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 $\mathbb{P}^3$, 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 $(2,2)$-isogenies of the underlying abelian surface.Comment: 27 pages; minor typos corrected in version