3,948 research outputs found
On the Propagation of Slip Fronts at Frictional Interfaces
The dynamic initiation of sliding at planar interfaces between deformable and
rigid solids is studied with particular focus on the speed of the slip front.
Recent experimental results showed a close relation between this speed and the
local ratio of shear to normal stress measured before slip occurs (static
stress ratio). Using a two-dimensional finite element model, we demonstrate,
however, that fronts propagating in different directions do not have the same
dynamics under similar stress conditions. A lack of correlation is also
observed between accelerating and decelerating slip fronts. These effects
cannot be entirely associated with static local stresses but call for a dynamic
description. Considering a dynamic stress ratio (measured in front of the slip
tip) instead of a static one reduces the above-mentioned inconsistencies.
However, the effects of the direction and acceleration are still present. To
overcome this we propose an energetic criterion that uniquely associates,
independently on the direction of propagation and its acceleration, the slip
front velocity with the relative rise of the energy density at the slip tip.Comment: 15 pages, 6 figure
A Cost-based Optimizer for Gradient Descent Optimization
As the use of machine learning (ML) permeates into diverse application
domains, there is an urgent need to support a declarative framework for ML.
Ideally, a user will specify an ML task in a high-level and easy-to-use
language and the framework will invoke the appropriate algorithms and system
configurations to execute it. An important observation towards designing such a
framework is that many ML tasks can be expressed as mathematical optimization
problems, which take a specific form. Furthermore, these optimization problems
can be efficiently solved using variations of the gradient descent (GD)
algorithm. Thus, to decouple a user specification of an ML task from its
execution, a key component is a GD optimizer. We propose a cost-based GD
optimizer that selects the best GD plan for a given ML task. To build our
optimizer, we introduce a set of abstract operators for expressing GD
algorithms and propose a novel approach to estimate the number of iterations a
GD algorithm requires to converge. Extensive experiments on real and synthetic
datasets show that our optimizer not only chooses the best GD plan but also
allows for optimizations that achieve orders of magnitude performance speed-up.Comment: Accepted at SIGMOD 201
Control theory for principled heap sizing
We propose a new, principled approach to adaptive heap sizing based on control theory. We review current state-of-the-art heap sizing mechanisms, as deployed in Jikes RVM and HotSpot. We then formulate heap sizing as a control problem, apply and tune a standard controller algorithm, and evaluate its performance on a set of well-known benchmarks. We find our controller adapts the heap size more responsively than existing mechanisms. This responsiveness allows tighter virtual machine memory footprints while preserving target application throughput, which is ideal for both embedded and utility computing domains. In short, we argue that formal, systematic approaches to memory management should be replacing ad-hoc heuristics as the discipline matures. Control-theoretic heap sizing is one such systematic approach
Differential stabilization of reaction intermediates: specificity checkpoints for M.EcoRI revealed by transient fluorescence and fluorescence lifetime studies
M.EcoRI, a bacterial sequence-specific S-adenosyl-l-methionine-dependent DNA methyltransferase, relies on a complex conformational mechanism to achieve its remarkable specificity, including DNA bending, base flipping and intercalation into the DNA. Using transient fluorescence and fluorescence lifetime studies with cognate and noncognate DNA, we have characterized several reaction intermediates involving the WT enzyme. Similar studies with a bending-impaired, enhanced-specificity M.EcoRI mutant show minimal differences with the cognate DNA, but significant differences with noncognate DNA. These results provide a plausible explanation of the way in which destabilization of reaction intermediates can lead to changes in substrate specificity
Teknologi Litik di Situs Talimbue, Sulawesi Tenggara: Teknologi Berlanjut dari Masa Pleistosen Akhir Hingga Holosen.
Abstract. The Lithic Technology at Talimbue Site, Southeast Sulawesi: Continuing Technology from Late Pleistocene up to Holocene Periods. The Talimbue site at Southeast Sulawesi is packed with lithic and these offer a new perspective on the lithic technology of Sulawesi. The absence of information on the prehistoric lithic technology of Southeast Sulawesi is a factor of interest that makes research on knowledge of the Talimbue site necessary. Lithic artefacts were manufactured from the terminal Pleistocene to the Late Holocene. This research will disentangle the details of the lithic technology at the Talimbue Site. The analyzed flaked stone artefacts fall into 3 categories, which are retouched flakes, debitage and cores. For its part, debitage was classified into 3 categories, which are complete flakes, broken flakes and debris. The retouch index was also measured so as to provide a quantitative estimate of the level of retouch intensity of the retouched flakes. The results of the analysis indicate changes in the stone flake technology during the period of occupation of the Talimbue Site. The change of technology occurs because the process of adaptation caused by a change of environment. Abstrak. Temuan litik yang sangat padat di Situs Talimbue di Sulawesi Tenggara menunjukkan sebuah persepektif baru dalam kajian teknologi litik di Sulawesi. Kekosongan informasi teknologi litik masa prasejarah di wilayah Sulawesi Tenggara adalah hal yang menarik dikaji dalam penelitian di Situs Talimbue. Artefak litik digunakan dari masa Pleistosen Akhir hingga masa Holosen Akhir. Penelitian ini akan menguraikan secara detail bagaimana teknologi litik di Situs Talimbue. Artefak batu diserpih yang dianalisis menjadi 3 kategori, yaitu serpih diretus, serpihan dan batu inti. Serpihan kemudian diklasifikasi menjadi 3 kategori, yaitu serpih utuh, serpih rusak dan tatal. Pengukuran indeks retus juga dilakukan bertujuan untuk mengestimasi secara kuantitatif tingkat intensitas retus terhadap serpih yang telah diretus. Hasil penelitian menunjukkan perubahan teknologi artefak batu diserpih terjadi selama masa hunian di Situs Talimbue. Perubahan teknologi terjadi karena adanya proses adaptasi yang disebabkan oleh perubahan lingkungan
