2,573 research outputs found
Analytical evaluation of the X-ray scattering contribution to imaging degradation in grazing-incidence X-ray telescopes
The focusing performance of X-ray optics (conveniently expressed in terms of
HEW, Half Energy Width) strongly depend on both mirrors deformations and photon
scattering caused by the microroughness of reflecting surfaces. In particular,
the contribution of X-ray Scattering (XRS) to the HEW of the optic is usually
an increasing function H(E) of the photon energy E. Therefore, in future hard
X-ray imaging telescopes of the future (SIMBOL-X, NeXT, Constellation-X, XEUS),
the X-ray scattering could be the dominant problem since they will operate also
in the hard X-ray band (i.e. beyond 10 keV). [...]
Several methods were proposed in the past years to estimate the scattering
contribution to the HEW, dealing with the surface microroughness expressed in
terms of its Power Spectral Density (PSD), on the basis of the well-established
theory of X-ray scattering from rough surfaces. We faced that problem on the
basis on the same theory, but we tried a new approach: the direct, analytical
translation of a given surface roughness PSD into a H(E) trend, and - vice
versa - the direct translation of a H(E) requirement into a surface PSD. This
PSD represents the maximum tolerable microroughness level in order to meet the
H(E) requirement in the energy band of a given X-ray telescope.
We have thereby found a new, analytical and widely applicable formalism to
compute the XRS contribution to the HEW from the surface PSD, provided that the
PSD had been measured in a wide range of spatial frequencies. The inverse
problem was also solved, allowing the immediate evaluation of the mirror
surface PSD from a measured function H(E). The same formalism allows
establishing the maximum allowed PSD of the mirror in order to fulfill a given
H(E) requirement. [...]Comment: 10 pages, 6 figures, published in Astronomy & Astrophysics, sect.
"Astronomical Instrumentation". In this version, a typo in two equations has
been corrected. After the correction, the other results, formulae and
conclusions in the paper remain unchange
Minimal Synthesis of String To String Functions From Examples
We study the problem of synthesizing string to string transformations from a
set of input/output examples. The transformations we consider are expressed
using deterministic finite automata (DFA) that read pairs of letters, one
letter from the input and one from the output. The DFA corresponding to these
transformations have additional constraints, ensuring that each input string is
mapped to exactly one output string.
We suggest that, given a set of input/output examples, the smallest DFA
consistent with the examples is a good candidate for the transformation the
user was expecting. We therefore study the problem of, given a set of examples,
finding a minimal DFA consistent with the examples and satisfying the
functionality and totality constraints mentioned above.
We prove that, in general, this problem (the corresponding decision problem)
is NP-complete. This is unlike the standard DFA minimization problem which can
be solved in polynomial time. We provide several NP-hardness proofs that show
the hardness of multiple (independent) variants of the problem.
Finally, we propose an algorithm for finding the minimal DFA consistent with
input/output examples, that uses a reduction to SMT solvers. We implemented the
algorithm, and used it to evaluate the likelihood that the minimal DFA indeed
corresponds to the DFA expected by the user.Comment: SYNT 201
Density-Dependence as a Size-Independent Regulatory Mechanism
The growth function of populations is central in biomathematics. The main
dogma is the existence of density dependence mechanisms, which can be modelled
with distinct functional forms that depend on the size of the population. One
important class of regulatory functions is the -logistic, which
generalises the logistic equation. Using this model as a motivation, this paper
introduces a simple dynamical reformulation that generalises many growth
functions. The reformulation consists of two equations, one for population
size, and one for the growth rate. Furthermore, the model shows that although
population is density-dependent, the dynamics of the growth rate does not
depend either on population size, nor on the carrying capacity. Actually, the
growth equation is uncoupled from the population size equation, and the model
has only two parameters, a Malthusian parameter and a competition
coefficient . Distinct sign combinations of these parameters reproduce
not only the family of -logistics, but also the van Bertalanffy,
Gompertz and Potential Growth equations, among other possibilities. It is also
shown that, except for two critical points, there is a general size-scaling
relation that includes those appearing in the most important allometric
theories, including the recently proposed Metabolic Theory of Ecology. With
this model, several issues of general interest are discussed such as the growth
of animal population, extinctions, cell growth and allometry, and the effect of
environment over a population.Comment: 41 Pages, 5 figures Submitted to JT
Comparison of demographic and direct methods to calculate probabilistic maturation reaction norms for Flemish Cap cod (Gadus morhua)
Age and length at maturation have declined in many fish populations and this has been hypothesized to be a genetic change caused by high fishing mortality. Probabilistic Maturation Reaction Norms (PMRNs) have been used as a tool to gain a better understanding of the possible genetic nature of these changes. The demographic and direct methods are two ways to calculate PMRNs. The data requirements are more often met for the demographic method than for the direct method which requires the identification of recruit spawners. However, the demographic method relies on more assumptions than the direct method, typically assuming equality of growth and mortality rates for immature and mature individuals within an age class. This study provides the first direct comparison of demographic and direct methods and shows that both methods produce comparable results. Differences between methods are hypothesized to be owed to possible differences in growth rate between mature and immature individuals in Flemish Cap cod
An asymptotic form of the reciprocity theorem with applications in x-ray scattering
The emission of electromagnetic waves from a source within or near a
non-trivial medium (with or without boundaries, crystalline or amorphous, with
inhomogeneities, absorption and so on) is sometimes studied using the
reciprocity principle. This is a variation of the method of Green's functions.
