2,618 research outputs found
Uniform Local Amenability
The main results of this paper show that various coarse (`large scale')
geometric properties are closely related. In particular, we show that property
A implies the operator norm localisation property, and thus that norms of
operators associated to a very large class of metric spaces can be effectively
estimated.
The main tool is a new property called uniform local amenability. This
property is easy to negate, which we use to study some `bad' spaces. We also
generalise and reprove a theorem of Nowak relating amenability and asymptotic
dimension in the quantitative setting
Expanders and Property A
We give a cohomological characterisation of expander graphs, and use it to
give a direct proof that expander graphs do not have Yu's property A
Reconciling the Classical-Field Method with the Beliaev Broken Symmetry Approach
We present our views on the issues raised in the chapter by Griffin and
Zaremba [A. Griffin and E. Zaremba, in Quantum Gases: Finite Temperature and
Non-Equilibrium Dynamics, N. P. Proukakis, S. A. Gardiner, M. J. Davis, and M.
H. Szymanska, eds., Imperial College Press, London (in press)]. We review some
of the strengths and limitations of the Bose symmetry-breaking assumption, and
explain how such an approach precludes the description of many important
phenomena in degenerate Bose gases. We discuss the theoretical justification
for the classical-field (c-field) methods, their relation to other
non-perturbative methods for similar systems, and their utility in the
description of beyond-mean-field physics. Although it is true that present
implementations of c-field methods cannot accurately describe certain
collective oscillations of the partially condensed Bose gas, there is no
fundamental reason why these methods cannot be extended to treat such
scenarios. By contrast, many regimes of non-equilibrium dynamics that can be
described with c-field methods are beyond the reach of generalised mean-field
kinetic approaches based on symmetry-breaking, such as the ZNG formalism.Comment: 8 pages. Unedited version of chapter to appear in Quantum Gases:
Finite Temperature and Non-Equilibrium Dynamics (Vol. 1 Cold Atoms Series).
N.P. Proukakis, S.A. Gardiner, M.J. Davis and M.H. Szymanska, eds. Imperial
College Press, London (in press). See
http://www.icpress.co.uk/physics/p817.html v2: Added arXiv cross-reference
DNA → RNA: What Do Students Think the Arrow Means?
The central dogma of molecular biology, a model that has remained intact for decades, describes the transfer of genetic information from DNA to protein though an RNA intermediate. While recent work has illustrated many exceptions to the central dogma, it is still a common model used to describe and study the relationship between genes and protein products. We investigated understanding of central dogma concepts and found that students are not primed to think about information when presented with the canonical figure of the central dogma. We also uncovered conceptual errors in student interpretation of the meaning of the transcription arrow in the central dogma representation; 36% of students (n = 128; all undergraduate levels) described transcription as a chemical conversion of DNA into RNA or suggested that RNA existed before the process of transcription began. Interviews confirm that students with weak conceptual understanding of information flow find inappropriate meaning in the canonical representation of central dogma. Therefore, we suggest that use of this representation during instruction can be counterproductive unless educators are explicit about the underlying meanin
The stellar activity-rotation relationship
Using a new catalog of 824 solar and late-type stars with measured X-ray luminosities and rotation periods we have studied the relationship between rotation and stellar activity that is believed to be a probe of the underlying stellar dynamo. Using an unbiased subset of the sample we calculate the power law slope of the unsaturated regime of the activity-rotation relationship as LX/Lbol α Roβ, where β = –2.70 ± 0.13. Notably this is inconsistent with the canonical β = –2 slope to a confidence of 5σ are argues for an interface-type dynamo. Super-saturation is observed for the fastest rotators in our sample and its parametric dependencies are explored. Significant correlations are found with both the corotation radius and the excess polar updraft, the latter theory being preferred as it is supported by other observations. We also present a new X-ray population synthesis model of the mature stellar component of our Galaxy and use it to reproduce deep observations of a high Galactic latitude field. The model, XStar, can be used to test models of stellar spin-down and dynamo decay, as well as for estimating stellar X-ray contamination rates for non-stellar studies. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Development of the Central Dogma Concept Inventory (CDCI) Assessment Tool
