2,674 research outputs found
A static cost analysis for a higher-order language
We develop a static complexity analysis for a higher-order functional
language with structural list recursion. The complexity of an expression is a
pair consisting of a cost and a potential. The former is defined to be the size
of the expression's evaluation derivation in a standard big-step operational
semantics. The latter is a measure of the "future" cost of using the value of
that expression. A translation function tr maps target expressions to
complexities. Our main result is the following Soundness Theorem: If t is a
term in the target language, then the cost component of tr(t) is an upper bound
on the cost of evaluating t. The proof of the Soundness Theorem is formalized
in Coq, providing certified upper bounds on the cost of any expression in the
target language.Comment: Final versio
Ternary cluster decay within the liquid drop model
Longitudinal ternary and binary fission barriers of Ar, Ni and
Cf nuclei have been determined within a rotational liquid drop model
taking into account the nuclear proximity energy. For the light nuclei the
heights of the ternary fission barriers become competitive with the binary ones
at high angular momenta since the maximum lies at an outer position and has a
much higher moment of inertia.Comment: Talk presented at the 9th International Conference on Clustering
Aspects of Nuclear Structure and Dynamics (CLUSTERS'07
Compact star clusters of the LMC HII region N11C
Based on imaging and spectroscopy obtained at the ESO NTT telescope and using
an efficient image analysis algorithm, we study the core of the LMC OB
association LH13, particularly the two compact stellar clusters Sk-6641 and HNT
in the HII, region N11C. We resolve Sk-6641 into 15 components and for the
first time the HNT cluster into 70 stars, and derive photometry for the
members. Moreover, from medium resolution spectroscopy we determine the
spectral types for sixteen stars in N11C. We compare the color-magnitude
diagrams of the clusters with that of the field stars and discuss the cluster
ages. With an age of ~100 Myr, the HNT cluster appears significantly older than
the very young (< 5 Myr) Sk-6641 starburst. We suggest that most of the `field'
O-stars in the core of N11C have actually been ejected from Sk-6641 through
dynamical interactions in the compact cluster. The properties of the Sk-6641
and HNT clusters suggest that we are viewing different star formation regions
lying at different distances along the same line of sight.Comment: 12 pages, 8 figure
Invloed van cadmiumconcentraties in minerale of plantaardige grondstoffen op deze gehaltes in bloed en nieren van vleesvarkens
De mogelijke cadmiumbelasting in varkens kan worden verklaard door de bijdrage van de plaatselijke landbouw, maar ook door het gebruik van minerale grondstoffen die niet lokaal worden geproduceerd. Het doel van deze studie is een vergelijking tussen de effecten van plantaardige of minerale grondstoffen met een Cd-gehalte dicht onder de toegestane norm op de concentratie in de nieren van vleesvarkens. Het Cd-gehalte in bloed geeft de recente blootstelling van de varkens weer, terwijl het Cd-gehalte in de nieren de cumulatieve blootstelling weergeeft. Het bloedgehalte en de uiteindelijke nierconcentratie van Cd zijn niet verschillend voor varkens die door de minerale fractie of de plantaardige fractie van een mengvoer blootgesteld zijn aan een verhoogd Cd-gehalte
FUSE observations of G226-29: First detection of the H_2 quasi-molecular satellite at 1150A
We present new FUV observations of the pulsating DA white dwarf G226-29
obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). This ZZ Ceti
star is the brightest one of its class and the coolest white dwarf observed by
FUSE. We report the first detection of the broad quasi-molecular
collision-induced satellite of Ly-beta at 1150 A, an absorption feature that is
due to transitions which take place during close collisions of hydrogen atoms.
