4,659 research outputs found
Parallel algorithms and concentration bounds for the Lovasz Local Lemma via witness DAGs
The Lov\'{a}sz Local Lemma (LLL) is a cornerstone principle in the
probabilistic method of combinatorics, and a seminal algorithm of Moser &
Tardos (2010) provides an efficient randomized algorithm to implement it. This
can be parallelized to give an algorithm that uses polynomially many processors
and runs in time on an EREW PRAM, stemming from
adaptive computations of a maximal independent set (MIS). Chung et al. (2014)
developed faster local and parallel algorithms, potentially running in time
, but these algorithms require more stringent conditions than the
LLL.
We give a new parallel algorithm that works under essentially the same
conditions as the original algorithm of Moser & Tardos but uses only a single
MIS computation, thus running in time on an EREW PRAM. This can
be derandomized to give an NC algorithm running in time as well,
speeding up a previous NC LLL algorithm of Chandrasekaran et al. (2013).
We also provide improved and tighter bounds on the run-times of the
sequential and parallel resampling-based algorithms originally developed by
Moser & Tardos. These apply to any problem instance in which the tighter
Shearer LLL criterion is satisfied
Exterior power operations on higher K-groups via binary complexes
We use Grayson's binary multicomplex presentation of algebraic K-theory to give a new construction of exterior power operations on the higher K-groups of a (quasi-compact) scheme. We show that these operations satisfy the axioms of a lambda-ring, including the product and composition laws. To prove the composition law we show that the Grothendieck group of the exact category of integral polynomial functors is the universal lambda-ring on one generator
In Vivo Time-Lapse Imaging of Cell Divisions during Neurogenesis in the Developing Zebrafish Retina
AbstractTwo-photon excitation microscopy was used to reconstruct cell divisions in living zebrafish embryonic retinas. Contrary to proposed models for vertebrate asymmetric divisions, no apico-basal cell divisions take place in the zebrafish retina during the generation of postmitotic neurons. However, a surprising shift in the orientation of cell division from central-peripheral to circumferential occurs within the plane of the ventricular surface. In the sonic you (syu) and lakritz (lak) mutants, the shift from central-peripheral to circumferential divisions is absent or delayed, correlating with the delay in neuronal differentiation and neurogenesis in these mutants. The reconstructions here show that mitotic cells always remain in contact with the opposite basal surface by means of a thin basal process that can be inherited asymmetrically
Светоизлучающие диоды белого света: состояние и основные тенденции развития
Проведен обзор состояния и тенденций развития технологии изготовления светоизлучающих диодов белого света. Систематизированы параметры сверхъярких белых светодиодов, светодиодных модулей и источников света
Molecular organization of selected prokaryotic S-Iayer proteins
Regular crystalline surface layers (S-layers) are widespread among prokaryotes and probably represent the earliest cell wall structures. S-layer genes have been found in approximately 400 different species of the prokaryotic domains bacteria and archaea. S-layers usually consist of a single (glyco-rprotein species with molecular masses ranging from about 40 to 200 kDa that form lattices of oblique, tetragonal, or hexagonal architecture. The primary sequences of hyperthermophilic archaeal species exhibit some characteristic signatures, Further adaptations to their specific environments occur by various post-translational modifications, such as linkage of glycans, lipids, phosphate, and sulfate groups to the protein or by proteolytic processing. Specific domains direct the anchoring of the S-layer to the underlying cell wall components and transport across the cytoplasma memhrane. In addition to their presumptive original role as protective coats in archaea and bacteria, they have adapted new functions, e.g., as molecular sieves, attachment sites for extracellular enzymes, and virulence factors.Peer reviewe
Global Rebalancing of Cellular Resources by Pleiotropic Point Mutations Illustrates a Multi-scale Mechanism of Adaptive Evolution
Pleiotropic regulatory mutations affect diverse cellular processes, posing a challenge to our understanding of genotype-phenotype relationships across multiple biological scales. Adaptive Laboratory Evolution (ALE) allows for such mutations to be found and characterized in the context of clear selection pressures. Here, several ALE-selected single-mutation variants in Escherichia coli's RNA polymerase (RNAP) are detailed using an integrated multi-scale experimental and computational approach. While these mutations increase cellular growth rates in steady environments, they reduce tolerance to stress and environmental fluctuations. We detail structural changes in the RNAP that rewire the transcriptional machinery to rebalance proteome and energy allocation towards growth and away from several hedging and stress functions. We find that while these mutations occur in diverse locations in the RNAP, they share a common adaptive mechanism. In turn, these findings highlight the resource allocation tradeoffs organisms face and suggest how the structure of the regulatory network enhances evolvability
