8,218 research outputs found
Structure Functions, Form Factors, and Lattice QCD
We present results towards the calculation of the pion electric form factor
and structure function on a lattice using charge overlap. By
sacrificing Fourier transform information in two directions, it is seen that
the longitudinal four point function can be extracted with reasonable error
bars at low momentum.Comment: 3 pages (contribution to "Lattice 93"), UNIX SHAR file includes the
LaTeX source and three encapsulated PS figures (which will print on
appropriate drivers but can not be previewed), BU-HEP-93-0
Deflated Iterative Methods for Linear Equations with Multiple Right-Hand Sides
A new approach is discussed for solving large nonsymmetric systems of linear
equations with multiple right-hand sides. The first system is solved with a
deflated GMRES method that generates eigenvector information at the same time
that the linear equations are solved. Subsequent systems are solved by
combining restarted GMRES with a projection over the previously determined
eigenvectors. This approach offers an alternative to block methods, and it can
also be combined with a block method. It is useful when there are a limited
number of small eigenvalues that slow the convergence. An example is given
showing significant improvement for a problem from quantum chromodynamics. The
second and subsequent right-hand sides are solved much quicker than without the
deflation. This new approach is relatively simple to implement and is very
efficient compared to other deflation methods.Comment: 13 pages, 5 figure
The sensing and perception subsystem of the NASA research telerobot
A useful space telerobot for on-orbit assembly, maintenance, and repair tasks must have a sensing and perception subsystem which can provide the locations, orientations, and velocities of all relevant objects in the work environment. This function must be accomplished with sufficient speed and accuracy to permit effective grappling and manipulation. Appropriate symbolic names must be attached to each object for use by higher-level planning algorithms. Sensor data and inferences must be presented to the remote human operator in a way that is both comprehensible in ensuring safe autonomous operation and useful for direct teleoperation. Research at JPL toward these objectives is described
High temperature deformation of dispersion strengthened nickel alloys. 1 - The influence of initial structure on tensile and creep deformation of TD nickel. 2 - The effect of matrix stacking fault energy on creep of Ni-Cr-ThO2 alloys Final report, 9 Feb. 1967 - 9 Feb. 1968
High temperature creep studies on unaltered, and recrystallized nickel alloy
The role of grain size and shape in the strengthening of dispersion hardened nickel alloys
Thermomechanical processing was used to develop various microsstructures in Ni, Ni-2ThO2, Ni-20Cr, Ni-20CR-2ThO2, Ni-20Cr-10W-and Ni-20Cr-10W-2ThO2. The yield strength at 25 C increased with substructure refinement according to the Hall-Petch relation, and substructure refinement was a much more potent means of strengthening than was dispersion hardening. At elevated temperature (1093 C), the most important microstructural feature affecting strength was the grain aspect ratio (grain length, L, divided by grain width, 1. The yield strength and creep strength increased linearly with increasing L/1
Deflated BiCGStab for linear equations in QCD problems
The large systems of complex linear equations that are generated in QCD
problems often have multiple right-hand sides (for multiple sources) and
multiple shifts (for multiple masses). Deflated GMRES methods have previously
been developed for solving multiple right-hand sides. Eigenvectors are
generated during solution of the first right-hand side and used to speed up
convergence for the other right-hand sides. Here we discuss deflating
non-restarted methods such as BiCGStab. For effective deflation, both left and
right eigenvectors are needed. Fortunately, with the Wilson matrix, left
eigenvectors can be derived from the right eigenvectors. We demonstrate for
difficult problems with kappa near kappa_c that deflating eigenvalues can
significantly improve BiCGStab. We also will look at improving solution of
twisted mass problems with multiple shifts. Projecting over previous solutions
is an easy way to reduce the work needed.Comment: 7 pages, 4 figures, presented at the XXV International Symposium on
Lattice Field Theory, 30 July - 4 August 2007, Regensburg, German
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