13,744 research outputs found
Total Synthesis and Structural Revision of the Alkaloid Incargranine B
Seeing double: Consideration of the biosynthetic origins of incargranineB, which was originally assigned an unprecedented indolo[1.7]naphthyridine structure, led to the proposal of a dipyrroloquinoline framework as a more biosynthetically feasible struct
A fixed point theorem for the infinite-dimensional simplex
We define the infinite dimensional simplex to be the closure of the convex
hull of the standard basis vectors in R^infinity, and prove that this space has
the 'fixed point property': any continuous function from the space into itself
has a fixed point. Our proof is constructive, in the sense that it can be used
to find an approximate fixed point; the proof relies on elementary analysis and
Sperner's lemma. The fixed point theorem is shown to imply Schauder's fixed
point theorem on infinite-dimensional compact convex subsets of normed spaces.Comment: 8 pages; related work at http://www.math.hmc.edu/~su/papers.htm
Algorithm based comparison between the integral method and harmonic analysis of the timing jitter of diode-based and solid-state pulsed laser sources
AbstractA comparison between two methods of timing jitter calculation is presented. The integral method utilizes spectral area of the single side-band (SSB) phase noise spectrum to calculate root mean square (rms) timing jitter. In contrast the harmonic analysis exploits the uppermost noise power in high harmonics to retrieve timing fluctuation. The results obtained show that a consistent timing jitter of 1.2ps is found by the integral method and harmonic analysis in gain-switched laser diodes with an external cavity scheme. A comparison of the two approaches in noise measurement of a diode-pumped Yb:KY(WO4)2 passively mode-locked laser is also shown in which both techniques give 2ps rms timing jitter
Analysis of anti-diabetic exosomes secreted from beige adipocytes
Accumulation of excess fat in white adipose tissue is associated with an increase in risk for type 2 diabetes. Within white fat tissue resides a population of “beige” adipocytes that are activated by cold exposure and expend energy contained in fats, which is released as heat. Increasing energy expenditure through beige adipocyte activation has been shown to reduce diabetic symptoms in rodent models of obesity. However, activation of beige adipocytes through exposure of humans to cold temperatures is uncomfortable and likely not a realistic strategy to control body weight. In addition to its fat burning potential, secreted factors derived from activated beige adipocytes may enter the circulation and reduce diabetic symptoms such as insulin resistance in other tissues. The mechanisms by which these secreted factors act on distant tissues may in part be due to their transport inside extracellular vesicles, known as exosomes. Exosomes carry a diverse array of signaling molecules, including microRNAs that are transported and released into recipient cells and tissues. The goal of this project was to determine if beige adipocytes grown in cell culture secrete exosomes that contain microRNAs that may harbor anti-diabetic properties. Unexpectedly, we found that during the activation of beige adipocytes, secreted exosomes contain elevated expression of a number of microRNAs known to be negative regulators of beige adipocyte activation, including mir27. This suggests that exosome secretion may be a way to increase beige adipocyte activation by decreasing the expression of specific microRNAs. Future testing of these microRNA candidates may translate to improved therapies for obese patients that develop diabetes
On the accuracy of conservation of adiabatic invariants in slow-fast systems
Let the adiabatic invariant of action variable in slow-fast Hamiltonian
system with two degrees of freedom have two limiting values along the
trajectories as time tends to infinity. The difference of two limits is
exponentially small in analytic systems. An iso-energetic reduction and
canonical transformations are applied to transform the slow-fast systems to
form of systems depending on slowly varying parameters in a complexified phase
space. On the basis of this method an estimate for the accuracy of conservation
of adiabatic invariant is given for such systems.Comment: 27 pages, 14 figure
Vector mesons in nuclear medium with small three momentum, a QCD sum rule approach
Using the QCD Operator Product Expansion, we derive the real part of the
transverse and longitudinal vector vector correlation function with the
quantum numbers to leading order in density and in at
. To dimension 6, only twist-2 and 4 operators
contribute. These OPE, through the energy dispersion relation, provide model
independent constraints for the dependence of the vector meson
spectral density in nuclear medium. We further make a QCD sum rule type of
analysis to extract the momentum dependence of the vector meson dispersion
relation in medium. The contributions from twist-2 operators are added up to
infinite order to check the validity of the OPE at the relevant Borel window.Comment: 6 pages, talk presented at the workshop "QCD at finite baryon
density" Zif, Bielefeld, April 27-30, 1998. Submitted to Nucl. Phys.
