3,685 research outputs found
Considerations about multistep community detection
The problem and implications of community detection in networks have raised a
huge attention, for its important applications in both natural and social
sciences. A number of algorithms has been developed to solve this problem,
addressing either speed optimization or the quality of the partitions
calculated. In this paper we propose a multi-step procedure bridging the
fastest, but less accurate algorithms (coarse clustering), with the slowest,
most effective ones (refinement). By adopting heuristic ranking of the nodes,
and classifying a fraction of them as `critical', a refinement step can be
restricted to this subset of the network, thus saving computational time.
Preliminary numerical results are discussed, showing improvement of the final
partition.Comment: 12 page
Multilayer metamaterial absorbers inspired by perfectly matched layers
We derive periodic multilayer absorbers with effective uniaxial properties
similar to perfectly matched layers (PML). This approximate representation of
PML is based on the effective medium theory and we call it an effective medium
PML (EM-PML). We compare the spatial reflection spectrum of the layered
absorbers to that of a PML material and demonstrate that after neglecting gain
and magnetic properties, the absorber remains functional. This opens a route to
create electromagnetic absorbers for real and not only numerical applications
and as an example we introduce a layered absorber for the wavelength of
~m made of SiO and NaCl. We also show that similar cylindrical
core-shell nanostructures derived from flat multilayers also exhibit very good
absorptive and reflective properties despite the different geometry
Infall of gas as the formation mechanism of stars up to 20 times more massive than the Sun
Theory predicts and observations confirm that low-mass stars (like the Sun)
in their early life grow by accreting gas from the surrounding material. But
for stars ~ 10 times more massive than the Sun (~10 M_sun), the powerful
stellar radiation is expected to inhibit accretion and thus limit the growth of
their mass. Clearly, stars with masses >10 M_sun exist, so there must be a way
for them to form. The problem may be solved by non-spherical accretion, which
allows some of the stellar photons to escape along the symmetry axis where the
density is lower. The recent detection of rotating disks and toroids around
very young massive stars has lent support to the idea that high-mass (> 8
M_sun) stars could form in this way. Here we report observations of an ammonia
line towards a high-mass star forming region. We conclude from the data that
the gas is falling inwards towards a very young star of ~20 M_sun, in line with
theoretical predictions of non-spherical accretion.Comment: 11 pages, 2 figure
Emergence of non-centrosymmetric topological insulating phase in BiTeI under pressure
The spin-orbit interaction affects the electronic structure of solids in
various ways. Topological insulators are one example where the spin-orbit
interaction leads the bulk bands to have a non-trivial topology, observable as
gapless surface or edge states. Another example is the Rashba effect, which
lifts the electron-spin degeneracy as a consequence of spin-orbit interaction
under broken inversion symmetry. It is of particular importance to know how
these two effects, i.e. the non-trivial topology of electronic states and
Rashba spin splitting, interplay with each other. Here we show, through
sophisticated first-principles calculations, that BiTeI, a giant bulk Rashba
semiconductor, turns into a topological insulator under a reasonable pressure.
This material is shown to exhibit several unique features such as, a highly
pressure-tunable giant Rashba spin splitting, an unusual pressure-induced
quantum phase transition, and more importantly the formation of strikingly
different Dirac surface states at opposite sides of the material.Comment: 5 figures are include
Effects of Acute Tryptophan Depletion on Brain Serotonin Function and Concentrations of Dopamine and Norepinephrine in C57BL/6J and BALB/cJ Mice
Acute tryptophan depletion (ATD) is a method of lowering brain serotonin (5-HT). Administration of large neutral amino acids (LNAA) limits the transport of endogenous tryptophan (TRP) across the blood brain barrier by competition with other LNAAs and subsequently decreases serotonergic neurotransmission. A recent discussion on the specificity and efficacy of the ATD paradigm for inhibition of central nervous 5-HT has arisen. Moreover, side effects such as vomiting and nausea after intake of amino acids (AA) still limit its use. ATD Moja-De is a revised mixture of AAs which is less nauseating than conventional protocols. It has been used in preliminary clinical studies but its effects on central 5-HT mechanisms and other neurotransmitter systems have not been validated in an animal model. We tested ATD Moja-De (TRP−) in two strains of mice: C57BL/6J, and BALB/cJ, which are reported to have impaired 5-HT synthesis and a more anxious phenotype relative to other strains of mice. ATD Moja-De lowered brain TRP, significantly decreased 5-HT synthesis as indexed by 5-HTP levels after decarboxlyase inhibition, and lowered 5-HT and 5-HIAA in both strains of mice, however more so in C57BL/6J than in BALB/cJ. Dopamine and its metabolites as well as norepinephrine were not affected. A balanced (TRP+) control mixture did not raise 5-HT or 5-HIAA. The present findings suggest that ATD Moja-De effectively and specifically suppresses central serotonergic function. These results also demonstrate a strain- specific effect of ATD Moja-De on anxiety-like behavior
Optimality of mutation and selection in germinal centers
The population dynamics theory of B cells in a typical germinal center could
play an important role in revealing how affinity maturation is achieved.
