3,120 research outputs found
Efficiently Computing Minimal Sets of Critical Pairs
In the computation of a Gr"obner basis using Buchberger's algorithm, a key
issue for improving the efficiency is to produce techniques for avoiding as
many unnecessary critical pairs as possible. A good solution would be to avoid
_all_ non-minimal critical pairs, and hence to process only a_minimal_ set of
generators of the module generated by the critical syzygies. In this paper we
show how to obtain that desired solution in the homogeneous case while
retaining the same efficiency as with the classical implementation. As a
consequence, we get a new Optimized Buchberger Algorithm.Comment: LaTeX using elsart.cls, 27 page
Physisorption of an electron in deep surface potentials off a dielectric surface
We study phonon-mediated adsorption and desorption of an electron at
dielectric surfaces with deep polarization-induced surface potentials where
multi-phonon transitions are responsible for electron energy relaxation.
Focusing on multi-phonon processes due to the nonlinearity of the coupling
between the external electron and the acoustic bulk phonon triggering the
transitions between surface states, we calculate electron desorption times for
graphite, MgO, CaO, (\text{Al}_2\text{O}_3), and (\text{SiO}_2) and electron
sticking coefficients for (\text{Al}_2\text{O}_3), CaO, and (\text{SiO}_2). To
reveal the kinetic stages of electron physisorption, we moreover study the time
evolution of the image state occupancy and the energy-resolved desorption flux.
Depending on the potential depth and the surface temperature we identify two
generic scenarios: (i)adsorption via trapping in shallow image states followed
by relaxation to the lowest image state and desorption from that state via a
cascade through the second strongly bound image state in not too deep
potentials and (ii)adsorption via trapping in shallow image states but followed
by a relaxation bottleneck retarding the transition to the lowest image state
and desorption from that state via a one step process to the continuum in deep
potentials.Comment: 12 pages, 7 figure
Nonequilibrium fluctuation-dissipation relations for one- and two-particle correlation functions in steady-state quantum transport
We study the non-equilibrium (NE) fluctuation-dissipation (FD) relations in
the context of quantum thermoelectric transport through a two-terminal
nanodevice in the steady-state. The FD relations for the one- and two-particle
correlation functions are derived for a model of the central region consisting
of a single electron level. Explicit expressions for the FD relations of the
Green's functions (one-particle correlations) are provided. The FD relations
for the current-current and charge-charge (two-particle) correlations are
calculated numerically. We use self-consistent NE Green's functions
calculations to treat the system in the absence and in the presence of
interaction (electron-phonon) in the central region. We show that, for this
model, there is no single universal FD theorem for the NE steady state. There
are different FD relations for each different class of problems. We find that
the FD relations for the one-particle correlation function are strongly
dependent on both the NE conditions and the interactions, while the FD
relations of the current-current correlation function are much less dependent
on the interaction. The latter property suggests interesting applications for
single-molecule and other nanoscale transport experiments.Comment: This revised version is now accepted for publication in the Journal
of Chemical Physics (March 2014). arXiv admin note: text overlap with
arXiv:1305.507
Three particles in a finite volume: The breakdown of spherical symmetry
Lattice simulations of light nuclei necessarily take place in finite volumes,
thus affecting their infrared properties. These effects can be addressed in a
model-independent manner using Effective Field Theories. We study the model
case of three identical bosons (mass m) with resonant two-body interactions in
a cubic box with periodic boundary conditions, which can also be generalized to
the three-nucleon system in a straightforward manner. Our results allow for the
removal of finite volume effects from lattice results as well as the
determination of infinite volume scattering parameters from the volume
dependence of the spectrum. We study the volume dependence of several states
below the break-up threshold, spanning one order of magnitude in the binding
energy in the infinite volume, for box side lengths L between the two-body
scattering length a and L = 0.25a. For example, a state with a three-body
energy of -3/(ma^2) in the infinite volume has been shifted to -10/(ma^2) at L
= a. Special emphasis is put on the consequences of the breakdown of spherical
symmetry and several ways to perturbatively treat the ensuing partial wave
admixtures. We find their contributions to be on the sub-percent level compared
to the strong volume dependence of the S-wave component. For shallow bound
states, we find a transition to boson-diboson scattering behavior when
decreasing the size of the finite volume.Comment: 21 pages, 4 figures, 2 table
The status of the quantum dissipation-fluctuation relation and Langevin equation
I examine the arguments which have been given for quantum
fluctuation-dissipation theorems. I distinguish between a weak form of the
theorem, which is true under rather general conditions, and a strong form which
requires a Langevin equation for its statement. I argue that the latter has not
been reliably derived.Comment: 9 page
Fano hypersurfaces and Calabi-Yau supermanifolds
In this paper, we study the geometrical interpretations associated with
Sethi's proposed general correspondence between N = 2 Landau-Ginzburg orbifolds
with integral \hat{c} and N = 2 nonlinear sigma models. We focus on the
supervarieties associated with \hat{c} = 3 Gepner models. In the process, we
test a conjecture regarding the superdimension of the singular locus of these
supervarieties. The supervarieties are defined by a hypersurface \widetilde{W}
= 0 in a weighted superprojective space and have vanishing super-first Chern
class. Here, \widetilde{W} is the modified superpotential obtained by adding as
necessary to the Gepner superpotential a boson mass term and/or fermion
bilinears so that the superdimension of the supervariety is equal to \hat{c}.
