3,738 research outputs found
On the exotic fishes given to... geometry
Polyhedral forms are extremely widespread both in animate and inanimate nature. Thus, crystals occur as polyhedra only. Besides, these forms are quite common with various primitive organisms, i.e. icosahedral viruses, radiolaria and algae. Here we discuss the cases of exotic Boxfish and Porcupinefish. The specific morphology of the Boxfish reveals in polygonal osseous blades, covering its body. As for the Porcupinefish, its polyhedral approximation was observed via certain geometrical techniques applied. Namely, their spine bases were considered the Delaunay point (R, r)-systems. Consequently, the respective Dirichlet tiling proved to be quasifullerenes and analogous to the Boxfish morphology. This unexpected geometrical dualism of the two families corroborates their taxonomic affinity within the Tetradontiformes order. The above biometrical method is highly recommended as a means of characterization of the Tetraodontiformes specimens in terms of the Delaunay (R, r)-systems
The inverse problem for the Gross - Pitaevskii equation
Two different methods are proposed for the generation of wide classes of
exact solutions to the stationary Gross - Pitaevskii equation (GPE). The first
method, suggested by the work by Kondrat'ev and Miller (1966), applies to
one-dimensional (1D) GPE. It is based on the similarity between the GPE and the
integrable Gardner equation, all solutions of the latter equation (both
stationary and nonstationary ones) generating exact solutions to the GPE, with
the potential function proportional to the corresponding solutions. The second
method is based on the "inverse problem" for the GPE, i.e. construction of a
potential function which provides a desirable solution to the equation.
Systematic results are presented for 1D and 2D cases. Both methods are
illustrated by a variety of localized solutions, including solitary vortices,
for both attractive and repulsive nonlinearity in the GPE. The stability of the
1D solutions is tested by direct simulations of the time-dependent GPE
In-cell characterization of the stable tyrosyl radical in E. coli ribonucleotide reductase using advanced EPR spectroscopy
The E. coli ribonucleotide reductase (RNR), a paradigm for class Ia enzymes including human RNR, catalyzes the biosynthesis of DNA building blocks and requires a di-iron tyrosyl radical (Y122.) cofactor for activity. The knowledge on the in vitro Y122. structure and its radical distribution within the β2 subunit has accumulated over the years; yet little information exists on the in vivo Y122.. Here, we characterize this essential radical in whole cells. Multi-frequency EPR and electron-nuclear double resonance (ENDOR) demonstrate that the structure and electrostatic environment of Y122. are identical under in vivo and in vitro conditions. Pulsed dipolar EPR experiments shed light on a distinct in vivo Y122. per β2 distribution, supporting the key role of Y. concentrations in regulating RNR activity. Additionally, we spectroscopically verify the generation of an unnatural amino acid radical, F3Y122., in whole cells, providing a crucial step towards unique insights into the RNR catalysis under physiological conditions
Accretion with back reaction
We calculate analytically a back reaction of the stationary spherical
accretion flow near the event horizon and near the inner Cauchy horizon of the
charged black hole. It is shown that corresponding back-reaction corrections to
the black hole metric depend only on the fluid accretion rate and diverge in
the case of an extremely charged black hole. In result, the test fluid
approximation for stationary accretion is violated for extreme black holes.
This behavior of the accreting black hole is in accordance with the third law
of black hole thermodynamics, forbidding the practical attainability of the
extreme state.Comment: 5 pages, 2 figures; new figure and references adde
Lipoprotein lipase regulates hematopoietic stem progenitor cell maintenance through DHA supply.
