8,303 research outputs found
On implicit-factorization constraint preconditioners
Recently Dollar and Wathen [14] proposed a class of incomplete factorizations for saddle-point problems, based upon earlier work by Schilders [40]. In this paper, we generalize this class of preconditioners, and examine the spectral implications of our approach. Numerical tests indicate the efficacy of our preconditioners
Using constraint preconditioners with regularized saddle-point problems
The problem of finding good preconditioners for the numerical solution of a certain important class of indefinite linear systems is considered. These systems are of a 2 by 2 block (KKT) structure in which the (2,2) block (denoted by -C) is assumed to be nonzero. In Constraint preconditioning for indefinite linear systems , SIAM J. Matrix Anal. Appl., 21 (2000), Keller, Gould and Wathen introduced the idea of using constraint preconditioners that have a specific 2 by 2 block structure for the case of C being zero. We shall give results concerning the spectrum and form of the eigenvectors when a preconditioner of the form considered by Keller, Gould and Wathen is used but the system we wish to solve may have C \neq 0 . In particular, the results presented here indicate clustering of eigenvalues and, hence, faster convergence of Krylov subspace iterative methods when the entries of C are small; such situations arise naturally in interior point methods for optimization and we present results for such problems which validate our conclusions.\ud
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The first author's work was supported by the OUCL Doctorial Training Accoun
A Bramble-Pasciak-like method with applications in optimization
Saddle-point systems arise in many applications areas, in fact in any situation where an extremum principle arises with constraints. The Stokes problem describing slow viscous flow of an incompressible fluid is a classic example coming from partial differential equations and in the area of Optimization such problems are ubiquitous.\ud
In this manuscript we show how new approaches for the solution of saddle-point systems arising in Optimization can be derived from the Bramble-Pasciak Conjugate Gradient approach widely used in PDEs and more recent generalizations thereof. In particular we derive a class of new solution methods based on the use of Preconditioned Conjugate Gradients in non-standard inner products and demonstrate how these can be understood through more standard machinery. We show connections to Constraint Preconditioning and give the results of numerical computations on a number of standard Optimization test examples
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Replication enhancer elements within the open reading frame of tick-borne encephalitis virus and their evolution within the Flavivirus genus
We provide experimental evidence of a replication enhancer element (REE) within the capsid gene of tick-borne encephalitis virus (TBEV, genus Flavivirus). Thermodynamic and phylogenetic analyses predicted that the REE folds as a long stable stem–loop (designated SL6), conserved among all tick-borne flaviviruses (TBFV). Homologous sequences and potential base pairing were found in the corresponding regions of mosquito-borne flaviviruses, but not in more genetically distant flaviviruses. To investigate the role
of SL6, nucleotide substitutions were introduced which changed a conserved hexanucleotide motif, the conformation of the terminal loop and the base-paired dsRNA stacking. Substitutions were made within a TBEV reverse genetic system and recovered mutants were compared for plaque
morphology, single-step replication kinetics and cytopathic effect. The greatest phenotypic changes were observed in mutants with a destabilized stem. Point mutations in the conserved hexanucleotide motif of the terminal loop caused
moderate virus attenuation. However, all mutants
eventually reached the titre of wild-type virus late post-infection. Thus, although not essential for growth in tissue culture, the SL6 REE acts to up-regulate virus replication. We hypothesize that this modulatory role may be important for TBEV survival in nature, where the virus circulates by non-viraemic transmission between infected and
non-infected ticks, during co-feeding on local rodents
Chern-Simons number diffusion in (1+1)-dimensional Higgs theory
We study the Chern-Simons number diffusion rate in the (1+1)-dimensional
latticeAbelian Higgs model at temperatures much higher than, as well as
comparable to, the sphaleron energy. It is found that in the high-temperature
limit the rate is likely to grow as power 2/3 of the temperature. In the
intermediate-temperature regime, our numerical simulations show that very weak
temperature dependence of the rate, found in previous work, persists at smaller
lattice spacings. We discuss possibilities of relating the observed behavior of
the rate to static finite-temperature properties of the model.Comment: 9 pages, LATeX + 4 figures included as postscript files, to be
encapsulated using epsf. Text + figures uuencoded. Also available as a
compressed postscript file by anonymous ftp from maggia.ethz.ch (login ftp,
pw ftp; then: cd pub, binary, get ahm.ps.Z). IPS Research Report No. 94-0
Anomaluos RR Lyrae (V-I)_0 colors in Baade's Window
We compare (V-I)_0-(V-K)_0 color-color and (V-I)_0-log P period-color
diagrams for Baade's Window and local RRab Lyrae stars. We find that for a
fixed log P the Baade's Window RR Lyrae stars are ~0.17 magnitudes redder in
(V-I)_0 than the local RR Lyrae stars. We also show that there is no such
effect observed in (V-K)_0. We argue that an extinction misestimate towards
Baade's Window is not a plausible explanation of the discrepancy. Unlike
Baade's Window RR Lyrae stars, the local ones follow a black-body color-color
relation and are well approximated by theoretical models. We test two
parameters, metallicity and surface gravity, and find that their effects are
too small to explain the (V-I)_0 discrepancy between the two groups of stars.
