423 research outputs found
Noncyclic covers of knot complements
Hempel has shown that the fundamental groups of knot complements are
residually finite. This implies that every nontrivial knot must have a
finite-sheeted, noncyclic cover. We give an explicit bound, , such
that if is a nontrivial knot in the three-sphere with a diagram with
crossings and a particularly simple JSJ decomposition then the complement of
has a finite-sheeted, noncyclic cover with at most sheets.Comment: 29 pages, 8 figures, from Ph.D. thesis at Columbia University;
Acknowledgments added; Content correcte
A Process Sensor for Locating the Liquid-Solid Boundary through the Mold of a Casting
Accurate process control of single-crystal and directionally-solidified castings requires knowledge of the exact location of the solidifying front. If the front advances too rapidly, single crystal growth in a preferred orientation degenerates into the formation of polycrystals. A solidification front which moves more slowly than necessary is wasteful of the casting resources. A sensing technology is being developed which determines the location of the boundary between a solidifying crystal and liquid metal. The sensing method utilizes the ordered pattern of x-rays diffracted from the solid as an absolute indicator of the liquid-crystal interface.</p
DETERMINATION OF THE THIXOCASTING TEMPERATURES OF AZ91D AND OTHER Mg ALLOYS USING A QUENCHING METHOD
Real-time non-equilibrium dynamics of quantum glassy systems
We develop a systematic analytic approach to aging effects in quantum
disordered systems in contact with an environment. Within the closed-time
path-integral formalism we include dissipation by coupling the system to a set
of independent harmonic oscillators that mimic a quantum thermal bath. After
integrating over the bath variables and averaging over disorder we obtain an
effective action that determines the real-time dynamics of the system. The
classical limit yields the Martin-Siggia-Rose generating functional associated
to a colored noise. We apply this general formalism to a prototype model
related to the spin-glass. We show that the model has a dynamic phase
transition separating the paramagnetic from the spin-glass phase and that
quantum fluctuations depress the transition temperature until a quantum
critical point is reached. We show that the dynamics in the paramagnetic phase
is stationary but presents an interesting crossover from a region controlled by
the classical critical point to another one controlled by the quantum critical
point. The most characteristic property of the dynamics in a glassy phase,
namely aging, survives the quantum fluctuations. In the sub-critical region the
quantum fluctuation-dissipation theorem is modified in a way that is consistent
with the notion of effective temperatures introduced for the classical case. We
discuss these results in connection with recent experiments in dipolar quantum
spin-glasses and the relevance of the effective temperatures with respect to
the understanding of the low temperature dynamics.Comment: 56 pages, Revtex, 17 figures include
Epidemics on contact networks: a general stochastic approach
Dynamics on networks is considered from the perspective of Markov stochastic
processes. We partially describe the state of the system through network motifs
and infer any missing data using the available information. This versatile
approach is especially well adapted for modelling spreading processes and/or
population dynamics. In particular, the generality of our systematic framework
and the fact that its assumptions are explicitly stated suggests that it could
be used as a common ground for comparing existing epidemics models too complex
for direct comparison, such as agent-based computer simulations. We provide
many examples for the special cases of susceptible-infectious-susceptible (SIS)
and susceptible-infectious-removed (SIR) dynamics (e.g., epidemics propagation)
and we observe multiple situations where accurate results may be obtained at
low computational cost. Our perspective reveals a subtle balance between the
complex requirements of a realistic model and its basic assumptions.Comment: Main document: 16 pages, 7 figures. Electronic Supplementary Material
(included): 6 pages, 1 tabl
Theory and applications of atomic and ionic polarizabilities
Atomic polarization phenomena impinge upon a number of areas and processes in
physics. The dielectric constant and refractive index of any gas are examples
of macroscopic properties that are largely determined by the dipole
polarizability. When it comes to microscopic phenomena, the existence of
alkaline-earth anions and the recently discovered ability of positrons to bind
to many atoms are predominantly due to the polarization interaction. An
imperfect knowledge of atomic polarizabilities is presently looming as the
largest source of uncertainty in the new generation of optical frequency
standards. Accurate polarizabilities for the group I and II atoms and ions of
the periodic table have recently become available by a variety of techniques.
