942 research outputs found
Spectral multiplicity for powers of weakly mixing automorphisms
We study the behavior of maximal multiplicities for the powers of
a weakly mixing automorphism . For some special infinite set we show the
existence of a weakly mixing rank-one automorphism such that
and for all . Moreover, the cardinality
of the set of spectral multiplicities for is not bounded. We have
and , , . We
also construct another weakly mixing automorphism with the following
properties: for but ,
all powers have homogeneous spectrum, and the set of limit points of
the sequence is infinite
Sub-wavelength imaging: Resolution enhancement using metal wire gratings
An experimental evidence of subwavelength imaging with a "lens", which is a
uniaxial negative permittivity wire medium slab, is reported. The slab is
formed by gratings of long thin parallel conducting cylinders. Taking into
account the anisotropy and spatial dispersion in the wire medium we
theoretically show that there are no usual plasmons that could be exited on
surfaces of such a slab, and there is no resonant enhancement of evanescent
fields in the slab. The experimentally observed clear improvement of the
resolution in the presence of the slab is explained as filtering out the
harmonics with small wavenumbers. In other words, the wire gratings (the wire
medium) suppress strong traveling-mode components increasing the role of
evanescent waves in the image formation. This effect can be used in near-field
imaging and detection applications.Comment: 12 pages, 6 figure
Detection of organic materials by spectrometric radiography method
In this paper we report a spectrometric approach to dual-energy digital
radiography that has been developed and applied to identify specific organic
substances and discern small differences in their effective atomic number. An
experimental setup has been designed, and a theoretical description proposed
based on the experimental results obtained. The proposed method is based on
application of special reference samples made of materials with different
effective atomic number and thickness, parameters known to affect X-ray
attenuation in the low-energy range. The results obtained can be used in the
development of a new generation of multi-energy customs or medical X-ray
scanners.Comment: 6 pages, 2 tables, 5 figures, will be presented at the Workshop on
X-Ray Imaging, 22-24 October, 2008, Dresden, German
Ontology-based data access with a horn fragment of metric temporal logic
We advocate datalogMTL, a datalog extension of a Horn fragment of the metric temporal logic MTL, as a language for ontology-based access to temporal log data. We show that datalogMTL is EXPSPACE-complete even with punctual intervals, in which case MTL is known to be undecidable. Nonrecursive datalogMTL turns out to be PSPACE-complete for combined complexity and in AC0 for data complexity. We demonstrate by two real-world use cases that nonrecursive datalogMTL programs can express complex temporal concepts from typical user queries and thereby facilitate access to log data. Our experiments with Siemens turbine data and MesoWest weather data show that datalogMTL ontology-mediated queries are efficient and scale on large datasets of up to 11GB
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