133,448 research outputs found
A technique for accelerating iterative convergence in numerical integration, with application in transonic aerodynamics
A technique is described for the efficient numerical solution of nonlinear partial differential equations by rapid iteration. In particular, a special approach is described for applying the Aitken acceleration formula (a simple Pade approximant) for accelerating the iterative convergence. The method finds the most appropriate successive approximations, which are in a most nearly geometric sequence, for use in the Aitken formula. Simple examples are given to illustrate the use of the method. The method is then applied to the mixed elliptic-hyperbolic problem of steady, inviscid, transonic flow over an airfoil in a subsonic free stream
Library of high-resolution UES echelle spectra of F, G, K and M field dwarf stars
We present a library of Utrecht echelle spectrograph (UES) observations of a
sample of F, G, K and M field dwarf stars covering the spectral range from 4800
AA to 10600 AA with a resolution of 55000. These spectra include some of the
spectral lines most widely used as optical and near-infrared indicators of
chromospheric activity such as H_beta, Mg I b triplet, Na I D_1, D_2, He I D_3,
H_alpha, and Ca II IRT lines, as well as a large number of photospheric lines
which can also be affected by chromospheric activity. The spectra have been
compiled with the aim of providing a set of standards observed at
high-resolution to be used in the application of the spectral subtraction
technique to obtain the active-chromosphere contribution to these lines in
chromospherically active single and binary stars. This library can also be used
for spectral classification purposes. A digital version with all the spectra is
available via ftp and the World Wide Web (WWW) in both ASCII and FITS formats.Comment: Latex file with 5 figures using l-aa.sty and psfig.sty. Full
postscript (text and figures) available at
http://www.ucm.es/OTROS/Astrof/fgkmsl/UESfgkmsl.html Accepted for publication
in: Astronomy and Astrophysics Supplement
Learning to Understand by Evolving Theories
In this paper, we describe an approach that enables an autonomous system to
infer the semantics of a command (i.e. a symbol sequence representing an
action) in terms of the relations between changes in the observations and the
action instances. We present a method of how to induce a theory (i.e. a
semantic description) of the meaning of a command in terms of a minimal set of
background knowledge. The only thing we have is a sequence of observations from
which we extract what kinds of effects were caused by performing the command.
This way, we yield a description of the semantics of the action and, hence, a
definition.Comment: KRR Workshop at ICLP 201
Toward a theory of the integer quantum Hall transition: continuum limit of the Chalker-Coddington model
An N-channel generalization of the network model of Chalker and Coddington is
considered. The model for N = 1 is known to describe the critical behavior at
the plateau transition in systems exhibiting the integer quantum Hall effect.
Using a recently discovered equality of integrals, the network model is
transformed into a lattice field theory defined over Efetov's sigma model space
with unitary symmetry. The transformation is exact for all N, no saddle-point
approximation is made, and no massive modes have to be eliminated. The naive
continuum limit of the lattice theory is shown to be a supersymmetric version
of Pruisken's nonlinear sigma model with couplings sigma_xx = sigma_xy = N/2 at
the symmetric point. It follows that the model for N = 2, which describes a
spin degenerate Landau level and the random flux problem, is noncritical. On
the basis of symmetry considerations and inspection of the Hamiltonian limit, a
modified network model is formulated, which still lies in the quantum Hall
universality class. The prospects for deformation to a Yang-Baxter integrable
vertex model are briefly discussed.Comment: 25 pages, REVTEX, calculation of sigma_xx correcte
Coherent macroscopic quantum tunneling in boson-fermion mixtures
We show that the cold atom systems of simultaneously trapped Bose-Einstein
condensates (BEC's) and quantum degenerate fermionic atoms provide promising
laboratories for the study of macroscopic quantum tunneling. Our theoretical
studies reveal that the spatial extent of a small trapped BEC immersed in a
Fermi sea can tunnel and coherently oscillate between the values of the
separated and mixed configurations (the phases of the phase separation
transition of BEC-fermion systems). We evaluate the period, amplitude and
dissipation rate for Na and K-atoms and we discuss the
experimental prospects for observing this phenomenon.Comment: 4 pages, 3 figure
Seawater strontium isotopes at the Cretaceous-Tertiary boundary
Anomalously high values of Seawater Sr-87/Sr-86 near the Cretaceous-Tertiary (K-T) boundary have been reported. However, few of the data from the literature are from a single continuous section, and perhaps the most complete study of the boundary region, from a shallow marine limestone sequence in Alabama, showed elevated Sr-87/Sr-86 but no pronounced spike. Thus, in order to investigate the cause of the change in strontium isotopic composition, it is important to determine the exact nature and magnitude of the increase by studying in detail continuous sections through the boundary. If there is indeed a Sr isotope spike at the K-T boundary, it requires the addition of a large amount of radiogenic Sr to the oceans over a short time period, a phenomenon that may be linked to other large-scale environmental disturbances which occurred at that time. In order to address this question, a high-resolution strontium isotope study of foraminifera from three Deep Sea Drilling Project (DSDP) cores which recovered the K-T boundary section: Site 356 in the South Atlantic, Site 384 in the North Atlantic and Site 577 from the Shatsky Rise in the Pacific was initiated. The isotope measurements are being made on either single or small numbers of forams carefully picked and identified and in most cases examined by SEM before analysis. Because this work is not yet complete, conclusions drawn here must be viewed as tentative. They are briefly discussed
ANALYSIS OF A SPATIAL ROTATION PLAN FOR THE TULE LAKE NATIONAL WILDLIFE REFUGE
This paper examines the joint agro-wildfowl regulation of the Tule Lake National Wildlife Refuge in California. The area is jointly managed by the Bureau of Reclamation for both farming and wildfowl benefits. Production in both sectors has been declining recently, in farming due to nematode and soil pathogen buildup and in wildfowl production due to climax vegetation choking the lake. A novel spatial rotation plan has surfaced to solve both problems. We develop a simple model of the rotation option to identify critical variables and then we estimate some of these using data on lease bids.Resource /Energy Economics and Policy,
MARINE RESERVES WITH ENDOGENOUS PORTS: EMPIRICAL BIOECONOMICS OF THE CALIFORNIA SEA URCHIN FISHERY
Marine reserves are gaining substantial public support as tools for commercial fisheries management Harvest sector responses will influence policy performance, yet biological studies often depict harvester behavior as spread uniformly over fishing grounds and unresponsive to economic opportunities. Previous bioeconomic analyses show that these behavioral assumptions are inconsistent with empirical data and, more importantly, lead to overly optimistic predictions about harvest gains from reserves. This paper adds another layer of behavioral realism to the bioeconomics of marine reserves by endogenizing fisher home port choices with a partial adjustment share model. Estimated with Seemingly Unrelated Regression over monthly data, this approach allows simulation of both short- and long-run behavioral response to changes induced by marine reserve formation. The findings cast further doubt on the notion that marine reserves generate long-run harvest benefits.Resource /Energy Economics and Policy,
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