These include refined many-body perturbation theory and coupled-cluster
calculations sometimes combined with precise experimental data for selected
transitions, microwave spectroscopy of Rydberg atoms and ions, refractive index
measurements in microwave cavities, ab initio calculations of atomic structures
using explicitly correlated wave functions, interferometry with atom beams, and
velocity changes of laser cooled atoms induced by an electric field. This
review examines existing theoretical methods of determining atomic and ionic
polarizabilities, and discusses their relevance to various applications with
particular emphasis on cold-atom physics and the metrology of atomic frequency
standards.Comment: Review paper, 44 page
Reactive nitrogen partitioning and its relationship to winter ozone events in Utah
High wintertime ozone levels have been observed in the Uintah Basin, Utah, a
sparsely populated rural region with intensive oil and gas operations. The
reactive nitrogen budget plays an important role in tropospheric ozone
formation. Measurements were taken during three field campaigns in the
winters of 2012, 2013 and 2014, which experienced varying climatic
conditions. Average concentrations of ozone and total reactive nitrogen were
observed to be 2.5 times higher in 2013 than 2012, with 2014 an intermediate
year in most respects. However, photochemically active NO<sub><i>x</i></sub>
(NO + NO<sub>2</sub>) remained remarkably similar all three years. Nitric acid
comprised roughly half of NO<sub><i>z</i></sub> ( ≡  NO<sub><i>y</i></sub> − NO<sub><i>x</i></sub>) in 2013,
with nighttime nitric acid formation through heterogeneous uptake of
N<sub>2</sub>O<sub>5</sub> contributing approximately 6 times more than daytime formation. In
2012, N<sub>2</sub>O<sub>5</sub> and ClNO<sub>2</sub> were larger components of NO<sub><i>z</i></sub> relative to
HNO<sub>3</sub>. The nighttime N<sub>2</sub>O<sub>5</sub> lifetime between the high-ozone year 2013
and the low-ozone year 2012 is lower by a factor of 2.6, and much of this is
due to higher aerosol surface area in the high-ozone year of 2013. A
box-model simulation supports the importance of nighttime chemistry on the
reactive nitrogen budget, showing a large sensitivity of NO<sub><i>x</i></sub> and ozone
concentrations to nighttime processes
Ozone photochemistry in an oil and natural gas extraction region during winter: simulations of a snow-free season in the Uintah Basin, Utah
The Uintah Basin in northeastern Utah, a region of intense oil and gas extraction, experienced ozone (O3) concentrations above levels harmful to human health for multiple days during the winters of 2009–2010 and 2010–2011. These wintertime O3 pollution episodes occur during cold, stable periods when the ground is snow-covered, and have been linked to emissions from the oil and gas extraction process. The Uintah Basin Winter Ozone Study (UBWOS) was a field intensive in early 2012, whose goal was to address current uncertainties in the chemical and physical processes that drive wintertime O3 production in regions of oil and gas development. Although elevated O3 concentrations were not observed during the winter of 2011–2012, the comprehensive set of observations tests our understanding of O3 photochemistry in this unusual emissions environment. A box model, constrained to the observations and using the nearexplicit Master Chemical Mechanism (MCM) v3.2 chemistry scheme, has been used to investigate the sensitivities of O3 production during UBWOS 2012. Simulations identify the O3 production photochemistry to be highly radical limited (with a radical production rate significantly smaller than the NOx emission rate). Production of OH from O3 photolysis (through reaction of O(1D) with water vapor) contributed only 170 pptv day−1, 8% of the total primary radical source on average (primary radicals being those produced from non-radical precursors). Other radical sources, including the photolysis of formaldehyde (HCHO, 52 %), nitrous acid (HONO, 26 %), and nitryl chloride (ClNO2, 13 %) were larger. O3 production was also found to be highly sensitive to aromatic volatile organic compound (VOC) concentrations, due to radical amplification reactions in the oxidation scheme of these species. Radical production was shown to be small in comparison to the emissions of nitrogen oxides (NOx), such that NOx acted as the primary radical sink. Consequently, the system was highly VOC sensitive, despite the much larger mixing ratio of total non-methane hydrocarbons (230 ppbv (2080 ppbC), 6 week average) relative to NOx (5.6 ppbv average). However, the importance of radical sources which are themselves derived from NOx emissions and chemistry, such as ClNO2 and HONO, make the response of the system to changes in NOx emissions uncertain. Model simulations attempting to reproduce conditions expected during snow-covered cold-pool conditions show a significant increase in O3 production, although calculated concentrations do not achieve the highest seen during the 2010–2011 O3 pollution events in the Uintah Basin. These box model simulations provide useful insight into the chemistry controlling winter O3 production in regions of oil and gas extraction