The physical interpretation of this feature is based on recent progress of the
line broadening theory of the far wing of Ly-beta. This predicted feature had
never been observed before, even in laboratory spectra.Comment: Accepted for publication in ApJ Letters; 6 pages, 3 figure
Alpha-decay lifetimes semiempirical relationship including shell effects
A new version of the semiempirical formula based on fission approach of alpha
decay is derived, by using the optimum values of the fitting parameters
determined for even-even nuclei, combined with hindrance factors for even-odd,
odd-even, and odd-odd nuclides. The deviations from experimental data for two
regions of nuclear chart (493 alpha emitters with Z=52-118 and 142 transuranium
nuclei including superheavies (Z=92-118), respectively) are compared with those
obtained by using the universal curve and the Viola-Seaborg semiempirical
relationship.Comment: 11 pages, 3 figures, revtex
The Generalized Liquid Drop Model Alpha-Decay Formula: Predictability Analysis and Super-Heavy Element Alpha Half-Lives
The predictive accuracy of the generalized liquid drop model (GLDM) formula
for alpha decay half-lives has been investigated in a detailed manner and a
variant of the formula with improved coefficients is proposed. The method
employs the experimental alpha half-lives of the well-known alpha standards
(REFERENCE) to obtain the coefficients of the analytical formula using the
experimental Qalpha values (the DSR-E formula), as well as the finite range
droplet model (FRDM) derived Qalpha values (the FRDMFRDM formula). The
predictive accuracy of these formulae were checked against the experimental
alpha half-lives of an independent set of nuclei (TEST) that span approximately
the same Z,A region as the standards and possess reliable alpha spectroscopic
data, and were found to yield good results for the DSR-E formula but not for
the FRDM-FRDM formula. The two formulae were used to obtain the alpha
half-lives of super-heavy (SHE) and heavy nuclides where the relative accuracy
was found to markedly improve for the FRDM-FRDM, which corroborates the
appropriateness of the FRDM masses and the GLDM prescription for high Z,A
nuclides. Further improvement resulted, especially for the FRDM-FRDM formula,
after a simple linear optimization over the calculated and experimental
half-lives of TEST was used to re-calculate the half-lives of the SHE and heavy
nuclides. The advantage of this optimization was that it required no
recalculation of the coefficients of the basic DSR-E or FRDM-FRDM formulae. The
halflives for 324 medium-mass to super-heavy alpha decaying nuclides,
calculated using these formulae and the comparison with experimental
half-lives, are presented.Comment: 61 pages, 6 figures, PDF file, to appear in Atomic Data and Nuclear
Data Table
Droplet and cluster formation in freely falling granular streams
Particle beams are important tools for probing atomic and molecular
interactions. Here we demonstrate that particle beams also offer a unique
opportunity to investigate interactions in macroscopic systems, such as
granular media. Motivated by recent experiments on streams of grains that
exhibit liquid-like breakup into droplets, we use molecular dynamics
simulations to investigate the evolution of a dense stream of macroscopic
spheres accelerating out of an opening at the bottom of a reservoir. We show
how nanoscale details associated with energy dissipation during collisions
modify the stream's macroscopic behavior. We find that inelastic collisions
collimate the stream, while the presence of short-range attractive interactions
drives structure formation. Parameterizing the collision dynamics by the
coefficient of restitution (i.e., the ratio of relative velocities before and
after impact) and the strength of the cohesive interaction, we map out a
spectrum of behaviors that ranges from gas-like jets in which all grains drift
apart to liquid-like streams that break into large droplets containing hundreds
of grains. We also find a new, intermediate regime in which small aggregates
form by capture from the gas phase, similar to what can be observed in
molecular beams. Our results show that nearly all aspects of stream behavior
are closely related to the velocity gradient associated with vertical free
fall. Led by this observation, we propose a simple energy balance model to
explain the droplet formation process. The qualitative as well as many
quantitative features of the simulations and the model compare well with
available experimental data and provide a first quantitative measure of the
role of attractions in freely cooling granular streams
Addressing the Enduring Primary Care Physician Shortage in The United States: The Direct and Indirect Effects of Gender on the Medical Specialty Decision-Making Process
Background: There has been an enduring primary care (PC) physician shortage in the United States (U.S.) for decades, which is projected to worsen. With women entering PC at significantly higher rates than men, the aim of this study was to explore various pathways through which gender may affect the medical specialty decision-making process. Methods: Using data from the National Survey of Attitudes and Choices in Medical Education and Training (ACMET) II on a sample of 492 medical residents, this study employed structural equation modeling (SEM) to explore how gender shaped residents’ preferences for future practice and their perceptions of PC, and how their experiences with faculty affected the medical specialty decision-making process. Results: As expected, women were significantly more likely than men to report choosing PC. This study also found that there were several indirect pathways through which gender affects specialty choice, including through negative perceptions about PC and the time spent with PC faculty in medical school. Conclusion: Given the multiple pathways through which gender affects the medical specialty decision-making process, this study highlights a need for gender-specific interventions when addressing the enduring PC physician shortage in the U.S. Specifically, the results of this study suggest that increasing the time that male medical students spend in PC through structural changes in medical education might mitigate negative perceptions about PC and encourage males to enter PC at higher rates. Future research should assess the effectiveness of such gender-specific interventions
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