Scientific teaching requires scientifically constructed, field-tested instruments to accurately evaluate student thinking and gauge teacher effectiveness. We have developed a 23-question, multiple select–format assessment of student understanding of the essential concepts of the central dogma of molecular biology that is appropriate for all levels of undergraduate biology. Questions for the Central Dogma Concept Inventory (CDCI) tool were developed and iteratively revised based on student language and review by experts. The ability of the CDCI to discriminate between levels of understanding of the central dogma is supported by field testing (N = 54), and large-scale beta testing (N = 1733). Performance on the assessment increased with experience in biology; scores covered a broad range and showed no ceiling effect, even with senior biology majors, and pre/posttesting of a single class focused on the central dogma showed significant improvement. The multiple-select format reduces the chances of correct answers by random guessing, allows students at different levels to exhibit the extent of their knowledge, and provides deeper insight into the complexity of student thinking on each theme. To date, the CDCI is the first tool dedicated to measuring student thinking about the central dogma of molecular biology, and version 5 is ready to use
Facilitating the operational readiness of the NHS for the in-house manufacture and delivery of autologous cell therapy [210]
Facilitating the operational readiness of the NHS for the in-house manufacture and delivery of autologous cell therapy [210
The statistical mechanics of a polygenic characterunder stabilizing selection, mutation and drift
By exploiting an analogy between population genetics and statistical
mechanics, we study the evolution of a polygenic trait under stabilizing
selection, mutation, and genetic drift. This requires us to track only four
macroscopic variables, instead of the distribution of all the allele
frequencies that influence the trait. These macroscopic variables are the
expectations of: the trait mean and its square, the genetic variance, and of a
measure of heterozygosity, and are derived from a generating function that is
in turn derived by maximizing an entropy measure. These four macroscopics are
enough to accurately describe the dynamics of the trait mean and of its genetic
variance (and in principle of any other quantity). Unlike previous approaches
that were based on an infinite series of moments or cumulants, which had to be
truncated arbitrarily, our calculations provide a well-defined approximation
procedure. We apply the framework to abrupt and gradual changes in the optimum,
as well as to changes in the strength of stabilizing selection. Our
approximations are surprisingly accurate, even for systems with as few as 5
loci. We find that when the effects of drift are included, the expected genetic
variance is hardly altered by directional selection, even though it fluctuates
in any particular instance. We also find hysteresis, showing that even after
averaging over the microscopic variables, the macroscopic trajectories retain a
memory of the underlying genetic states.Comment: 35 pages, 8 figure
Beyond the consensus: dissecting within-host viral population diversity of foot-and-mouth disease virus using next-generation genome sequencing
The sequence diversity of viral populations within individual hosts is the
starting material for selection and subsequent evolution of RNA viruses such as
foot-and-mouth disease virus (FMDV). Using next-generation sequencing (NGS)
performed on a Genome Analyzer platform (Illumina), this study compared the
viral populations within two bovine epithelial samples (foot lesions) from a
single animal with the Inoculum used to initiate experimental infection.
Genomic sequences were determined in duplicate sequencing runs, and the
consensus sequence determined by NGS, for the Inoculum, was identical to that
previously determined using the Sanger method. However, NGS reveals the fine
polymorphic sub-structure of the viral population, from nucleotide variants
present at just below 50% frequency to those present at fractions of 1%. Some
of the higher frequency polymorphisms identified encoded changes within codons
associated with heparan sulphate binding and were present in both feet lesions
revealing intermediate stages in the evolution of a tissue-culture adapted
virus replicating within a mammalian host. We identified 2,622, 1,434 and 1,703
polymorphisms in the Inoculum, and in the two foot lesions respectively: most
of the substitutions occurred only in a small fraction of the population and
represent the progeny from recent cellular replication prior to onset of any
selective pressures. We estimated an upper limit for the genome-wide mutation
rate of the virus within a cell to be 7.8 x 10-4 per nt. The greater depth of
detection, achieved by NGS, demonstrates that this method is a powerful and
valuable tool for the dissection of FMDV populations within-hosts
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