15,366 research outputs found
Teaching an Old Dog New Tricks: Improved Estimation of the Parameters of Stochastic Differential Equations by Numerical Solution of the Fokker-Planck Equation
Many stochastic differential equations (SDEs) do not have readily available closed-form expressions for their transitional probability density functions (PDFs). As a result, a large number of competing estimation approaches have been proposed in order to obtain maximum-likelihood estimates of their parameters. Arguably the most straightforward of these is one in which the required estimates of the transitional PDF are obtained by numerical solution of the Fokker-Planck(or forward-Kolmogorov) partial differential equation. Despite the fact that this method produces accurate estimates and is completely generic, it has not proved popular in the applied literature. Perhaps this is attributable to the fact that this approach requires repeated solution of a parabolic partial differential equation to obtain the transitional PDF and is therefore computationally quite expensive. In this paper, three avenues for improving the reliability and speed of this estimation method are introduced and explored in the context of estimating the parameters of the popular Cox-Ingersoll-Ross and Ornstein-Uhlenbeck models. The recommended algorithm that emerges from this investigation is seen to offer substantial gains in reliability and computational time.stochastic di®erential equations, maximum likelihood, ¯nite di®erence, ¯nite element, cumulative
Chronic sequelae of foodborne disease.
In the past decade the complexity of foodborne pathogens, as well as their adaptability and ability to cause acute illness, and in some cases chronic (secondary) complications, have been newly appreciated. This overview examines long-term consequences of foodborne infections and intoxications to emphasize the need for more research and education
Role of Schizosaccharomyces pombe RecQ homolog recombination and checkpoint genes in UV Damage tolerance
The cellular responses to DNA damage are complex and include direct DNA repair pathways that remove the damage and indirect damage responses which allow cells to survive DNA damage that has not been, or cannot be, removed. We have identified the gene mutated in the rad12.502 strain as a Schizosaccharomyces pombe recQ homolog. The same gene (designated rqh1) is also mutated in the hus2.22 mutant. We show that Rqh1 is involved in a DNA damage survival mechanism which prevents cell death when UV-induced DNA damage cannot be removed. This pathway also requires the correct functioning of the recombination machinery and the six checkpoint tad gene products plus the Cds1 kinase. Our data suggest that Rqh1 operates during S phase as part of a mechanism which prevents DNA damage causing cell lethality. This process may involve the bypass of DNA damage sites by the replication fork. Finally, in contrast with the reported literature, we do not find that rqh1 (rad12) mutant cells are defective in UV dimer endonuclease activity
The use of historical accounts of species distribution to suggest restoration targets for UK upland mires within a ‘moorland’ landscape
Using contemporary accounts of vegetation in the South Pennine landscape of northern England from the late 18th Century to the present day, we describe the degradation and subsequent partial recovery of these upland mire systems in terms of their vegetation biodiversity. The historical sources highlight several species that were once common on these peatlands but which do not currently feature as positive indicator species in monitoring or restoration programmes. The use of archival sources may provide additional evidence that complements palaeoecological data when setting restoration targets. For example, the historical accounts support the palaeoecological timeline for the disappearance of Sphagnum from these landscapes. As a step toward a possible expanded set of targets for restoration works, we suggest an extended list of positive indicator species for the South Pennines which could also be applicable elsewhere in the UK. These include species such as lesser twayblade (Neottia cordata) and the club mosses (Lycopodiaceae), which were noted to be common in the 18th and 19th centuries, but which had become rare by the early 20th century. We highlight changes in land ownership and land use through the process of enclosure, as well as indirect effects from industrialisation, as the dominant interacting drivers of vegetation change
An iron(ii) spin-crossover metallacycle from a back-to-back bis-[dipyrazolylpyridine]
