1,169 research outputs found
Improving Inversions of the Overlap Operator
We present relaxation and preconditioning techniques which accelerate the
inversion of the overlap operator by a factor of four on small lattices, with
larger gains as the lattice size increases. These improvements can be used in
both propagator calculations and dynamical simulations.Comment: lattice2004(machines
Numerical Methods for the QCD Overlap Operator: I. Sign-Function and Error Bounds
The numerical and computational aspects of the overlap formalism in lattice
quantum chromodynamics are extremely demanding due to a matrix-vector product
that involves the sign function of the hermitian Wilson matrix. In this paper
we investigate several methods to compute the product of the matrix
sign-function with a vector, in particular Lanczos based methods and partial
fraction expansion methods. Our goal is two-fold: we give realistic comparisons
between known methods together with novel approaches and we present error
bounds which allow to guarantee a given accuracy when terminating the Lanczos
method and the multishift-CG solver, applied within the partial fraction
expansion methods.Comment: 30 pages, 2 figure
A comparative study of numerical methods for the overlap Dirac operator--a status report
Improvements of various methods to compute the sign function of the hermitian
Wilson-Dirac matrix within the overlap operator are presented. An optimal
partial fraction expansion (PFE) based on a theorem of Zolotarev is given.
Benchmarks show that this PFE together with removal of converged systems within
a multi-shift CG appears to approximate the sign function times a vector most
efficiently. A posteriori error bounds are given.Comment: 3 pages, poster contribution to Lattice2001(algorithms
A Mutation in Amino Acid Permease AAP6 Reduces the Amino Acid Content of the Arabidopsis Sieve Elements but Leaves Aphid Herbivores Unaffected.
The aim of this study was to investigate the role of the amino acid permease gene AAP6 in regulating phloem amino
acid composition and then to determine the effects of this altered diet on aphid performance. A genotype of
Arabidopsis thaliana (L.) was produced in which the function of the amino acid permease gene AAP6 (At5g49630)
was abolished. Plants homozygous for the insertionally inactivated AAP6 gene had a significantly larger mean
rosette width than the wild type and a greater number of cauline leaves. Seeds from the aap6 mutant were also
significantly larger than those from the wild-type plants. Sieve element (SE) sap was collected by aphid stylectomy
and the amino acids derivatized, separated, and quantified using Capillary Electrophoresis with Laser Induced
Fluorescence (CE-LIF). In spite of the large variation across samples, the total amino acid concentration of SE sap of the aap6 mutant plants was significantly lower than that of the wild-type plants. The concentrations of lysine,
phenylalanine, leucine, and aspartic acid were all significantly lower in concentration in the aap6 mutant plants compared with wild-type plants. This is the first direct demonstration of a physiological role for an amino acid transporter in regulating SE composition in vivo. The amino acid availability in sieve element sap is thought to be the major limiting factor for aphid growth and reproduction. Despite the changes in their diet, the aphid Myzus persicae(Sulzer) displayed only small changes in feeding behaviour on mutant plants when measured using the Electronic Penetration Graph (EPG) technique. Salivation by the aphid into the SE (E1 phase) was increased on mutant plants but there was no significant effect on other feeding EPG behaviours, or in the rate of honeydew production.
Consistent with the small effect on aphid feeding behaviour, there was only a small effect of reduced sieve element amino acid concentration on aphid reproduction. The data are discussed in relation to the regulation of phloem
composition and the role of phloem amino acids in regulating aphid performance
One-Flavour Hybrid Monte Carlo with Wilson Fermions
The Wilson fermion determinant can be written as product of the determinants
of two hermitian positive definite matrices. This formulation allows to
simulate non-degenerate quark flavors by means of the hybrid Monte Carlo
algorithm. A major numerical difficulty is the occurrence of nested inversions.
We construct a Uzawa iteration scheme which treats the nested system within one
iterative process.Comment: 11 pages, to appear in proceedings of the workshop "Numerical
Challenges in Lattice QCD", Springer Verla
- …