3,594 research outputs found
Half-Precision Kronecker Product SVD Preconditioner for Structured Inverse Problems
In this paper we investigate the use of half-precision Kronecker product
singular value decomposition (SVD) approximations as preconditioners for
large-scale Tikhonov regularized least squares problems. Half precision reduces
storage requirements and has the potential to greatly speedup computations on
certain GPU architectures. We consider both standard PCG and flexible PCG
algorithms, and investigate, through numerical experiments on image deblurring
problems, the trade-offs between potentially faster convergence with the
additional cost per iteration when using this preconditioning approach.
Moreover, we also investigate the use of several regularization parameter
choice methods, including generalized cross validation and the discrepancy
principle
Parton theory of magnetic polarons: Mesonic resonances and signatures in dynamics
When a mobile hole is moving in an anti-ferromagnet it distorts the
surrounding Neel order and forms a magnetic polaron. Such interplay between
hole motion and anti-ferromagnetism is believed to be at the heart of high-Tc
superconductivity in cuprates. We study a single hole described by the t-Jz
model with Ising interactions between the spins in 2D. This situation can be
experimentally realized in quantum gas microscopes. When the hole hopping is
much larger than couplings between the spins, we find strong evidence that
magnetic polarons can be understood as bound states of two partons, a spinon
and a holon carrying spin and charge quantum numbers respectively. We introduce
a microscopic parton description which is benchmarked by comparison with
results from advanced numerical simulations. Using this parton theory, we
predict a series of excited states that are invisible in the spectral function
and correspond to rotational excitations of the spinon-holon pair. This is
reminiscent of mesonic resonances observed in high-energy physics, which can be
understood as rotating quark antiquark pairs. We also apply the strong coupling
parton theory to study far-from equilibrium dynamics of magnetic polarons
observable in current experiments with ultracold atoms. Our work supports
earlier ideas that partons in a confining phase of matter represent a useful
paradigm in condensed-matter physics and in the context of high-Tc
superconductivity. While direct observations of spinons and holons in real
space are impossible in traditional solid-state experiments, quantum gas
microscopes provide a new experimental toolbox. We show that, using this
platform, direct observations of partons in and out-of equilibrium are
possible. Extensions of our approach to the t-J model are also discussed. Our
predictions in this case are relevant to current experiments with quantum gas
microscopes for ultracold atoms.Comment: 30 pages, 4 appendices, 26 figure
Changes in male rat urinary protein profile during puberty: a pilot study
BACKGROUND: Androgen-dependent proteins (lipocalins) circulate in blood of male rats and mice and, being small (~ 18 kDa), pass freely into glomerular filtrate. Some are salvaged in proximal nephrons but some escape in urine. Several organic molecules can bind to these proteins causing, where salvage occurs, nephropathy including malignancy in renal cortex. In urine, both free lipocalins and ligands contribute to an increasingly-recognised vital biological role in social communication between adults, especially in the dark where reliance is on smell and taste. Crystal structure of the first-characterised lipocalin of male rats, α2u-globulin, has been determined and peptide sequences for others are available, but no study of occurrence during early puberty has been made. We have followed temporal occurrence in urine of juveniles (n = 3) for non-invasive pilot study by high resolution gradient mini-gel electrophoresis, tryptic digest of excised protein bands, and LC-MS/MS of digest to identify peptide fragments and assign to specific lipocalins. Study objective refers directly to external availability for social communication but also indirectly to indicate kinetics of circulating lipocalins to which some xenobiotics may bind and constitute determinants of renal disease. RESULTS: Mini-gels revealed greater lipocalin complexity than hitherto recognised, possibly reflecting post-translational modifications. Earliest patterns comprised rat urinary protein 1, already evident in Sprague-Dawley and Wistar strains at 36 and 52 days, respectively. By 44 and 57 days major rat protein (α2u-globulin) occurred as the progressively more dominant protein, though as two forms with different electrophoretic mobility, characterised by seven peptide sequences. No significant change in urinary testosterone had occurred in Wistars when major rat protein became evident, but testosterone surged by 107 days concomitant with the marked abundance of excreted lipocalins. CONCLUSIONS: Qualitative temporal changes in the composition of excreted lipocalins early in puberty, and apparent increase in major urinary protein as two resolvable forms, should catalyse systematic non-invasive study of urinary lipocalin and testosterone dynamics from early age, to illuminate this aspect of laboratory rodent social physiology. It could also define the potential temporal onset of nephrotoxic ligand risk, applicable to young animals used as toxicological models
Secondary resonances of co-orbital motions
The size distribution of the stability region around the Lagrangian point L4
is investigated in the elliptic restricted three-body problem as the function
of the mass parameter and the orbital eccentricity of the primaries. It is
shown that there are minimum zones in the size distribution of the stability
regions, and these zones are connected with secondary resonances between the
frequencies of librational motions around L4. The results can be applied to
hypothetical Trojan planets for predicting values of the mass parameter and the
eccentricity for which such objects can be expected or their existence is less