Diversity of Artists in Major U.S. Museums
The U.S. art museum sector is grappling with diversity. While previous work
has investigated the demographic diversity of museum staffs and visitors, the
diversity of artists in their collections has remained unreported. We conduct
the first large-scale study of artist diversity in museums. By scraping the
public online catalogs of 18 major U.S. museums, deploying a sample of 10,000
artist records comprising over 9,000 unique artists to crowdsourcing, and
analyzing 45,000 responses, we infer artist genders, ethnicities, geographic
origins, and birth decades. Our results are threefold. First, we provide
estimates of gender and ethnic diversity at each museum, and overall, we find
that 85% of artists are white and 87% are men. Second, we identify museums that
are outliers, having significantly higher or lower representation of certain
demographic groups than the rest of the pool. Third, we find that the
relationship between museum collection mission and artist diversity is weak,
suggesting that a museum wishing to increase diversity might do so without
changing its emphases on specific time periods and regions. Our methodology can
be used to broadly and efficiently assess diversity in other fields.Comment: 15 pages, 2 figures, minor revisions of and enhancements to tex
Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped Cuprate Superconductors
We calculate the c-axis infrared conductivity in
underdoped cuprate superconductors for spinfluctuation exchange scattering
within the CuO-planes including a phenomenological d-wave pseudogap of
amplitude . For temperatures decreasing below a temperature , a gap for develops in in the
incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting'
behavior, i.e. it increases for low temperatures above the constant behavior
for . We find that the pseudogap structure in the in-plane optical
conductivity is about twice as big as in the interplane conductivity
, in qualitative agreement with experiment. This is a
consequence of the fact that the spinfluctuation exchange interaction is
suppressed at low frequencies as a result of the opening of the pseudogap.
While the c-axis conductivity in the underdoped regime is described best by
incoherent transmission, in the overdoped regime coherent conductance gives a
better description.Comment: to be published in Phys. Rev. B (November 1, 1999
Plasticity in the foraging behavior of male Southern Rockhopper Penguins (Eudyptes chrysocome) during incubation in the Falkland/Malvinas Islands
Environmental changes often affect the persistence of species or populations at different spatial and temporal scales. Thus, species must either adapt to these changes or experience negative impacts at the individual or population levels. Southern Rockhopper Penguins Eudyptes chrysocome are distributed throughout the Southern Ocean and have experienced substantial declines in the past which were linked to various anthropogenic and environmental factors. The aim of this study was to investigate the foraging behavior of male Southern Rockhopper Penguins at Berkeley Sound, East Falkland, Falkland/Malvinas Islands, during incubation, a period at-sea which is crucial for replenishing body condition between two extended fasting periods ashore. Thus, birds are forced to forage efficiently during that time to balance their energy demands. We linked their at-sea distribution and foraging behavior to satellite-derived sea surface temperatures and temperature-depth profiles which were recorded by devices attached to the birds. While Southern Rockhopper Penguins usually travel several hundreds of km out into the open sea on multiple-day trips during incubation, we found in our study that most birds foraged close inshore, less than 9 km away from their colony, and regularly returned to their breeding site. We propose that this behavior occurred in response to the close proximity of the 8 °C SST isotherm and the vertical stratification of the waters therein. Also, while usually feeding pelagically in open waters, there are strong indications that Southern Rockhopper Penguins performed benthic or, at least, near-bottom dives to catch their prey during these short trips. The consequences of this behavioral plasticity in response to variations in sea temperatures and inferred prey availability are discussed, especially with regard to predicted global climate change
- …