If one is only interested in the asymptotic radiation fields the generality of
these methods may actually be a shortcoming: obtaining expressions valid for
the uninteresting near fields is not just a wasted effort but may be
prohibitively difficult. In this work we obtain a modified form the reciprocity
principle which gives the asymptotic radiation field directly. The method may
be used to obtain the radiation from a prescribed source, and also to study
scattering problems. To illustrate the power of the method we study a few
pedagogical examples and then, as a more challenging application we tackle two
related problems. We calculate the specular reflection of x rays by a rough
surface and by a smoothly graded surface taking polarization effects into
account. In conventional treatments of reflection x rays are treated as scalar
waves, polarization effects are neglected. This is a good approximation at
grazing incidence but becomes increasingly questionable for soft x rays and UV
at higher incidence angles.
PACs: 61.10.Dp, 61.10.Kw, 03.50.DeComment: 19 pages, 4 figure
Long-term home cage activity scans reveal lowered exploratory behaviour in symptomatic female Rett mice
<p>Numerous experimental models have been developed to reiterate endophenotypes of Rett syndrome, a neurodevelopmental disorder with a multitude of motor, cognitive and vegetative symptoms. Here, female Mecp2Stop mice [1] were characterised at mild symptomatic conditions in tests for anxiety (open field, elevated plus maze) and home cage observation systems for food intake, locomotor activity and circadian rhythms.</p>
<p>Aged 8–9 months, Mecp2Stop mice presented with heightened body weight, lower overall activity in the open field, but no anxiety phenotype. Although home cage activity scans conducted in two different observation systems, PhenoMaster and PhenoTyper, confirmed normal circadian activity, they revealed severely compromised habituation to a novel environment in all parameters registered including those derived from a non-linear decay model such as initial exploration maximum, decay half-life of activity and span, as well as plateau. Furthermore, overall activity was significantly reduced in nocturnal periods due to reductions in both fast ambulatory movements, but also a slow lingering. In contrast, light-period activity profiles during which the amount of sleep was highest remained normal in Mecp2Stop mice.</p>
<p>These data confirm the slow and progressive development of Rett-like symptoms in female Mecp2Stop mice resulting in a prominent reduction of overall locomotor activity, while circadian rhythms are maintained. Alterations in the time-course of habituation may indicate deficiencies in cognitive processing.</p>
The influence of maternal and infant nutrition on cardiometabolic traits: novel findings and future research directions from four Canadian birth cohort studies
A mother's nutritional choices while pregnant may have a great influence on her baby's development in the womb and during infancy. There is evidence that what a mother eats during pregnancy interacts with her genes to affect her child's susceptibility to poor health outcomes including childhood obesity, pre-diabetes, allergy and asthma. Furthermore, after what an infant eats can change his or her intestinal bacteria, which can further influence the development of these poor outcomes. In the present paper, we review the importance of birth cohorts, the formation and early findings from a multi-ethnic birth cohort alliance in Canada and summarise our future research directions for this birth cohort alliance. We summarise a method for harmonising collection and analysis of self-reported dietary data across multiple cohorts and provide examples of how this birth cohort alliance has contributed to our understanding of gestational diabetes risk; ethnic and diet-influences differences in the healthy infant microbiome; and the interplay between diet, ethnicity and birth weight. Ongoing work in this birth cohort alliance will focus on the use of metabolomic profiling to measure dietary intake, discovery of unique diet–gene and diet–epigenome interactions, and qualitative interviews with families of children at risk of metabolic syndrome. Our findings to-date and future areas of research will advance the evidence base that informs dietary guidelines in pregnancy, infancy and childhood, and will be relevant to diverse and high-risk populations of Canada and other high-income countries
Oxidative status and fitness components in the Seychelles warbler
1. Oxidative damage, caused by reactive oxygen species during aerobic respiration, is thought to be an important mediator of life-history trade-offs. To mitigate oxidative damage, antioxidant defence mechanisms are deployed, often at the cost of resource allocation to other body functions. Both reduced resource allocation to body functions and direct oxidative damage may decrease individual fitness, through reducing survival and/or reproductive output. 2. The oxidative costs of reproduction have gained much attention recently, but few studies have investigated the long-term consequences of oxidative damage on survival and (future) reproductive output under natural conditions. 3. Using a wild population of the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis), we tested the prediction that high levels of reactive oxygen species, or high antioxidant investments to avoid oxidative damage, have fitness consequences because they reduce survival and/or reproductive output. 4. We found that individuals with higher circulating non-enzymatic antioxidant capacity had a lower probability of surviving until the next year. However, neither current reproductive output, nor future reproductive output in the surviving individuals, was associated with circulating non-enzymatic antioxidant capacity or oxidative damage. 5. The negative relationship between antioxidant capacity and survival that we observed concurs with the findings of an extensive comparative study on birds, however the mechanisms underlying this association remain to be resolved
Does reproduction cause oxidative stress? An open question
There has been substantial recent interest in the possible role of oxidative stress as a mechanism underlying life-history trade-offs, particularly with regard to reproductive costs. Several recent papers have found no evidence that reproduction increases oxidative damage and so have questioned the basis of the hypothesis that oxidative damage mediates the reproduction–lifespan trade-off. However, we suggest here that the absence of the predicted relationships could be due to a fundamental problem in the design of all of the published empirical studies, namely a failure to manipulate reproductive effort. We conclude by suggesting experimental approaches that might provide a more conclusive test of the hypothesis
Transport spin polarization of Ni_xFe_{1-x}: electronic kinematics and band structure
We present measurements of the transport spin polarization of Ni_xFe_{1-x}
(0<x<1) using the recently-developed Point Contact Andreev Reflection
technique, and compare them with our first principles calculations of the spin
polarization for this system. Surpisingly, the measured spin polarization is
almost composition-independent. The results clearly demonstrate that the sign
of the transport spin polarization does not coincide with that of the
difference of the densities of states at the Fermi level. Calculations indicate
that the independence of the spin polarization of the composition is due to
compensation of density of states and Fermi velocity in the s- and d- bands
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