We do not provide any explanation for the anomalous (V-I)_0 behavior of the
Baade's Window RR Lyrae stars. We note that a similar effect for clump giant
stars has been recently reported by Paczynski and we caution that RR Lyrae
stars and clump giants, often used as standard candles, can be subject to the
same type of systematics.Comment: 10 pages, 7 figures, submitted to Ap
Early-type Stars: Most Favorable Targets for Astrometrically Detectable Planets in the Habitable Zone
Early-type stars appear to be a difficult place to look for planets
astrometrically. First, they are relatively heavy, and for fixed planetary mass
the astrometric signal falls inversely as the stellar mass. Second, they are
relatively rare (and so tend to be more distant), and for fixed orbital
separation the astrometric signal falls inversely as the distance.
Nevertheless, because early-type stars are relatively more luminous, their
habitable zones are at larger semi-major axis. Since astrometric signal scales
directly as orbital size, this gives early-type stars a strong advantage, which
more than compensates for the other two factors. Using the Hipparcos catalog,
we show that early-type stars constitute the majority of viable targets for
astrometric searches for planets in the habitable zone. We contrast this
characteristic to transit searches, which are primarily sensitive to habitable
planets around late-type stars.Comment: Submitted to ApJ Letters, 8 pages including 2 figure
The relationship between isometric mid-thigh pull force-time characteristics and swing performance in high-level youth golfers
The production of vertical ground reaction force has been suggested to relate
directly to club head speed (CHS) in golfers, providing a rationale for the
implementation of strength and conditioning to enhance performance. The aim
of the study was to determine the relationship between isometric mid-thigh pull
(IMTP) force-time characteristics and measures of swing performance (CHS and
Carry) in high-level youth golfers. Thirteen high-level youth golfers selected for
their National Squad performed IMTP and swing testing using a TrackMan
launch monitor across two testing sessions. Results revealed significant
correlations between IMTP Peak Force (PF) and 6-iron Carry (r=0.91, p<0.001),
Driver Carry (r=0.91, p<0.001), 6-iron CHS (r=0.89, p<0.001) and Driver CHS
(r=0.88, p<0.001). All other variables showed trivial to large non-significant
correlations. Findings support the use of the IMTP as a testing tool in high-level
youth golfers and emphasise the importance of strength training within this
population
The Star Blended with the MOA-2008-BLG-310 Source Is Not the Exoplanet Host Star
High resolution Hubble Space Telescope (HST) image analysis of the
MOA-2008-BLG-310 microlens system indicates that the excess flux at the
location of the source found in the discovery paper cannot primarily be due to
the lens star because it does not match the lens-source relative proper motion,
, predicted by the microlens models. This excess flux is most
likely to be due to an unrelated star that happens to be located in close
proximity to the source star. Two epochs of HST observations indicate proper
motion for this blend star that is typical of a random bulge star, but is not
consistent with a companion to the source or lens stars if the flux is
dominated by only one star, aside from the lens. We consider models in which
the excess flux is due to a combination of an unrelated star and the lens star,
and this yields 95\% confidence level upper limit on the lens star brightness
of and . A Bayesian analysis using a standard
Galactic model and these magnitude limits yields a host star mass , a planet mass of at a projected separation of AU. This result illustrates excess flux in a high
resolution image of a microlens-source system need not be due to the lens. It
is important to check that the lens-source relative proper motion is consistent
with the microlensing prediction. The high resolution image analysis techniques
developed in this paper can be used to verify the WFIRST exoplanet microlensing
survey mass measurements.Comment: Submitted to AJ on March 18, 201
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