8,203 research outputs found
Using Action Research to Facilitate School Improvement
This study describes a joint project between a mid-sized urban school district and local university. Specifically, district and university personnel collaborated to provide action research training for building leadership teams. A multi-method approach was used to identify the barriers and facilitators of the use of action research training as a method for teachers to create school improvement. A content analysis of the action research plans that were a summative product of the training is provided. Findings include the importance of both technical research skills and attention to the provision of time as a resource for those involved with school improvement efforts
Active contractility in actomyosin networks
Contractile forces are essential for many developmental processes involving
cell shape change and tissue deformation. Recent experiments on reconstituted
actomyosin networks, the major component of the contractile machinery, have
shown that active contractility occurs above a threshold motor concentration
and within a window of crosslink concentration. We present a microscopic
dynamic model that incorporates two essential aspects of actomyosin
self-organization: the asymmetric load response of individual actin filaments
and the correlated motor-driven events mimicking myosin-induced filament
sliding. Using computer simulations we examine how the concentration and
susceptibility of motors contribute to their collective behavior and interplay
with the network connectivity to regulate macroscopic contractility. Our model
is shown to capture the formation and dynamics of contractile structures and
agree with the observed dependence of active contractility on microscopic
parameters including the contractility onset. Cooperative action of
load-resisting motors in a force-percolating structure integrates local
contraction/buckling events into a global contractile state via an active
coarsening process, in contrast to the flow transition driven by uncorrelated
kicks of susceptible motors.Comment: 15 pages, 4 main figures, 4 supplementary figure
Self-synchronization and dissipation-induced threshold in collective atomic recoil lasing
Networks of globally coupled oscillators exhibit phase transitions from incoherent to coherent states. Atoms interacting with the counterpropagating modes of a unidirectionally pumped high-finesse ring cavity form such a globally coupled network. The coupling mechanism is provided by collective atomic recoil lasing, i.e., cooperative Bragg scattering of laser light at an atomic density grating, which is self-induced by the laser light. Under the rule of an additional friction force, the atomic ensemble is expected to undergo a phase transition to a state of synchronized atomic motion. We present the experimental investigation of this phase transition by studying the threshold behavior of this lasing process
A stochastic model of Min oscillations in Escherichia coli and Min protein segregation during cell division
The Min system in Escherichia coli directs division to the centre of the cell
through pole-to-pole oscillations of the MinCDE proteins. We present a one
dimensional stochastic model of these oscillations which incorporates membrane
polymerisation of MinD into linear chains. This model reproduces much of the
observed phenomenology of the Min system, including pole-to-pole oscillations
of the Min proteins. We then apply this model to investigate the Min system
during cell division. Oscillations continue initially unaffected by the closing
septum, before cutting off rapidly. The fractions of Min proteins in the
daughter cells vary widely, from 50%-50% up to 85%-15% of the total from the
parent cell, suggesting that there may be another mechanism for regulating
these levels in vivo.Comment: 19 pages, 12 figures (25 figure files); published at
http://www.iop.org/EJ/journal/physbi
The non-Gaussian tail of cosmic-shear statistics
Due to gravitational instability, an initially Gaussian density field
develops non-Gaussian features as the Universe evolves. The most prominent
non-Gaussian features are massive haloes, visible as clusters of galaxies. The
distortion of high-redshift galaxy images due to the tidal gravitational field
of the large-scale matter distribution, called cosmic shear, can be used to
investigate the statistical properties of the LSS. In particular, non-Gaussian
properties of the LSS will lead to a non-Gaussian distribution of cosmic-shear
statistics. The aperture mass () statistics, recently introduced as
a measure for cosmic shear, is particularly well suited for measuring these
non-Gaussian properties. In this paper we calculate the highly non-Gaussian
tail of the aperture mass probability distribution, assuming Press-Schechter
theory for the halo abundance and the `universal' density profile of haloes as
obtained from numerical simulations. We find that for values of
much larger than its dispersion, this probability distribution is closely
approximated by an exponential, rather than a Gaussian. We determine the
amplitude and shape of this exponential for various cosmological models and
aperture sizes, and show that wide-field imaging surveys can be used to
distinguish between some of the currently most popular cosmogonies. Our study
here is complementary to earlier cosmic-shear investigations which focussed
more on two-point statistical properties.Comment: 9 pages, 5 figures, submitted to MNRA
On the monotone stability approach to BSDEs with jumps: Extensions, concrete criteria and examples
We show a concise extension of the monotone stability approach to backward
stochastic differential equations (BSDEs) that are jointly driven by a Brownian
motion and a random measure for jumps, which could be of infinite activity with
a non-deterministic and time inhomogeneous compensator. The BSDE generator
function can be non convex and needs not to satisfy global Lipschitz conditions
in the jump integrand. We contribute concrete criteria, that are easy to
verify, for results on existence and uniqueness of bounded solutions to BSDEs
with jumps, and on comparison and a-priori -bounds. Several
examples and counter examples are discussed to shed light on the scope and
applicability of different assumptions, and we provide an overview of major
applications in finance and optimal control.Comment: 28 pages. Added DOI
https://link.springer.com/chapter/10.1007%2F978-3-030-22285-7_1 for final
publication, corrected typo (missing gamma) in example 4.1
The Camassa-Holm equation as the long-wave limit of the improved Boussinesq equation and of a class of nonlocal wave equations
In the present study we prove rigorously that in the long-wave limit, the
unidirectional solutions of a class of nonlocal wave equations to which the
improved Boussinesq equation belongs are well approximated by the solutions of
the Camassa-Holm equation over a long time scale. This general class of
nonlocal wave equations model bidirectional wave propagation in a nonlocally
and nonlinearly elastic medium whose constitutive equation is given by a
convolution integral. To justify the Camassa-Holm approximation we show that
approximation errors remain small over a long time interval. To be more
precise, we obtain error estimates in terms of two independent, small, positive
parameters and measuring the effect of nonlinearity and
dispersion, respectively. We further show that similar conclusions are also
valid for the lower order approximations: the Benjamin-Bona-Mahony
approximation and the Korteweg-de Vries approximation.Comment: 24 pages, to appear in Discrete and Continuous Dynamical System
Coarse-grained model of entropic allostery
Many signaling functions in molecular biology require proteins to bind to substrates such as DNA in response to environmental signals such as the simultaneous binding to a small molecule. Examples are repressor proteins which may transmit information via a conformational change in response to the ligand binding. An alternative entropic mechanism of "allostery" suggests that the inducer ligand changes the intramolecular vibrational entropy, not just the mean static structure. We present a quantitative, coarse-grained model of entropic allostery, which suggests design rules for internal cohesive potentials in proteins employing this effect. It also addresses the issue of how the signal information to bind or unbind is transmitted through the protein. The model may be applicable to a wide range of repressors and also to signaling in trans-membrane proteins
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