probable.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Development of nociceptive synaptic inputs to the neonatal rat dorsal horn: glutamate release by capsaicin and menthol
To study the postnatal development of nociceptive synaptic inputs in the superficial dorsal horn of the neonatal rat spinal cord, we examined the effect of capsaicin and menthol on glutamatergic mEPSCs in postnatal day (P) 0-1, P5-6 and P9-11 slices of spinal cord. Capsaicin (100 nM to 2 muM) increased the mEPSC frequency in a concentration-dependent manner at all ages tested, with a significant enhancement of the effect between P5 and P10. This effect was sensitive to vanilloid receptor (VR) antagonists. The elevation in mEPSC frequency occurred at concentrations of capsaicin (100 nM) that did not alter the distribution of mEPSC amplitudes and was abolished by a dorsal rhizotomy, demonstrating that capsaicin acts via presynaptic VR1 receptors localized on primary afferents. Menthol significantly increased the mEPSC frequency with a similar developmental pattern to capsaicin without consistently affecting mEPSC amplitude. The increase in mEPSC frequency following capsaicin did not depend on transmembrane calcium influx since it persisted in zero [Ca2+](o). The facilitation of spontaneous glutamate release by capsaicin was sufficient to evoke action potentials in neonatal dorsal horn neurons but was accompanied by a block of EPSCs evoked by electrical stimulation of the dorsal root. These results indicate that VR1-expressing nociceptive primary afferents; form functional synaptic connections in the superficial dorsal horn from birth and that activation of the VR1 receptor increases spontaneous glutamate release via an undetermined mechanism. In addition, the data suggest that immature primary afferents express functional menthol receptors that are capable of modulating transmitter release. These results have important functional implications for infant pain processing
Functional renormalization group with a compactly supported smooth regulator function
The functional renormalization group equation with a compactly supported
smooth (CSS) regulator function is considered. It is demonstrated that in an
appropriate limit the CSS regulator recovers the optimized one and it has
derivatives of all orders. The more generalized form of the CSS regulator is
shown to reduce to all major type of regulator functions (exponential,
power-law) in appropriate limits. The CSS regulator function is tested by
studying the critical behavior of the bosonized two-dimensional quantum
electrodynamics in the local potential approximation and the sine-Gordon scalar
theory for d<2 dimensions beyond the local potential approximation. It is shown
that a similar smoothing problem in nuclear physics has already been solved by
introducing the so called Salamon-Vertse potential which can be related to the
CSS regulator.Comment: JHEP style, 11 pages, 2 figures, proofs corrected, accepted for
publication by JHE
Phase Structure and Compactness
In order to study the influence of compactness on low-energy properties, we
compare the phase structures of the compact and non-compact two-dimensional
multi-frequency sine-Gordon models. It is shown that the high-energy scaling of
the compact and non-compact models coincides, but their low-energy behaviors
differ. The critical frequency at which the sine-Gordon model
undergoes a topological phase transition is found to be unaffected by the
compactness of the field since it is determined by high-energy scaling laws.
However, the compact two-frequency sine-Gordon model has first and second order
phase transitions determined by the low-energy scaling: we show that these are
absent in the non-compact model.Comment: 21 pages, 5 figures, minor changes, final version, accepted for
publication in JHE
Proton-Proton Near-Forward Hard Elastic Scattering
We calculate the leading twist contribution to near-forward proton-proton
(and proton-antiproton) elastic scattering with large momentum transfer, in the
multiple scattering (Landshoff) mechanism. The amplitude in the near-forward
region is dominated by singlet exchange for all three valence quark-quark
scatterings. We assume the existence of a hard singlet quark-quark amplitude,
which we estimate to be . For a three-quark state whose
transverse size is less than , Sudakov resummation
accounts for both approximate \d \sigma^{pp}/ \d t \sim t^{-8} at moderate
, and \d \sigma^{pp}/ \d t \sim t^{-10} at larger . The transition from
approximate to behavior is strongly correlated with the
transverse size of the valence three-quark state in the proton.Comment: 34 pages, LaTe
On the assessment of passive devices for structural control via real-time dynamic substructuring
In this work, the applicability of a dynamic testing technique known as real-time dynamic substructuring (RTDS) for the assessment of passive vibration suppression systems in seismic protection of buildings is analysed. RTDS is an efficient method for the assessment of dynamic and rate-dependent behaviour of systems subjected to dynamic excitation at real scale and in real scenarios. The actuators used in RTDS test introduce additional undesirable dynamics into the system, which are often not fully compensated for in the actuator controller—these dynamics are commonly approximated as a feedback delay. To guarantee the validity and accuracy of an RTDS simulation, a stability analysis of the substructured system that includes the feedback delay should be carried out. In this paper, we present explicit analyses that provide a dynamic characterization of the delay-induced phenomena in RTDS simulations when considering passive vibration suppression systems with strong nonlinearities. We present a complete set of closed-form expressions to describe the main phenomena because of delay in terms of dynamic stability in an RTDS simulation. Through an experimental study, we confirm the existence of self-sustained oscillations caused by very small delay in the feedback loop. This lead the system to instability in the form of high-frequency oscillations. Copyright © 2011 John Wiley & Sons, Ltd
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