1,771 research outputs found
Recommended from our members
Robust Inference on Seasonal Unit Roots via a Bootstrap Applied to OECD Macroeconomic Series
Recent experimental results presented in Burridge and Taylor (2001a,b, and 2003) show that, as usually implemented, the Hylleberg et al. (1990) seasonal unit root tests can be rather liberal, with true level often substantially higher than nominal level. This effect is due to the presence of any of three things: data-based lag selection in the implementation of the tests, and either or both periodic heteroscedasticity and serial correlation in the driving shocks. Burridge and Taylor (2003) demonstrate that under experimental conditions a carefully implemented bootstrap substantially corrects test level without loss of power. The present study applies their technique to a large number of publicly available series, and demonstrates conclusively that the bootstrap produces less liberal, and, given the experimental results cited above, more reliable inference. We report results for Sweden, the UK and the US, which are typical of the fifteen countries in our panel. Other results, the GAUSS code, and raw data are all available at: www.staff.city.ac.uk/p.burridge
Actin-membrane interaction in fibroblasts: what proteins are involved in this association?
In this review we discuss some of the proteins for which a role in
linking actin to the fibroblast plasma membrane has been suggested. We
focus on the family of proteins related to erythrocyte spectrin, proteins
that have generally been viewed as having an organization and a function in
actin-membrane attachment similar to those of erythrocyte spectrin.
Experiments in which we precipitated the nonerythrocyte spectrin within
living fibroblasts have led us to question this supposed similarity of
organization and function of the nonerythrocyte and erythrocyte spectrins.
Intracellular precipitation of fibroblast spectrin does not affect the
integrity of the major actin-containing structures, the stress fiber
microfilament bundles. Unexpectedly, however, we found that the
precipitation of spectrin results in a condensation and altered
distribution of the vimentin class of intermediate filaments in most cells
examined. Although fibroblast spectrin may have a role in the attachment of
some of the cortical, submembranous actin, it is surprising how little the
intracellular immunoprecipitation of the spectrin affects the cells.
Several proteins have been found concentrated at the ends of stress fibers,
where the actin filaments terminate at focal contacts. Two of these
proteins, alpha-actinin and fimbrin, have properties that suggest that they
are not involved in the attachment of the ends of the bundles to the
membrane but are more probably involved in the organization and
cross-linking of the filaments within the bundles. On the other hand,
vinculin and talin are two proteins that interact with each other and may
form part of a chain of attachments between the ends of the microfilament
bundles and the focal contact membrane. Their role in this attachment,
however, has not been established and further work is needed to examine
their interaction with actin and to identify any other components with
which they may interact, particularly in the plasma membrane
The complex scaling behavior of non--conserved self--organized critical systems
The Olami--Feder--Christensen earthquake model is often considered the
prototype dissipative self--organized critical model. It is shown that the size
distribution of events in this model results from a complex interplay of
several different phenomena, including limited floating--point precision.
Parallels between the dynamics of synchronized regions and those of a system
with periodic boundary conditions are pointed out, and the asymptotic avalanche
size distribution is conjectured to be dominated by avalanches of size one,
with the weight of larger avalanches converging towards zero as the system size
increases.Comment: 4 pages revtex4, 5 figure
Network of recurrent events for the Olami-Feder-Christensen model
We numerically study the dynamics of a discrete spring-block model introduced
by Olami, Feder and Christensen (OFC) to mimic earthquakes and investigate to
which extent this simple model is able to reproduce the observed spatiotemporal
clustering of seismicty. Following a recently proposed method to characterize
such clustering by networks of recurrent events [Geophys. Res. Lett. {\bf 33},
L1304, 2006], we find that for synthetic catalogs generated by the OFC model
these networks have many non-trivial statistical properties. This includes
characteristic degree distributions -- very similar to what has been observed
for real seismicity. There are, however, also significant differences between
the OFC model and earthquake catalogs indicating that this simple model is
insufficient to account for certain aspects of the spatiotemporal clustering of
seismicity.Comment: 11 pages, 16 figure
Asperity characteristics of the Olami-Feder-Christensen model of earthquakes
Properties of the Olami-Feder-Christensen (OFC) model of earthquakes are
studied by numerical simulations. The previous study indicated that the model
exhibits ``asperity''-like phenomena, {\it i.e.}, the same region ruptures many
times near periodically [T.Kotani {\it et al}, Phys. Rev. E {\bf 77}, 010102
(2008)]. Such periodic or characteristic features apparently coexist with
power-law-like critical features, {\it e.g.}, the Gutenberg-Richter law
observed in the size distribution. In order to clarify the origin and the
nature of the asperity-like phenomena, we investigate here the properties of
the OFC model with emphasis on its stress distribution. It is found that the
asperity formation is accompanied by self-organization of the highly
concentrated stress state. Such stress organization naturally provides the
mechanism underlying our observation that a series of asperity events repeat
with a common epicenter site and with a common period solely determined by the
transmission parameter of the model. Asperity events tend to cluster both in
time and in space
The Network of Epicenters of the Olami-Feder-Christensen Model of Earthquakes
We study the dynamics of the Olami-Feder-Christensen (OFC) model of
earthquakes, focusing on the behavior of sequences of epicenters regarded as a
growing complex network. Besides making a detailed and quantitative study of
the effects of the borders (the occurrence of epicenters is dominated by a
strong border effect which does not scale with system size), we examine the
degree distribution and the degree correlation of the graph. We detect sharp
differences between the conservative and nonconservative regimes of the model.