Lipoprotein lipase (LPL) mediates hydrolysis of triglycerides (TGs) to supply free fatty acids (FFAs) to tissues. Here, we show that LPL activity is also required for hematopoietic stem progenitor cell (HSPC) maintenance. Knockout of Lpl or its obligatory cofactor Apoc2 results in significantly reduced HSPC expansion during definitive hematopoiesis in zebrafish. A human APOC2 mimetic peptide or the human very low-density lipoprotein, which carries APOC2, rescues the phenotype in apoc2 but not in lpl mutant zebrafish. Creating parabiotic apoc2 and lpl mutant zebrafish rescues the hematopoietic defect in both. Docosahexaenoic acid (DHA) is identified as an important factor in HSPC expansion. FFA-DHA, but not TG-DHA, rescues the HSPC defects in apoc2 and lpl mutant zebrafish. Reduced blood cell counts are also observed in Apoc2 mutant mice at the time of weaning. These results indicate that LPL-mediated release of the essential fatty acid DHA regulates HSPC expansion and definitive hematopoiesis
Human Capital Investments Among Veterinarians
Professional labor markets can be affected by alternating periods of excess or scarcity in labor. The phenomenon is most prevalent in labor markets where a substantial lag occurs between occupational choice and labor market entry. In this paper, a unique longitudinal dataset from veterinary labor markets is used to identify factors significantly associated with volatility in labor supply. Our econometric analysis establishes a statistically significant relationship between boom-bust cycles in labor and certain pertinent variables: entry-level earnings, a demand proxy, and supply-side features. Results support the notion that decision-makers gauge the expected levels of these variables when making career choices
Moving Domain Computational Fluid Dynamics to Interface with an Embryonic Model of Cardiac Morphogenesis
Peristaltic contraction of the embryonic heart tube produces time- and spatial-varying wall shear stress (WSS) and pressure gradients (∇P) across the atrioventricular (AV) canal. Zebrafish (Danio rerio) are a genetically tractable system to investigate cardiac morphogenesis. The use of Tg(fli1a:EGFP)y1 transgenic embryos allowed for delineation and two-dimensional reconstruction of the endocardium. This time-varying wall motion was then prescribed in a two-dimensional moving domain computational fluid dynamics (CFD) model, providing new insights into spatial and temporal variations in WSS and ∇P during cardiac development. The CFD simulations were validated with particle image velocimetry (PIV) across the atrioventricular (AV) canal, revealing an increase in both velocities and heart rates, but a decrease in the duration of atrial systole from early to later stages. At 20-30 hours post fertilization (hpf), simulation results revealed bidirectional WSS across the AV canal in the heart tube in response to peristaltic motion of the wall. At 40-50 hpf, the tube structure undergoes cardiac looping, accompanied by a nearly 3-fold increase in WSS magnitude. At 110-120 hpf, distinct AV valve, atrium, ventricle, and bulbus arteriosus form, accompanied by incremental increases in both WSS magnitude and ∇P, but a decrease in bi-directional flow. Laminar flow develops across the AV canal at 20-30 hpf, and persists at 110-120 hpf. Reynolds numbers at the AV canal increase from 0.07±0.03 at 20-30 hpf to 0.23±0.07 at 110-120 hpf (p< 0.05, n=6), whereas Womersley numbers remain relatively unchanged from 0.11 to 0.13. Our moving domain simulations highlights hemodynamic changes in relation to cardiac morphogenesis; thereby, providing a 2-D quantitative approach to complement imaging analysis. © 2013 Lee et al
Disorder-induced cavities, resonances, and lasing in randomly-layered media
We study, theoretically and experimentally, disorder-induced resonances in
randomly-layered samples,and develop an algorithm for the detection and
characterization of the effective cavities that give rise to these resonances.
This algorithm enables us to find the eigen-frequencies and pinpoint the
locations of the resonant cavities that appear in individual realizations of
random samples, for arbitrary distributions of the widths and refractive
indices of the layers. Each cavity is formed in a region whose size is a few
localization lengths. Its eigen-frequency is independent of the location inside
the sample, and does not change if the total length of the sample is increased
by, for example, adding more scatterers on the sides. We show that the total
number of cavities, , and resonances, , per
unit frequency interval is uniquely determined by the size of the disordered
system and is independent of the strength of the disorder. In an active,
amplifying medium, part of the cavities may host lasing modes whose number is
less than . The ensemble of lasing cavities behaves as
distributed feedback lasers, provided that the gain of the medium exceeds the
lasing threshold, which is specific for each cavity. We present the results of
experiments carried out with single-mode optical fibers with gain and
randomly-located resonant Bragg reflectors (periodic gratings). When the fiber
was illuminated by a pumping laser with an intensity high enough to overcome
the lasing threshold, the resonances revealed themselves by peaks in the
emission spectrum. Our experimental results are in a good agreement with the
theory presented here.Comment: minor correction
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