57,558 research outputs found
Off-Campus Library Services in Higher Education in the United Kingdom
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How to keep the noise down without using the ‘Shush’ word!
In autumn 2005 Leeds Metropolitan University’s Civic Quarter Library began a pilot to ‘zone’ study space to meet the differing demands of students. Requests asking for increased group study areas as more assignments require collaborative work had to be balanced by silent areas to support more traditional use of the library. Previously staff had often found themselves in an unpleasant situation trying to keep noise levels down on the floors: we hoped that by offering students somewhere else to go within the library it would be less stressful for all concerned
Identifying Keystone Species in the Human Gut Microbiome from Metagenomic Timeseries using Sparse Linear Regression
Human associated microbial communities exert tremendous influence over human
health and disease. With modern metagenomic sequencing methods it is possible
to follow the relative abundance of microbes in a community over time. These
microbial communities exhibit rich ecological dynamics and an important goal of
microbial ecology is to infer the interactions between species from sequence
data. Any algorithm for inferring species interactions must overcome three
obstacles: 1) a correlation between the abundances of two species does not
imply that those species are interacting, 2) the sum constraint on the relative
abundances obtained from metagenomic studies makes it difficult to infer the
parameters in timeseries models, and 3) errors due to experimental uncertainty,
or mis-assignment of sequencing reads into operational taxonomic units, bias
inferences of species interactions. Here we introduce an approach, Learning
Interactions from MIcrobial Time Series (LIMITS), that overcomes these
obstacles. LIMITS uses sparse linear regression with boostrap aggregation to
infer a discrete-time Lotka-Volterra model for microbial dynamics. We tested
LIMITS on synthetic data and showed that it could reliably infer the topology
of the inter-species ecological interactions. We then used LIMITS to
characterize the species interactions in the gut microbiomes of two individuals
and found that the interaction networks varied significantly between
individuals. Furthermore, we found that the interaction networks of the two
individuals are dominated by distinct "keystone species", Bacteroides fragilis
and Bacteroided stercosis, that have a disproportionate influence on the
structure of the gut microbiome even though they are only found in moderate
abundance. Based on our results, we hypothesize that the abundances of certain
keystone species may be responsible for individuality in the human gut
microbiome
The Nonlinear Redshift Space Power Spectrum: Omega from Redshift Surveys
We examine the anisotropies in the power spectrum by the mapping of real to
redshift space. Using the Zel'dovich approximation, we obtain an analytic
expression for the nonlinear redshift space power spectrum in the distant
observer limit. For a given unbiased galaxy distribution in redshift space, the
anisotropies in the power spectrum depend on the parameter , where is the density parameter. We quantify these
anisotropies by the ratio, , of the quadrupole to monopole angular moments
of the power spectrum. In contrast to linear theory, the Zel'dovich
approximation predicts a decline in with decreasing scale. This departure
from linear theory is due to nonlinear dynamics and not a result of incoherent
random velocities. The rate of decline depends strongly on and the
initial power spectrum. However, we find a {\it universal} relation between the
quantity (where the linear theory value of ) and the
dimensionless variable , where is a wavenumber determined by
the scale of nonlinear structures. The universal relation is in good agreement
with a large N-body simulation. This universal relation greatly extends the
scales over which redshift distortions can be used as a probe of . A
preliminary application to the 1.2 Jy IRAS yields if IRAS
galaxies are unbiased.Comment: uuencoded compressed postscript. The preprint is also available at
http://www.ast.cam.ac.uk/preprint/PrePrint.htm
The Small Scale Velocity Dispersion of Galaxies: A Comparison of Cosmological Simulations
The velocity dispersion of galaxies on small scales ( Mpc),
, can be estimated from the anisotropy of the galaxy-galaxy
correlation function in redshift space. We apply this technique to
``mock-catalogs'' extracted from N-body simulations of several different
variants of Cold Dark Matter dominated cosmological models to obtain results
which may be consistently compared to similar results from observations. We
find a large variation in the value of in different
regions of the same simulation. We conclude that this statistic should not be
considered to conclusively rule out any of the cosmological models we have
studied. We attempt to make the statistic more robust by removing clusters from
the simulations using an automated cluster-removing routine, but this appears
to reduce the discriminatory power of the statistic. However, studying
as clusters with different internal velocity dispersions are
removed leads to interesting information about the amount of power on cluster
and subcluster scales. We also compute the pairwise velocity dispersion
directly and compare this to the values obtained using the Davis-Peebles
method, and find that the agreement is fairly good. We evaluate the models used
for the mean streaming velocity and the pairwise peculiar velocity distribution
in the original Davis-Peebles method by comparing the models with the results
from the simulations.Comment: 20 pages, uuencoded (Latex file + 8 Postscript figures), uses AAS
macro
Water Management in a River Basin with Downstream Externalities
Most water for agricultural and municipal uses comes from river basins. Traditionally the analysis of trade-offs has been confined to evaluating the marginal value of water in urban and municipal uses. However, return flows from these uses often end up in coastal waters that support downstream shrimp and other fish habitats. We examine this problem by developing a conceptual model of a river basin with multiple uses. We develop equilibrium conditions for water allocation to these alternative uses. A unique feature of this model is that the demand for water may vary due to exogenous conditions, such as drought. We apply this framework to secondary data from a coastal river basin in Georgia to show that only under very unique conditions may this trade-off become meaningful, i.e., upstream water withdrawals may need to be limited to protect downstream benefits. Working Paper Number 2005-001
A massive high density effective theory
We derive an effective theory for dense, cold and massive quark matter. To
this end, we employ a general effective action formalism where antiquarks and
quarks far from the Fermi surface, as well as hard gluons, are integrated out
explicitly. We show that the resulting effective action depends crucially on
the projectors used to separate quarks from antiquarks. If one neglects the
quark masses in these projectors, the Feynman rules of the effective theory
involve quark mass insertions which connect quark with antiquark propagators.
Including the quark masses into these projectors, mass insertions do not appear
and the Feynman rules are identical to those found in the zero-mass limit.Comment: 7 pages, 6 figure
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