The syntheses of 4-mercapto-2,6-di(pyrazol-1-yl)pyridine (bppSH) and bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]disulfide (bppSSbpp) are reported. In contrast to previously published “back-to-back” bis-[2,6-di(pyrazol-1-yl)pyridine] derivatives, which form coordination polymers with transition ions that are usually insoluble, bppSSbpp yields soluble oligomeric complexes with iron(II) and zinc(II). Mass spectrometry and DOSY data show that [{Fe(μ-bppSSbpp)}n]2n+ and [{Zn(μ-bppSSbpp)}n]2n+ form tetranuclear metallacycles in nitromethane solution (n = 4), although 1H NMR and conductivity measurements imply the iron compound may undergo more fragmentation than its zinc congener. Both [{Fe(bppSH)2]2+ and [{Fe(μ-bppSSbpp)}n]2n+ exhibit thermal spin-crossover in CD3NO2 solution, with midpoint temperatures near 245 K. The similarity of these equilibria implies there is little cooperativity between the iron centres in the metallacyclic structures
Design data for brazed Rene 41 honeycomb sandwich
Strength data, creep data and residual strength data after cyclic thermal exposure were obtained at temperatures from 78 K to 1144 K (-320 F to 1600 F). The influences of face thickness, core depth, core gage, cell size and thermal/stress exposure conditions on the mechanical design properties were investigated. A braze alloy and process was developed that is adequate to fully develop the strength of the honeycomb core while simultaneously solution treating and aging the Rene 41 fact sheets. New test procedures and test specimen configurations were developed to avoid excessive thermal stresses during cyclic thermal exposure
Radial Color Gradients in K+A Galaxies in Distant Clusters of Galaxies
Galaxies in rich clusters with z 0.3 are observed to have a higher
fraction of photometrically blue galaxies than their nearby counterparts. This
raises the important question of what environmental effects can cause the
termination of star formation between z 0.3 and the present. The star
formation may be truncated due to ram-pressure stripping, or the gas in the
disk may be depleted by an episode of star formation caused by some external
perturbation. To help resolve this issue, surface photometry was carried out
for a total of 70 early-type galaxies in the cluster Cl1358+62, at z
0.33, using two-color images from the Hubble Archive. The galaxies were divided
into two categories based on spectroscopic criteria: 24 are type K+A (e.g.,
strong Balmer lines, with no visible emission lines), while the remaining 46
are in the control sample with normal spectra. Radial color profiles were
produced to see if the K+A galaxies show bluer nuclei in relation to their
surrounding disks. Specifically, a linear gradient was fit to the radial color
profile of each galaxy. We find that the K+A galaxies on average tend to have
slightly bluer gradients towards the center than the normals. A
Kolmogorov-Smirnov two-sample test has been applied to the two sets of color
gradients. The result of the test indicates that there is only a 2%
probability that the K+A and normal samples are drawn from the same parent
distribution. There is a possible complication from a trend in the apparent
magnitude vs. color gradient relation, but overall our results favor the
centralized star formation scenario as an important process in the evolution of
galaxies in dense clusters.Comment: 16 pages, 12 figures, accepted for publication in A
Physically founded phonon dispersions of few-layer materials, and the case of borophene
An increasing number of theoretical calculations on few-layer materials have
been reporting a non-zero sound velocity for all three acoustic phonon modes.
In contrast with these reports, here we show that the lowest phonon dispersion
branch of atomistically described few-layer materials should be quadratic, and
this can have dramatic consequencies on calculated properties, such as the
thermal conductivity. By reformulating the interatomic force constants (IFC) in
terms of internal coordinates, we find that a delicate balance between the IFCs
is responsible for this quadraticity. This balance is hard to obtain in
ab-initio calculations even if all the symmetries are numerically enforced a
posteriori, but it arises naturally in our approach. We demonstrate the
phenomenon in the case of borophene, where a very subtle correction to the
ab-initio IFCs yields the physically correct quadratic dispersion, while
leaving the rest of the spectrum virtually unmodified. Such quadraticity
nevertheless has a major effect on the computed lattice thermal conductivity,
which in the case of borophene changes by more than a factor 2, and reverses
its anisotropy, when the subtle IFC correction is put in place
- …