However, the existing models encountered some conflicts with experiments. To
resolve these conflicts, we present a coarse-grained model to calculate the B
cell population development in affinity maturation, which allows a
comprehensive analysis of its parameter space to look for optimal values of
mutation rate, selection strength, and initial antibody-antigen binding level
that maximize the affinity improvement. With these optimized parameters, the
model is compatible with the experimental observations such as the ~100-fold
affinity improvements, the number of mutations, the hypermutation rate, and the
"all or none" phenomenon. Moreover, we study the reasons behind the optimal
parameters. The optimal mutation rate, in agreement with the hypermutation rate
in vivo, results from a tradeoff between accumulating enough beneficial
mutations and avoiding too many deleterious or lethal mutations. The optimal
selection strength evolves as a balance between the need for affinity
improvement and the requirement to pass the population bottleneck. These
findings point to the conclusion that germinal centers have been optimized by
evolution to generate strong affinity antibodies effectively and rapidly. In
addition, we study the enhancement of affinity improvement due to B cell
migration between germinal centers. These results could enhance our
understandings to the functions of germinal centers.Comment: 5 figures in main text, and 4 figures in Supplementary Informatio
Simultaneous determination of natural and synthetic steroid estrogens and their conjugates in aqueous matrices by liquid chromatography / mass spectrometry
An analytical method for the simultaneous determination of nine free and conjugated steroid estrogens was developed with application to environmental aqueous matrices. Solid phase extraction (SPE) was employed for isolation and concentration, with detection by liquid chromatography/mass spectrometry (LC/MS) using electrospray ionisation (ESI) in the negative mode. Method recoveries for various aqueous matrices (wastewater, lake and drinking water) were determined, recoveries proving to be sample dependent. When spiked at 50 ng/l concentrations in sewage influent, recoveries ranged from 62-89 % with relative standard deviations (RSD) < 8.1 %. In comparison, drinking water spiked at the same concentrations had recoveries between 82-100 % with an RSD < 5%. Ion suppression is a known phenomenon when using ESI; hence its impact on method recovery was elucidated for raw sewage. Both ion suppression from matrix interferences and the extraction procedure has bearing on the overall method recovery. Analysis of municipal raw sewage identified several of the analytes of interest at ng/l concentrations, estriol (E3) being the most abundant. Only one conjugate, estrone 3-sulphate (E1-3S) was observe
A role for core planar polarity proteins in cell contact-mediated orientation of planar cell division across the mammalian embryonic skin
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. © The Author(s) 2017. Supplementary information accompanies this paper at doi:10.1038/s41598-017-01971-2.The question of how cell division orientation is determined is fundamentally important for understanding tissue and organ shape in both healthy or disease conditions. Here we provide evidence for cell contact-dependent orientation of planar cell division in the mammalian embryonic skin. We propose a model where the core planar polarity proteins Celsr1 and Frizzled-6 (Fz6) communicate the long axis orientation of interphase basal cells to neighbouring basal mitoses so that they align their horizontal division plane along the same axis. The underlying mechanism requires a direct, cell surface, planar polarised cue, which we posit depends upon variant post-translational forms of Celsr1 protein coupled to Fz6. Our hypothesis has parallels with contact-mediated division orientation in early C. elegans embryos suggesting functional conservation between the adhesion-GPCRs Celsr1 and Latrophilin-1. We propose that linking planar cell division plane with interphase neighbour long axis geometry reinforces axial bias in skin spreading around the mouse embryo body.Peer reviewe
- …