Unique Organization of Extracellular Amylases into Amylosomes in the Resistant Starch-Utilizing Human Colonic Firmicutes Bacterium Ruminococcus bromii
ACKNOWLEDGMENTS We acknowledge support from BBSRC grant no. BB/L009951/1, from the Scottish government Food, Land and People program, and from the Society for Applied Microbiology. E.A.B. is supported by a grant (no. 1349/13) from the Israel Science Foundation (ISF), Jerusalem, Israel, and by a grant from the United States-Israel Binational Science Foundation (BSF). E.A.B. is the incumbent of the Maynard I. and Elaine Wishner Chair of Bio-organic Chemistry. Thanks are due to Fergus Nicol for proteomic analysis and to Auriane Bernard for enzyme assays on stationary-phase cultures. We also thank Julian Parkhill and Keith Turner (Wellcome Trust Sanger Institute, Cambridge, United Kingdom) for making the R. bromii L2-63 genome sequence available for analysis.Peer reviewedPublisher PD
The transcription factor Foxg1 regulates telencephalic progenitor proliferation cell autonomously, in part by controlling Pax6 expression levels
<p>Abstract</p> <p>Background</p> <p>The transcription factor Foxg1 is an important regulator of telencephalic cell cycles. Its inactivation causes premature lengthening of telencephalic progenitor cell cycles and increased neurogenic divisions, leading to severe hypoplasia of the telencephalon. These proliferation defects could be a secondary consequence of the loss of Foxg1 caused by the abnormal expression of several morphogens (Fibroblast growth factor 8, bone morphogenetic proteins) in the telencephalon of <it>Foxg1 </it>null mutants. Here we investigated whether Foxg1 has a cell autonomous role in the regulation of telencephalic progenitor proliferation. We analysed <it>Foxg1<sup>+/+</sup></it>↔<it>Foxg1<sup>-/- </sup></it>chimeras, in which mutant telencephalic cells have the potential to interact with, and to have any cell non-autonomous defects rescued by, normal wild-type cells.</p> <p>Results</p> <p>Our analysis showed that the <it>Foxg1<sup>-/- </sup></it>cells are under-represented in the chimeric telencephalon and the proportion of them in S-phase is significantly smaller than that of their wild-type neighbours, indicating that their under-representation is caused by a cell autonomous reduction in their proliferation. We then analysed the expression of the cell-cycle regulator Pax6 and found that it is cell-autonomously downregulated in <it>Foxg1<sup>-/- </sup></it>dorsal telencephalic cells. We went on to show that the introduction into <it>Foxg1<sup>-/- </sup></it>embryos of a transgene designed to reverse Pax6 expression defects resulted in a partial rescue of the telencephalic progenitor proliferation defects.</p> <p>Conclusions</p> <p>We conclude that Foxg1 exerts control over telencephalic progenitor proliferation by cell autonomous mechanisms that include the regulation of Pax6, which itself is known to regulate proliferation cell autonomously in a regional manner.</p
SN2010jp (PTF10aaxi): A Jet-Driven Type II Supernova
We present photometry and spectroscopy of the peculiar TypeII supernova (SN)
2010jp, also named PTF10aaxi. The light curve exhibits a linear decline with a
relatively low peak absolute magnitude of only -15.9, and a low radioactive
decay luminosity at late times that suggests a nickel mass below 0.003
. Spectra of SN2010jp display an unprecedented triple-peaked
H line profile, showing: (1) a narrow (800 km/s) central component that
suggests shock interaction with dense CSM; (2) high-velocity blue and red
emission features centered at -12600 and +15400 km/s; and (3) broad wings
extending from -22000 to +25000 km/s. These features persist during 100 days
after explosion. We propose that this line profile indicates a bipolar
jet-driven explosion, with the central component produced by normal SN ejecta
and CSM interaction at mid latitudes, while the high-velocity bumps and broad
line wings arise in a nonrelativistic bipolar jet. Two variations of the jet
interpretation seem plausible: (1) A fast jet mixes 56Ni to high velocities in
polar zones of the H-rich envelope, or (2) the reverse shock in the jet
produces blue and red bumps in Balmer lines when a jet interacts with dense
CSM. Jet-driven SNeII are predicted for collapsars resulting from a wide range
of initial masses above 25 at sub-solar metallicity. This seems
consistent with the SN host environment, which is either an extremely
low-luminosity dwarf galaxy or very remote parts of an interacting pair of
star-forming galaxies. It also seems consistent with the low 56Ni mass that may
accompany black hole formation. We speculate that the jet survives to produce
observable signatures because the star's H envelope was mostly stripped away by
previous eruptive mass loss.Comment: 11 pages, 9 figures, submitted to MNRA
Approaches for advancing scientific understanding of macrosystems
The emergence of macrosystems ecology (MSE), which focuses on regional- to continental-scale ecological patterns and processes, builds upon a history of long-term and broad-scale studies in ecology. Scientists face the difficulty of integrating the many elements that make up macrosystems, which consist of hierarchical processes at interacting spatial and temporal scales. Researchers must also identify the most relevant scales and variables to be considered, the required data resources, and the appropriate study design to provide the proper inferences. The large volumes of multi-thematic data often associated with macrosystem studies typically require validation, standardization, and assimilation. Finally, analytical approaches need to describe how cross-scale and hierarchical dynamics and interactions relate to macroscale phenomena. Here, we elaborate on some key methodological challenges of MSE research and discuss existing and novel approaches to meet them
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