Magnetisation distribution in the tetragonal phase of BaFe2As2
We have determined the spatial distribution of the magnetisation induced by a
field of 9 T in the tetragonal phase of BaFe2As2 using polarised neutron
diffraction. Magnetic structure factors derived from the polarisation
dependence of the intensities of Bragg reflections were used to make a maximum
entropy reconstruction of the distribution projected on the 110 plane. The
reconstruction shows clearly that the magnetisation is confined to the region
around the iron atoms and that there is no significant magnetisation associated
with either the As or Ba atoms. The distribution of magnetisation around the Fe
atom is significantly non-spherical with a shape which is extended in the
directions in the projection. These results show that the electrons which give
rise to the paramagnetic susceptibility are confined to the Fe atoms their
distribution suggests that they occupy 3d t_2g type orbitals with about 60% in
those of xy symmetry
Magnetization distribution and orbital moment in the non-Superconducting Chalcogenide Compound K0.8Fe1.6Se2
We have used polarized and unpolarized neutron diffraction to determine the
spatial distribution of the magnetization density induced by a magnetic field
of 9 T in the tetragonal phase of K0.8Fe1.6Se2. The maximum entropy
reconstruction shows clearly that most of the magnetization is confined to the
region around the iron atoms whereas there is no significant magnetization
associated with either Se or K atoms. The distribution of magnetization around
the Fe atom is slightly nonspherical with a shape which is extended along the
[0 0 1] direction in the projection. Multipolar refinement results show that
the electrons which give rise to the paramagnetic susceptibility are confined
to the Fe atoms and their distribution suggests that they occupy 3d t2g-type
orbitals with around 66% in those of xz/yz symmetry. Detail modeling of the
magnetic form factor indicates the presence of an orbital moment to the total
paramagnetic moment of Fe2+Comment: 7 pages, accepted for publication in Physical Review
The spectral difference between solar flare HXR coronal and footpoint sources due to wave-particle interactions
Investigate the spatial and spectral evolution of hard X-ray (HXR) emission
from flare accelerated electron beams subject to collisional transport and
wave-particle interactions in the solar atmosphere. We numerically follow the
propagation of a power-law of accelerated electrons in 1D space and time with
the response of the background plasma in the form of Langmuir waves using the
quasilinear approximation.}{We find that the addition of wave-particle
interactions to collisional transport for a transient initially injected
electron beam flattens the spectrum of the footpoint source. The coronal source
is unchanged and so the difference in the spectral indices between the coronal
and footpoint sources is \Delta \gamma > 2, which is larger than expected from
purely collisional transport. A steady-state beam shows little difference
between the two cases, as has been previously found, as a transiently injected
electron beam is required to produce significant wave growth, especially at
higher velocities. With this transiently injected beam the wave-particle
interactions dominate in the corona whereas the collisional losses dominate in
the chromosphere. The shape of the spectrum is different with increasing
electron beam density in the wave-particle interaction case whereas with purely
collisional transport only the normalisation is changed. We also find that the
starting height of the source electron beam above the photosphere affects the
spectral index of the footpoint when Langmuir wave growth is included. This may
account for the differing spectral indices found between double footpoints if
asymmetrical injection has occurred in the flaring loop.Comment: 10 pages, 10 FIgures, accepted for publication in A&
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