10,822 research outputs found
The Effect of Minimum Wage on Youth Employment and Unemployment in Taiwan
Minimum wage, Employment rate, Labor participation rate, Unemployment rate
Cognitive Radio from Hell: Flipping Attack on Direct-Sequence Spread Spectrum
In this paper, we introduce a strong adversarial attack, referred to as the
flipping attack, on Direct-Sequence Spread Spectrum (DSSS) systems. In this
attack, the attacker, which is appropriately positioned between the transmitter
and the receiver, instantaneously flips the transmitted symbols in the air at
50% rate, thereby driving the channel capacity to zero. Unlike the traditional
jamming attack, this attack, when perfectly executed, cannot be detected at the
receiver using signal-to-noise-ratio measurements. However, this attack
necessitates the attacker to perfectly know the realizations of all the
channels in the model. We first introduce the consequences of the flipping
attack on narrowband frequency-flat channels, and subsequently discuss its
feasibility in wideband frequency-selective channels. From the legitimate
users' perspective, we present a method to detect this attack and also propose
heuristics to improve the error-performance under the attack. We emphasize that
future cyber-physical systems that employ DSSS should design transceivers to
detect the proposed flipping attack, and then apply appropriate
countermeasures
Phase Information and the Evolution of Cosmological Density Perturbations
The Fourier transform of cosmological density perturbations can be
represented in terms of amplitudes and phases for each Fourier mode. We
investigate the phase evolution of these modes using a mixture of analytical
and numerical techniques. Using a toy model of one-dimensional perturbations
evolving under the Zel'dovich approximation as an initial motivation, we
develop a statistic that quantifies the information content of the distribution
of phases. Using numerical simulations beginning with more realistic Gaussian
random-phase initial conditions, we show that the information content of the
phases grows from zero in the initial conditions, first slowly and then rapidly
when structures become non-linear. This growth of phase information can be
expressed in terms of an effective entropy: Gaussian initial conditions are a
maximum entropy realisation of the initial power spectrum, gravitational
evolution decreases the phase entropy. We show that our definition of phase
entropy results in a statistic that explicitly quantifies the information
stored in the phases of density perturbations (rather than their amplitudes)
and that this statistic displays interesting scaling behaviour for self-similar
initial conditions.Comment: Accepted for publication in MNRAS with added comments on future work.
For high-resolution Figure 1, or postscript file, please see
http://www-star.qmw.ac.uk/~lyc
Non-linearity and Non-Gaussianity through Phase Information
In the standard picture of structure formation, initially random-phase
fluctuations are amplified by non-linear gravitational instability to produce a
final distribution of mass which is highly non-Gaussian and has highly coupled
Fourier phases. Second-order statistics, such as the power spectrum, are blind
to this kind of phase association. We discuss the information contained in the
phases of cosmological density fluctuations and their possible use in
statistical analysis tools. In particular, we show how the bispectrum measures
a particular form of phase association called quadratic phase coupling, show
how to visualise phase association using colour models. These techniques offer
the prospect of more complete tests of initial non-Gaussianity than those
available at present.Comment: 7 pages, 1 figure (two parts). To appear in the proceedings of The
MPA/ESO/MPE Joint Astronomy Conference "Mining the Sky" held in Garching,
Germany, July 31 - August 4 2000. To be published in the Springer-Verlag
series "ESO Astrophysics Symposia
Cytotoxicity of Mahanimbine, Murryafoline A and S-Benzyldithiocarbazate on Human Leukemic Cell Line, CEM-SS
Mahanimbine, a carbazole alkaloid was isolated from an ether extract of the stem bark
of Murraya koenigii whilst Murrayafoline A was isolated from petroleum ether extract
of the roots of Murraya koenigii. S-Benzyldithiocarbazate is a dithiocarbazic acid Schiff
base derived from S-alkyl esters. They were found to exhibit cytotoxic activity against
CEM-SS human T-lymphoblastic leukemic cells. The cytotoxic activity of
Mahanimbine, Murrayafoline A and S-Benzyldithiocarbazate that inhibit 50 % growth
(IC₅₀) of CEM-SS were 6 µg/ml, S µg/ml and 7.S µg/ml respectively. For comparative
purposes, the IC₅₀ of several commercial cytotoxic drugs against CEM-SS were
determined. The inhibition effect of Mahanimbine, Murrayafoline A and SBenzyldithiocarbazate
were better than Methotrexate (IC₅₀ > 30 µg/ml), Doxorubicine
(IC₅₀ = 21 µg/ml), Cytarabine (IC₅₀ > 30 µg/ml) and Colchecine (IC₅₀ = 8 µg/ml).These compounds were found to be less active than cis-diamine dichloroplatinwn and
Vinorelbine tartrate with a IC₅₀ value of 3 µg/ml. In contrast, these three compounds
were found to be less active against normal mouse fibroblasts cell, 3T3 with the IC₅₀
value of 11 µg/ml (Mahanimbine), 17 µg/ml (Murrayafoline A) and 10 µg/ml (SBenzyldithiocarbazate)
respectively. The study showed that the proliferation of cells
was inhibited before the cells were being killed. In addition, Mahanimbine,
MurrayafolineA and S-Benzyldithiocarbazate caused programmed cell death by
showing apoptotic features such as nucleus fragmentation, cell shrinkage, membrane
blebbing and formation of apoptotic bodies. These were further confirmed with DNA
laddering in agarose gel electrophoresis assay due to DNA fragmentation. DNA
laddering was obtained after 24 hours of treatment by these three compounds in a doseindependent
but time-dependent way. Mahanimbine and Murrayafoline A were shown
to arrest CEM-SS cells at G1 phase of cell cycle using flowcytometry method. As a
result, Mahanimbine, Murrayafoline A and S-Benzyldithiocarbazate were found as
potent antitumor agents
Kummer sandwiches and Greene-Plesser construction
In the context of K3 mirror symmetry, the Greene-Plesser orbifolding method
constructs a family of K3 surfaces, the mirror of quartic hypersurfaces in
, starting from a special one-parameter family of K3 varieties
known as the quartic Dwork pencil. We show that certain K3 double covers
obtained from the three-parameter family of quartic Kummer surfaces associated
with a principally polarized abelian surface generalize the relation of the
Dwork pencil and the quartic mirror family. Moreover, for the three-parameter
family we compute a formula for the rational point-count of its generic member
and derive its transformation behavior with respect to -isogenies of the
underlying abelian surface.Comment: 27 pages; minor typos corrected in version
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