-correcting codes.Comment: 18 pages, 15 figures, submitted to Proc. R. Soc.
Bayesian forecasting using stochastic search variable selection in a VAR subject to breaks
This paper builds a model which has two extensions over a standard VAR. The …rst of these is stochastic search variable selection, which is an automatic model selection device which allows for coefficients in a possibly over-parameterized VAR to be set to zero. The second allows for an unknown number of structual breaks in the VAR parameters. We investigate the in-sample and forecasting performance of our model in an application involving a commonly-used US macro-economic data set. We …nd that, in-sample, these extensions clearly are warranted. In a recursive forecasting exercise, we …nd moderate improvements over a standard VAR, although most of these improvements are due to the use of stochastic search variable selection rather than the inclusion of breaks
Change-Point Testing and Estimation for Risk Measures in Time Series
We investigate methods of change-point testing and confidence interval
construction for nonparametric estimators of expected shortfall and related
risk measures in weakly dependent time series. A key aspect of our work is the
ability to detect general multiple structural changes in the tails of time
series marginal distributions. Unlike extant approaches for detecting tail
structural changes using quantities such as tail index, our approach does not
require parametric modeling of the tail and detects more general changes in the
tail. Additionally, our methods are based on the recently introduced
self-normalization technique for time series, allowing for statistical analysis
without the issues of consistent standard error estimation. The theoretical
foundation for our methods are functional central limit theorems, which we
develop under weak assumptions. An empirical study of S&P 500 returns and US
30-Year Treasury bonds illustrates the practical use of our methods in
detecting and quantifying market instability via the tails of financial time
series during times of financial crisis
- …