1,998 research outputs found
Exact calculation of Fourier series in nonconforming spectral-element methods
In this note is presented a method, given nodal values on multidimensional
nonconforming spectral elements, for calculating global Fourier-series
coefficients. This method is ``exact'' in that given the approximation inherent
in the spectral-element method (SEM), no further approximation is introduced
that exceeds computer round-off error. The method is very useful when the SEM
has yielded an adaptive-mesh representation of a spatial function whose global
Fourier spectrum must be examined, e.g., in dynamically adaptive fluid-dynamics
simulations.Comment: 7 pages, 4 figures, submitted to J. Comp. Phys. 2005 June
Discrete Hubbard-Stratonovich transformations for systems with orbital degeneracy
A discrete Hubbard-Stratonovich transformation is presented for systems with
an orbital degeneracy and a Hubbard Coulomb interaction without multiplet
effects. An exact transformation is obtained by introducing an external field
which takes values. Alternative approximate transformations are
presented, where the field takes fewer values, for instance two values
corresponding to an Ising spin.Comment: 4 pages, revtex, 1 eps figure, additional material avalable at
http://librix.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
The matrix rate of return
In this paper we give definitions of matrix rates of return which do not
depend on the choice of basis describing baskets. We give their economic
interpretation. The matrix rate of return describes baskets of arbitrary type
and extends portfolio analysis to the complex variable domain. This allows us
for simultaneous analysis of evolution of baskets parameterized by complex
variables in both continuous and discrete time models.Comment: APFA5 Conference, Torino, 200
A New 3D Potential-Density Basis Set
A set of bi-orthogonal potential-density basis functions is introduced to
model the density and its associated gravitational field of three dimensional
stellar systems. Radial components of our basis functions are weighted integral
forms of spherical Bessel functions. We discuss the properties of our basis
functions and demonstrate their shapes for the latitudinal Fourier number
.Comment: 2 pages. To appear in the proceedings of IAU Symposium 245,
"Formation and Evolution of Galaxy Bulges," M. Bureau, E. Athanassoula, and
B. Barbuy, ed
The Eastern Oyster Microbiome and its Implications in the Marine Nitrogen Cycle
Microbial communities associated with a particular space or habitat, or microbiomes, play significant roles in host health and the regulation of biogeochemical cycles. In oysters these microbiomes may be important contributors in the removal of biologically available nitrogen (N) from the coastal and marine environment through the process of denitrification. Denitrification is the microbially mediated step-wise reduction of nitrate (NO3-) or nitrite (NO2-) to N2 gas. Excess nitrogen in the Chesapeake Bay has been implicated in the increase of eutrophication and other detrimental effects including harmful algal blooms, hypoxia, and loss of benthic communities. Oyster reefs have been shown to enhance the rates of denitrification in nearby sediments, but little is known about the oyster microbiomes or associated microbes responsible for denitrification (denitrifiers). Furthermore, the identification of the oyster core microbiome, or set of resident microbes continually present in the oyster, is relatively unknown. Assessing the stable underlying core is necessary to evaluate and predict the effect of varying environmental conditions on the oyster microbiome and oyster denitrification. A combined 16S targeted metagenomic and metabolic inference approach was used in this study to investigate the gill, gut and shell microbiomes of the eastern oyster (Crassostrea virginica) and their associated denitrifiers in response to spatial and temporal changes. Denitrification activity was linked to community structure using methods such as quantitative PCR of nitrous oxide reductase genes (nosZ) and 15N isotope pairing technique with experimental flow-through design. The oyster gill, gut, and shell microbiomes all showed distinct and unique core microbiomes, suggesting an importance of the core to oyster function or health. Denitrifier abundance and activities were most consistent in the shell microbiomes indicating a stable, pool of potential denitrifiers for oyster denitrification. In comparison, oyster gill and gut denitrifier abundances and activities were highly variable and likely related to transient denitrifiers ingested with food particles. Additionally, denitrifiers demonstrated niche differentiation between the different oyster microbiomes, indicating different groups of denitrifiers are responsible for performing denitrification in the oyster. Assessing the stability and variability of the oyster microbiome and associated denitrifiers provides a greater understanding of the oyster’s role in denitrification and the mitigation of excess N in marine and coastal environments
A simple electrostatic model applicable to biomolecular recognition
An exact, analytic solution for a simple electrostatic model applicable to
biomolecular recognition is presented. In the model, a layer of high dielectric
constant material (representative of the solvent, water) whose thickness may
vary separates two regions of low dielectric constant material (representative
of proteins, DNA, RNA, or similar materials), in each of which is embedded a
point charge. For identical charges, the presence of the screening layer always
lowers the energy compared to the case of point charges in an infinite medium
of low dielectric constant. Somewhat surprisingly, the presence of a
sufficiently thick screening layer also lowers the energy compared to the case
of point charges in an infinite medium of high dielectric constant. For charges
of opposite sign, the screening layer always lowers the energy compared to the
case of point charges in an infinite medium of either high or low dielectric
constant. The behavior of the energy leads to a substantially increased
repulsive force between charges of the same sign. The repulsive force between
charges of opposite signs is weaker than in an infinite medium of low
dielectric constant material but stronger than in an infinite medium of high
dielectric constant material. The presence of this behavior, which we name
asymmetric screening, in the simple system presented here confirms the
generality of the behavior that was established in a more complicated system of
an arbitrary number of charged dielectric spheres in an infinite solvent.Comment: 15 pages, 6 figure
Langevin equation with Coulomb friction
We propose a Langevin model with Coulomb friction. Through the analysis of
the corresponding Fokker-Planck equation, we have obtained the steady velocity
distribution function under the influence of the external field.Comment: 10 pages, 1 figure. to be published in Physica
Theory and Simulations of Whistler Wave Propagation
A linear theory of whistler wave is developed wihtin the paradigm of a two
dimensional incompressible electron magnetohydrodynamics model. Exact analytic
wave solutions are obtained for a small amplitude whistler wave that exhibit
magnetic field topological structures consistent with the observations and our
simulations in linear regime. In agreement with experiment, we find that the
parallel group velocity of the wave is large compared to its perpendicular
counterpart. Numerical simulations of collisional interactions demonstrate that
the wave magnetic field either coalesces or repels depending upon the polarity
of the associated current. In the nonlinear regime, our simulations demonstrate
that the evolution of wave magnetic field is governed essentially by the
nonlinear Hall force
Relation between energy shifts and relaxation rates for a small system coupled to a reservoir
For a small system the coupling to a reservoir causes energy shifts as well
as transitions between the system's energy levels. We show for a general
stationary situation that the energy shifts can essentially be reduced to the
relaxation rates. The effects of reservoir fluctuations and self reaction are
treated separately. We apply the results to a two-level atom coupled to a
reservoir which may be the vacuum of a radiation field.Comment: 6 pages, Latex, to appear in Phys. Lett.
Time Double-Slit Interference in Tunneling Ionization
We show that interference phenomena plays a big role for the electron yield
in ionization of atoms by an ultra-short laser pulse. Our theoretical study of
single ionization of atoms driven by few-cycles pulses extends the
photoelectron spectrum observed in the double-slit experiment by Lindner et al,
Phys. Rev. Lett. \textbf{95}, 040401 (2005) to a complete three-dimensional
momentum picture. We show that different wave packets corresponding to the same
single electron released at different times interfere, forming interference
fringes in the two-dimensional momentum distributions. These structures
reproduced by means of \textit{ab initio} calculations are understood within a
semiclassical model.Comment: 7 pages, 5 figure
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