5,570 research outputs found
Fractal Characterizations of MAX Statistical Distribution in Genetic Association Studies
Two non-integer parameters are defined for MAX statistics, which are maxima
of simpler test statistics. The first parameter, , is the
fractional number of tests, representing the equivalent numbers of independent
tests in MAX. If the tests are dependent, . The second
parameter is the fractional degrees of freedom of the chi-square
distribution that fits the MAX null distribution. These two
parameters, and , can be independently defined, and can be
non-integer even if is an integer. We illustrate these two parameters
using the example of MAX2 and MAX3 statistics in genetic case-control studies.
We speculate that is related to the amount of ambiguity of the model
inferred by the test. In the case-control genetic association, tests with low
(e.g. ) are able to provide definitive information about the disease
model, as versus tests with high (e.g. ) that are completely uncertain
about the disease model. Similar to Heisenberg's uncertain principle, the
ability to infer disease model and the ability to detect significant
association may not be simultaneously optimized, and seems to measure the
level of their balance
Moving boulders in flash floods and estimating flow conditions using boulders in ancient deposits
Boulders moving in flash floods cause considerable damage and casualties. More and bigger boulders move in flash floods than predicted from published theory. The interpretation of flow conditions from the size of large particles within flash flood deposits has, until now, generally assumed that the velocity (or discharge) is unchanging in time (i.e. flow is steady), or changes instantaneously between periods of constant conditions. Standard practice is to apply theories developed for steady flow conditions to flash floods, which are however inherently very unsteady flows. This is likely to lead to overestimates of peak flow velocity (or discharge). Flash floods are characterised by extremely rapid variations in flow that generate significant transient forces in addition to the mean-flow drag. These transient forces, generated by rapid velocity changes, are generally ignored in published theories, but they are briefly so large that they could initiate the motion of boulders. This paper develops a theory for the initiation of boulder movement due to the additional impulsive force generated by unsteady flow, and discusses the implications. Keywords
Persistence of the immune response induced by BCG vaccination.
BACKGROUND: Although BCG vaccination is recommended in most countries of the world, little is known of the persistence of BCG-induced immune responses. As novel TB vaccines may be given to boost the immunity induced by neonatal BCG vaccination, evidence concerning the persistence of the BCG vaccine-induced response would help inform decisions about when such boosting would be most effective. METHODS: A randomised control study of UK adolescents was carried out to investigate persistence of BCG immune responses. Adolescents were tested for interferon-gamma (IFN-gamma) response to Mycobacterium tuberculosis purified protein derivative (M.tb PPD) in a whole blood assay before, 3 months, 12 months (n = 148) and 3 years (n = 19) after receiving teenage BCG vaccination or 14 years after receiving infant BCG vaccination (n = 16). RESULTS: A gradual reduction in magnitude of response was evident from 3 months to 1 year and from 1 year to 3 years following teenage vaccination, but responses 3 years after vaccination were still on average 6 times higher than before vaccination among vaccinees. Some individuals (11/86; 13%) failed to make a detectable antigen-specific response three months after vaccination, or lost the response after 1 (11/86; 13%) or 3 (3/19; 16%) years. IFN-gamma response to Ag85 was measured in a subgroup of adolescents and appeared to be better maintained with no decline from 3 to 12 months. A smaller group of adolescents were tested 14 years after receiving infant BCG vaccination and 13/16 (81%) made a detectable IFN-gamma response to M.tb PPD 14 years after infant vaccination as compared to 6/16 (38%) matched unvaccinated controls (p = 0.012); teenagers vaccinated in infancy were 19 times more likely to make an IFN-gamma response of > 500 pg/ml than unvaccinated teenagers. CONCLUSION: BCG vaccination in infancy and adolescence induces immunological memory to mycobacterial antigens that is still present and measurable for at least 14 years in the majority of vaccinees, although the magnitude of the peripheral blood response wanes from 3 months to 12 months and from 12 months to 3 years post vaccination. The data presented here suggest that because of such waning in the response there may be scope for boosting anti-tuberculous immunity in BCG vaccinated children anytime from 3 months post-vaccination. This supports the prime boost strategies being employed for some new TB vaccines currently under development
Pressure-induced superconductivity in the giant Rashba system BiTeI
At ambient pressure, BiTeI is the first material found to exhibit a giant
Rashba splitting of the bulk electronic bands. At low pressures, BiTeI
undergoes a transition from trivial insulator to topological insulator. At
still higher pressures, two structural transitions are known to occur. We have
carried out a series of electrical resistivity and AC magnetic susceptibility
measurements on BiTeI at pressure up to ~40 GPa in an effort to characterize
the properties of the high-pressure phases. A previous calculation found that
the high-pressure orthorhombic P4/nmm structure BiTeI is a metal. We find that
this structure is superconducting with Tc values as high as 6 K. AC magnetic
susceptibility measurements support the bulk nature of the superconductivity.