When Sethi's proposal calls for adding fermion bilinears, setting the bosonic
part of \widetilde{W} (denoted by \widetilde{W}_{bos}) equal to zero defines a
Fano hypersurface embedded in a weighted projective space. In this case, if the
Newton polytope of \widetilde{W}_{bos} admits a nef partition, then the
Landau-Ginzburg orbifold can be given a geometrical interpretation as a
nonlinear sigma model on a complete intersection Calabi-Yau manifold. The
complete intersection Calabi-Yau manifold should be equivalent to the
Calabi-Yau supermanifold prescribed by Sethi's proposal.Comment: 24 pages, uses JHEP3.cls; v2: minor corrections, references adde
Ruminal methane inhibition potential of various pure compounds in comparison with garlic oil as determined with a rumen simulation technique (Rusitec)
Ruminants represent an important source of methane (CH4) emissions; therefore, CH4 mitigation by diet supplementation is a major goal in the current ruminant research. The objective of the present study was to use a rumen simulation technique to evaluate the CH4-mitigating potential of pure compounds in comparison with that achieved with garlic oil, a known anti-methanogenic supplement. A basal diet (15g DM/d) consisting of ryegrass hay, barley and soyabean meal (1:0·7:0·3) was incubated with the following additives: none (negative control); garlic oil (300mg/l incubation liquid; positive control); allyl isothiocyanate (75mg/l); lovastatin (150mg/l); chenodeoxycholic acid (150mg/l); 3-azido-propionic acid ethyl ester (APEE, 150mg/l); levulinic acid (300mg/l); 4-[(pyridin-2-ylmethyl)-amino]-benzoic acid (PABA, 300mg/l). Fermentation profiles (SCFA, microbial counts and N turnover) and H2 and CH4 formation were determined. Garlic oil, allyl isothiocyanate, lovastatin and the synthetic compound APEE decreased the absolute daily CH4 formation by 91, 59, 42 and 98%, respectively. The corresponding declines in CH4 emitted per mmol of SCFA were 87, 32, 40 and 99%, respectively, compared with the negative control; the total SCFA concentration was unaffected. Garlic oil decreased protozoal numbers and increased bacterial counts, while chenodeoxycholic acid completely defaunated the incubation liquid. In vitro, neutral-detergent fibre disappearance was lower following chenodeoxycholic acid and PABA treatments (−26 and −18%, respectively). In conclusion, garlic oil and APEE were extremely efficient at mitigating CH4 without noticeably impairing microbial nutrient fermentation. Other promising substances were allyl isothiocyanate and lovastati
Lung cancer in lifetime nonsmoking men – results of a case-control study in Germany
Epidemiological studies of lung cancer among nonsmoking men are few. This case–control study was conducted among lifetime nonsmoking men between 1990 and 1996 in Germany to examine lung cancer risk in relation to occupation, environmental tobacco smoke, residential radon, family history of cancer and previous lung disease. A total of 58 male cases with confirmed primary lung cancer and 803 male population controls who had never smoked more than 400 cigarettes in their lifetime were personally interviewed by a standardized questionnaire. In addition, 1-year radon measurements in the living and bedroom of the subjects' last dwelling were carried out. Unconditional logistic regression was used to calculate odds ratios (OR) and 95% confidence intervals (CI). Having ever worked in a job with known lung carcinogens was associated with a two-fold significantly increased lung cancer risk (OR = 2.2; Cl = 1.0–5.0), adjusted for age and region. The linear trend test for lung-cancer risk associated with radon exposure was close to statistical significance, demonstrating an excess relative risk for an increase in exposure of 100 Bq m−3 of 0.43 (P = 0.052). Nonsignificantly elevated effects of exposure to environmental tobacco smoke in public transportation and in social settings were observed. No associations with a family history of cancer or previous lung diseases were found. Our results indicate that occupational carcinogens and indoor radon may play a role in some lung cancers in nonsmoking men. © 2001 Cancer Research Campaign http://www.bjcancer.co
Statistical mechanical theory of an oscillating isolated system. The relaxation to equilibrium
In this contribution we show that a suitably defined nonequilibrium entropy
of an N-body isolated system is not a constant of the motion in general and its
variation is bounded, the bounds determined by the thermodynamic entropy, i.e.,
the equilibrium entropy. We define the nonequilibrium entropy as a convex
functional of the set of n-particle reduced distribution functions
(n=0,......., N) generalizing the Gibbs fine-grained entropy formula.
Additionally, as a consequence of our microscopic analysis we find that this
nonequilibrium entropy behaves as a free entropic oscillator. In the approach
to the equilibrium regime we find relaxation equations of the Fokker-Planck
type, particularly for the one-particle distribution function
Challenges and insights in the exploration of the low abundance human ocular surface microbiome.
PURPOSE
The low microbial abundance on the ocular surface results in challenges in the characterization of its microbiome. The purpose of this study was to reveal factors introducing bias in the pipeline from sample collection to data analysis of low-abundant microbiomes.
METHODS
Lower conjunctiva and lower lid swabs were collected from six participants using either standard cotton or flocked nylon swabs. Microbial DNA was isolated with two different kits (with or without prior host DNA depletion and mechanical lysis), followed by whole-metagenome shotgun sequencing with a high sequencing depth set at 60 million reads per sample. The relative microbial compositions were generated using the two different tools MetaPhlan3 and Kraken2.
RESULTS
The total amount of extracted DNA was increased by using nylon flocked swabs on the lower conjunctiva. In total, 269 microbial species were detected. The most abundant bacterial phyla were Actinobacteria, Firmicutes and Proteobacteria. Depending on the DNA extraction kit and tool used for profiling, the microbial composition and the relative abundance of viruses varied.
CONCLUSION
The microbial composition on the ocular surface is not dependent on the swab type, but on the DNA extraction method and profiling tool. These factors have to be considered in further studies about the ocular surface microbiome and other sparsely colonized microbiomes in order to improve data reproducibility. Understanding challenges and biases in the characterization of the ocular surface microbiome may set the basis for microbiome-altering interventions for treatment of ocular surface associated diseases
- …