Removing border effects, the conservative regime exhibits a Poisson-like degree
statistics and is uncorrelated, while the nonconservative has a broad
power-law-like distribution of degrees (if the smallest events are ignored),
which reproduces the observed behavior of real earthquakes. In this regime the
graph has also a unusually strong degree correlation among the vertices with
higher degree, which is the result of the existence of temporary attractors for
the dynamics: as the system evolves, the epicenters concentrate increasingly on
fewer sites, exhibiting strong synchronization, but eventually spread again
over the lattice after a series of sufficiently large earthquakes. We propose
an analytical description of the dynamics of this growing network, considering
a Markov process network with hidden variables, which is able to account for
the mentioned properties.Comment: 9 pages, 10 figures. Smaller number of figures, and minor text
corrections and modifications. For version with full resolution images see
http://fig.if.usp.br/~tpeixoto/cond-mat-0602244.pd
Phase-slip avalanches in the superflow of He through arrays of nanopores
Recent experiments by Sato et al. [1] have explored the dynamics of He
superflow through an array of nanopores. These experiments have found that, as
the temperature is lowered, phase-slippage in the pores changes its character,
from synchronous to asynchronous. Inspired by these experiments, we construct a
model to address the characteristics of phase-slippage in superflow through
nanopore arrays. We focus on the low-temperature regime, in which the
current-phase relation for a single pore is linear, and thermal fluctuations
may be neglected. Our model incorporates two basic ingredients: (1) each pore
has its own random value of critical velocity (due, e.g., to atomic-scale
imperfections), and (2) an effective inter-pore coupling, mediated through the
bulk superfluid. The inter-pore coupling tends to cause neighbours of a pore
that has already phase-slipped also to phase-slip; this process may cascade,
creating an avalanche of synchronously slipping phases. As the temperature is
lowered, the distribution of critical velocities is expected to effectively
broaden, owing to the reduction in the superfluid healing length, leading to a
loss of synchronicity in phase-slippage. Furthermore, we find that competition
between the strength of the disorder in the critical velocities and the
strength of the inter-pore interaction leads to a phase transition between
non-avalanching and avalanching regimes of phase-slippage.
[1] Sato, Y., Hoskinson, E. Packard, R. E. cond-mat/0605660.Comment: 8 pages, 5 figure
A comparison of line-sources of buoyancy placed near and far from a wall
Experiments are presented on turbulent buoyant free - line and wall plumes, whereby the buoyancy source is emitted from a horizontal line source, in one case free of the presence of a wall and in the other placed immediately adjacent to a wall. The dynamics of turbulent entrainment, whereby ambie nt fluid is mixed in to the plume, are explored. The velocity field and scalar edge of the plumes are measur ed. From this the time - averaged plume - width and volume flux are compared. The spreading rate, and therefo re the entrainment, of the wall plume is fo und to be half that of the free - line plume, indicating that the wall has a signif icant effect on the entrainment process. Further, the volume flux of the wall plume is found to be half that of the free - line plume, indicating that larger maximum scalar conc entrations are present in the wall plume. The effect that the reduced entrainment rate has on a typical heated room, via a line source of buoyancy, is demonstrated by comparing a numerical model of the develo ping temperature stratification within a sealed enclosure in the case of the line source near a wall and away from a wall , where in particular it is found that higher maximum temperatures are present for the case of the line source near a wall
Simulation study of spatio-temporal correlations of earthquakes as a stick-slip frictional instability
Spatio-temporal correlations of earthquakes are studied numerically on the
basis of the one-dimensional spring-block (Burridge-Knopoff) model. As large
events approach, the frequency of smaller events gradually increases, while,
just before the mainshock, it is dramatically suppressed in a close vicinity of
the epicenter of the upcoming mainshock, a phenomenon closely resembling the
``Mogi doughnut'
Missing physics in stick-slip dynamics of a model for peeling of an adhesive tape
It is now known that the equations of motion for the contact point during
peeling of an adhesive tape mounted on a roll introduced earlier are singular
and do not support dynamical jumps across the two stable branches of the peel
force function. By including the kinetic energy of the tape in the Lagrangian,
we derive equations of motion that support stick-slip jumps as a natural
consequence of the inherent dynamics. In the low mass limit, these equations
reproduce solutions obtained using a differential-algebraic algorithm
introduced for the earlier equations. Our analysis also shows that mass of the
ribbon has a strong influence on the nature of the dynamics.Comment: Accepted for publication in Phys. Rev. E (Rapid Communication
- …