7,176 research outputs found
A Concise Total Synthesis of (--)-Maoecrystal Z
The first total synthesis of (--)-maoecrystal Z
is described. The key steps of the synthesis include a
diastereoselective Ti^(III)-mediated reductive epoxide coupling reaction and a diastereoselective Sm^(II)-mediated reductive cascade cyclization reaction. These transformations enabled the preparation of (--)-maoecrystal Z in only 12 steps from (--)-Ī³-cyclogeraniol
Superfluid-insulator transition of the Josephson junction array model with commensurate frustration
We have studied the rationally frustrated Josephson-junction array model in
the square lattice through Monte Carlo simulations of D XY-model. For
frustration , the model at zero temperature shows a continuous
superfluid-insulator transition. From the measurement of the correlation
function and the superfluid stiffness, we obtain the dynamical critical
exponent and the correlation length critical exponent . While the dynamical critical exponent is the same as that for cases
, 1/2, and 1/3, the correlation length critical exponent is surprisingly
quite different. When , we have the nature of a first-order transition.Comment: RevTex 4, to appear in PR
Deflection microwave amplifier with fieldāemitter arrays
A new class of microwave and millimeterāwave amplifiers, based on the deflection of collimated microscopic electron beams with highācurrent density and low voltage from fieldāemitter arrays, was analyzed. The deflection concept may be applied in two ways; as microelectronic amplifiers or as bunched beam cathodes to power conventional amplifier configurations such as klystrodes and traveling wave tubes. We found that the frequency fT, at which the shortācircuit current gain attains unit magnitude, depends only on the electron beam energy, current density, and emittance, and is independent of beamwidth and total current. The results indicate that fieldāemitter arrays have the potential of making wideband deflection amplifiers at frequencies up to tens of GHz. Ā© 1994 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69885/2/APPLAB-65-22-2881-1.pd
Design and Evaluation of a Protection Relay for a Wind Generator Based on the Positive- and Negative-Sequence Fault Components
BRST Quantization of the Proca Model based on the BFT and the BFV Formalism
The BRST quantization of the Abelian Proca model is performed using the
Batalin-Fradkin-Tyutin and the Batalin-Fradkin-Vilkovisky formalism. First, the
BFT Hamiltonian method is applied in order to systematically convert a second
class constraint system of the model into an effectively first class one by
introducing new fields. In finding the involutive Hamiltonian we adopt a new
approach which is more simpler than the usual one. We also show that in our
model the Dirac brackets of the phase space variables in the original second
class constraint system are exactly the same as the Poisson brackets of the
corresponding modified fields in the extended phase space due to the linear
character of the constraints comparing the Dirac or Faddeev-Jackiw formalisms.
Then, according to the BFV formalism we obtain that the desired resulting
Lagrangian preserving BRST symmetry in the standard local gauge fixing
procedure naturally includes the St\"uckelberg scalar related to the explicit
gauge symmetry breaking effect due to the presence of the mass term. We also
analyze the nonstandard nonlocal gauge fixing procedure.Comment: 29 pages, plain Latex, To be published in Int. J. Mod. Phys.
Association between vitamin D and pressure ulcers in older ambulatory adults: results of a matched caseācontrol study
Usha R Kalava1, Stephen S Cha2, Paul Y Takahashi1,31Department of Internal Medicine, Division of Primary Care Internal Medicine, 2Department of Biostatistics, 3Kogod Center of Aging, Mayo Clinic, Rochester, MN, USABackground: Pressure ulcers are common among older adults, but knowledge about nutritional risk factors is still developing. Vitamin D deficiency is common in the elderly population and is required for normal skin proliferation. The role of vitamin D in pressure ulceration and wound healing is not known. The purpose of this case–control study was to determine the association between vitamin D levels and pressure ulceration in an older community-dwelling cohort.Methods: All cases and controls were community-dwelling elderly older than 60 years in a primary care panel in Olmsted County, MN. Pressure ulcer cases were defined clinically. The controls were age-matched and gender-matched to controls without pressure ulceration. The main exposure variable was 25-hydroxyvitamin D levels in both groups. The other exposure variable was the Charlson Comorbidity Index used to measure medical comorbidity. The analysis included univariate and conditional logistic regression for 25-hydroxyvitamin D levels.Results: The average (standard deviation) age of the study participants with a pressure ulcer was 80.46 years (±8.67), and the average vitamin D level was 30.92 ng/mL (±12.46). In univariate analysis, Vitamin D deficiency (levels < 25 ng/mL) was associated with pressure ulcers (odds ratio: 1.871, P = 0.0154). Comorbidities of the subjects calculated using the Charlson Comorbidity Index were also associated with pressure ulcers (odds ratio: 1.136, P < 0.001). In the final conditional logistical regression model, the association of Vitamin D and pressure ulcers became nonsignificant after adjustment for comorbid illness.Conclusion: Medical comorbidities increased the risk of pressure ulceration. Vitamin D deficiency was not an independent risk factor for pressure ulceration, and may be a marker of comorbid illness.Keywords: pressure ulcer, vitamin D, case–control study, aging, geriatric
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Analysis of magnetic energy stored in superconducting coils with and without ferromagnetic inserts
Inductance and energy of superconducting coils are calculated by (1) a long solenoid approximation, (2) a finite element model, and (3) working formulas and tables. The results of the finite element model compare favorably with that of the working formulas. The long solenoid approximation overpredicts the energy and the inductance compared to the other two methods. The difference decreases with increasing length to diameter ratio
"God's Companions: Reimagining Christian Ethics" by Samuel Wells
Review of Samuel Wells, God's Companions: Reimagining Christian Ethics (Oxford: Blackwell, 2006
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