5,107 research outputs found
Simple strong glass forming models: mean-field solution with activation
We introduce simple models, inspired by previous models for froths and
covalent glasses, with trivial equilibrium properties but dynamical behaviour
characteristic of strong glass forming systems. These models are also a
generalization of backgammon or urn models to a non--constant number of
particles, where entropic barriers are replaced by energy barriers, allowing
for the existence of activated processes. We formulate a mean--field version of
the models, which keeps most of the features of the finite dimensional ones,
and solve analytically the out--of--equilibrium dynamics in the low temperature
regime where activation plays an essential role.Comment: 18 pages, 9 figure
Probability Models for Degree Distributions of Protein Interaction Networks
The degree distribution of many biological and technological networks has
been described as a power-law distribution. While the degree distribution does
not capture all aspects of a network, it has often been suggested that its
functional form contains important clues as to underlying evolutionary
processes that have shaped the network. Generally, the functional form for the
degree distribution has been determined in an ad-hoc fashion, with clear
power-law like behaviour often only extending over a limited range of
connectivities. Here we apply formal model selection techniques to decide which
probability distribution best describes the degree distributions of protein
interaction networks. Contrary to previous studies this well defined approach
suggests that the degree distribution of many molecular networks is often
better described by distributions other than the popular power-law
distribution. This, in turn, suggests that simple, if elegant, models may not
necessarily help in the quantitative understanding of complex biological
processes.
Reliable estimation of prediction uncertainty for physico-chemical property models
The predictions of parameteric property models and their uncertainties are
sensitive to systematic errors such as inconsistent reference data, parametric
model assumptions, or inadequate computational methods. Here, we discuss the
calibration of property models in the light of bootstrapping, a sampling method
akin to Bayesian inference that can be employed for identifying systematic
errors and for reliable estimation of the prediction uncertainty. We apply
bootstrapping to assess a linear property model linking the 57Fe Moessbauer
isomer shift to the contact electron density at the iron nucleus for a diverse
set of 44 molecular iron compounds. The contact electron density is calculated
with twelve density functionals across Jacob's ladder (PWLDA, BP86, BLYP, PW91,
PBE, M06-L, TPSS, B3LYP, B3PW91, PBE0, M06, TPSSh). We provide systematic-error
diagnostics and reliable, locally resolved uncertainties for isomer-shift
predictions. Pure and hybrid density functionals yield average prediction
uncertainties of 0.06-0.08 mm/s and 0.04-0.05 mm/s, respectively, the latter
being close to the average experimental uncertainty of 0.02 mm/s. Furthermore,
we show that both model parameters and prediction uncertainty depend
significantly on the composition and number of reference data points.
Accordingly, we suggest that rankings of density functionals based on
performance measures (e.g., the coefficient of correlation, r2, or the
root-mean-square error, RMSE) should not be inferred from a single data set.
This study presents the first statistically rigorous calibration analysis for
theoretical Moessbauer spectroscopy, which is of general applicability for
physico-chemical property models and not restricted to isomer-shift
predictions. We provide the statistically meaningful reference data set MIS39
and a new calibration of the isomer shift based on the PBE0 functional.Comment: 49 pages, 9 figures, 7 table
Calculations of the Local Density of States for some Simple Systems
A recently proposed convolution technique for the calculation of local
density of states is described more thouroughly and new results of its
application are presented. For separable systems the exposed method allows to
construct the ldos for a higher dimensionality out of lower dimensional parts.
Some practical and theoretical aspects of this approach are also discussed.Comment: 5 pages, 3 figure
RXTE Observations of 1A 1744-361: Correlated Spectral and Timing Behavior
We analyze Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array
(PCA) data of the transient low mass X-ray binary (LMXB) system 1A 1744-361. We
explore the X-ray intensity and spectral evolution of the source, perform
timing analysis, and find that 1A 1744-361 shows `atoll' behavior during the
outbursts. The color-color diagram indicates that this LMXB was observed in a
low intensity spectrally hard (low-hard) state and in a high intensity `banana'
state. The low-hard state shows a horizontal pattern in the color-color
diagram, and the previously reported `dipper QPO' appears only during this
state. We also perform energy spectral analyses, and report the first detection
of broad iron emission line and iron absorption edge from 1A 1744-361.Comment: 20 pages, 4 tables, 4 figures, accepted for publication in Ap
Prevention is better than cure, but...: Preventive medication as a risk to ordinariness?
Preventive health remains at the forefront of public health concerns; recent initiatives, such as the NHS health check, may lead to recommendations for medication in response to the identification of 'at risk' individuals. Little is known about lay views of preventive medication. This paper uses the case of aspirin as a prophylactic against heart disease to explore views among people invited to screening for a trial investigating the efficacy of such an approach. Qualitative interviews (N=46) and focus groups (N=5, participants 31) revealed dilemmas about preventive medication in the form of clashes between norms: first, in general terms, assumptions about the benefit of prevention were complicated by dislike of medication; second, the individual duty to engage in prevention was complicated by the need not to be over involved with one's own health; third, the potential appeal of this alternative approach to health promotion was complicated by unease about the implications of encouraging irresponsible behaviour among others. Though respondents made different decisions about using the drug, they reported very similar ways of trying to resolve these conflicts, drawing upon concepts of necessity and legitimisation and the special ordinariness of the particular dru
Liposomal clodronate inhibition of osteoclastogenesis and osteoinduction by submicrostructured beta-tricalcium phosphate
Bone graft substitutes such as calcium phosphates are subject to the innate inflammatory reaction, which may bear important consequences for bone regeneration. We speculate that the surface architecture of osteoinductive β-tricalcium phosphate (TCP) stimulates the differentiation of invading monocyte/macrophages into osteoclasts, and that these cells may be essential to ectopic bone formation. To test this, porous TCP cubes with either submicron-scale surface architecture known to induce ectopic bone formation (TCPs, positive control) or micron-scale, non-osteoinductive surface architecture (TCPb, negative control) were subcutaneously implanted on the backs of FVB strain mice for 12 weeks. Additional TCPs samples received local, weekly injections of liposome-encapsulated clodronate (TCPs + LipClod) to deplete invading monocyte/macrophages. TCPs induced osteoclast formation, evident by positive tartrate resistant acid phosphatase (TRAP) cytochemical staining and negative macrophage membrane marker F4/80 immunostaining. No TRAP positive cells were found in TCPb or TCPs + LipClod, only F4/80 positive macrophages and foreign body giant cells. TCPs stimulated subcutaneous bone formation in all implants, while no bone could be found in TCPb or TCPs + LipClod. In agreement, expression of bone and osteoclast gene markers was upregulated in TCPs versus both TCPb and TCPs + LipClod, which were equivalent. In summary, submicron-scale surface structure of TCP induced osteoclastogenesis and ectopic bone formation in a process that is blocked by monocyte/macrophage depletion
Reconstruction from Radon projections and orthogonal expansion on a ball
The relation between Radon transform and orthogonal expansions of a function
on the unit ball in \RR^d is exploited. A compact formula for the partial
sums of the expansion is given in terms of the Radon transform, which leads to
algorithms for image reconstruction from Radon data. The relation between
orthogonal expansion and the singular value decomposition of the Radon
transform is also exploited.Comment: 15 page
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