Ozone Photochemistry in an oil and natural gas extraction region during winter: simulations of a snow-free season in the Uintah Basin, Utah
The Uintah Basin in northeastern Utah, a region of intense oil and gas extraction, experienced ozone (O3) concentrations above levels harmful to human health for multiple days during the winters of 2009–2010 and 2010–2011. These wintertime O3 pollution episodes occur during cold, stable periods when the ground is snow-covered, and have been linked to emissions from the oil and gas extraction process. The Uintah Basin Winter Ozone Study (UBWOS) was a field intensive in early 2012, whose goal was to address current uncertainties in the chemical and physical processes that drive wintertime O3 production in regions of oil and gas development. Although elevated O3 concentrations were not observed during the winter of 2011–2012, the comprehensive set of observations tests our understanding of O3 photochemistry in this unusual emissions environment. A box model, constrained to the observations and using the near-explicit Master Chemical Mechanism (MCM) v3.2 chemistry scheme, has been used to investigate the sensitivities of O3 production during UBWOS 2012. Simulations identify the O3 production photochemistry to be highly radical limited (with a radical production rate significantly smaller than the NOx emission rate). Production of OH from O3 photolysis (through reaction of O(1D) with water vapor) contributed only 170 pptv day−1, 8% of the total primary radical source on average (primary radicals being those produced from non-radical precursors). Other radical sources, including the photolysis of formaldehyde (HCHO, 52%), nitrous acid (HONO, 26%), and nitryl chloride (ClNO2, 13%) were larger. O3 production was also found to be highly sensitive to aromatic volatile organic compound (VOC) concentrations, due to radical amplification reactions in the oxidation scheme of these species. Radical production was shown to be small in comparison to the emissions of nitrogen oxides (NOx), such that NOx acted as the primary radical sink. Consequently, the system was highly VOC sensitive, despite the much larger mixing ratio of total non-methane hydrocarbons (230 ppbv (2080 ppbC), 6 week average) relative to NOx (5.6 ppbv average). However, the importance of radical sources which are themselves derived from NOx emissions and chemistry, such as ClNO2 and HONO, make the response of the system to changes in NOx emissions uncertain. Model simulations attempting to reproduce conditions expected during snow-covered cold-pool conditions show a significant increase in O3 production, although calculated concentrations do not achieve the highest seen during the 2010–2011 O3 pollution events in the Uintah Basin. These box model simulations provide useful insight into the chemistry controlling winter O3 production in regions of oil and gas extraction
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Protein-coding variants implicate novel genes related to lipid homeostasis contributing to body-fat distribution.
Body-fat distribution is a risk factor for adverse cardiovascular health consequences. We analyzed the association of body-fat distribution, assessed by waist-to-hip ratio adjusted for body mass index, with 228,985 predicted coding and splice site variants available on exome arrays in up to 344,369 individuals from five major ancestries (discovery) and 132,177 European-ancestry individuals (validation). We identified 15 common (minor allele frequency, MAF ≥5%) and nine low-frequency or rare (MAF <5%) coding novel variants. Pathway/gene set enrichment analyses identified lipid particle, adiponectin, abnormal white adipose tissue physiology and bone development and morphology as important contributors to fat distribution, while cross-trait associations highlight cardiometabolic traits. In functional follow-up analyses, specifically in Drosophila RNAi-knockdowns, we observed a significant increase in the total body triglyceride levels for two genes (DNAH10 and PLXND1). We implicate novel genes in fat distribution, stressing the importance of interrogating low-frequency and protein-coding variants
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