Using electronic structure and phonon calculations, we compute Tc and find that
our data is consistent with phonon-mediated superconductivity.Comment: 7 pages, 7 figure
Determination of Effective Permittivity and Permeability of Metamaterials from Reflection and Transmission Coefficients
We analyze the reflection and transmission coefficients calculated from
transfer matrix simulations on finite lenghts of electromagnetic metamaterials,
to determine the effective permittivity and permeability. We perform this
analysis on structures composed of periodic arrangements of wires, split ring
resonators (SRRs) and both wires and SRRs. We find the recovered
frequency-dependent permittivity and permeability are entirely consistent with
analytic expressions predicted by effective medium arguments. Of particular
relevance are that a wire medium exhibits a frequency region in which the real
part of permittivity is negative, and SRRs produce a frequency region in which
the real part of permeability is negative. In the combination structure, at
frequencies where both the recovered real part of permittivity and permeability
are simultaneously negative, the real part of the index-of-refraction is found
also to be unambigously negative.Comment: *.pdf file, 5 figure
An examination of the relationship of governance structure and performance: Evidence from banking companies in Bangladesh
Corporate governance has become increasingly important in developed and developing countries just after a series of corporate scandals and failures in a number of countries. Corporate governance structure is often viewed as a means of corporate success despite prior studies reveal mixed, somewhere conflicting and ambiguous, and somewhere no relationship between governance structure and performance. This study empirically investigates the relationship between corporate governance mechanisms and financial performance of listed banking companies in Bangladesh by using two multiple regression models. The study reveals that a good number of companies do not comply with the regulatory requirements indicating remarkable shortfall in corporate governance practice. The companies are run by the professional managers having no duality and no ownership interest for which they are compensated by high remuneration to curb agency conflict. Apart from some inconsistent relationship between some corporate variables, the corporate governance mechanisms do not appear to have significant relationship with financial performances. The findings reveal an insignificant negative impact or somewhere no impact of independent directors and non-independent non-executive directors on the level of performance that strongly support the concept that the managers are essentially worthy of trust and earn returns for the owners as claimed by stewardship theory. The study provides support for the view that while much emphasis on corporate governance mechanisms is necessary to safeguard the interest of stakeholders; corporate governance on its own, as a set of codes or standards for corporate conformance, cannot make a company successful. Companies need to balance corporate governance mechanisms with performance by adopting strategic decision and risk management with the efficient utilization of the organization’s resources
Killing Tensors and Conformal Killing Tensors from Conformal Killing Vectors
Koutras has proposed some methods to construct reducible proper conformal
Killing tensors and Killing tensors (which are, in general, irreducible) when a
pair of orthogonal conformal Killing vectors exist in a given space. We give
the completely general result demonstrating that this severe restriction of
orthogonality is unnecessary. In addition we correct and extend some results
concerning Killing tensors constructed from a single conformal Killing vector.
A number of examples demonstrate how it is possible to construct a much larger
class of reducible proper conformal Killing tensors and Killing tensors than
permitted by the Koutras algorithms. In particular, by showing that all
conformal Killing tensors are reducible in conformally flat spaces, we have a
method of constructing all conformal Killing tensors (including all the Killing
tensors which will in general be irreducible) of conformally flat spaces using
their conformal Killing vectors.Comment: 18 pages References added. Comments and reference to 2-dim case.
Typos correcte
High pressure transport properties of the topological insulator Bi2Se3
We report x-ray diffraction, electrical resistivity, and magnetoresistance
measurements on Bi2Se3 under high pressure and low temperature conditions.
Pressure induces profound changes in both the room temperature value of the
electrical resistivity as well as the temperature dependence of the
resistivity. Initially, pressure drives Bi2Se3 towards increasingly insulating
behavior and then, at higher pressures, the sample appears to enter a fully
metallic state coincident with a change in the crystal structure. Within the
low pressure phase, Bi2Se3 exhibits an unusual field dependence of the
transverse magnetoresistance that is positive at low fields and becomes
negative at higher fields. Our results demonstrate that pressures below 8 GPa
provide a non-chemical means to controllably reduce the bulk conductivity of
Bi2Se3
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