6,895 research outputs found
Development and Implementation of NURBS Models of Quadratic Curves and Surfaces
This article goes into the development of NURBS models of
quadratic curves and surfaces. Curves and surfaces which could be represented
by one general equation (one for the curves and one for the surfaces) are
addressed. The research examines the curves: ellipse, parabola and hyperbola,
the surfaces: ellipsoid, paraboloid, hyperboloid, double hyperboloid,
hyperbolic paraboloid and cone, and the cylinders: elliptic, parabolic and
hyperbolic. Many real objects which have to be modeled in 3D applications
possess specific features. Because of this these geometric objects have been
chosen. Using the NURBS models presented here, specialized software modules (plug-ins) have been developed for a 3D graphic system. An analysis of their implementation and the primitives they create has been performed
Spectral shift function for operators with crossed magnetic and electric fields
We obtain a representation formula for the derivative of the spectral shift
function related to the operators and . We establish a limiting absorption principle
for and an estimate for
, provided , where $Q =
(D_x - By)^2 + D_y^2 + V(x,y).
Polyhedral units and network connectivity in calcium aluminosilicate glasses from high-energy x-ray diffraction
Structure factors for Cax/2AlxSi1-xO2 glasses (x=0,0.25,0.5,0.67) extended to
a wave vector of magnitude Q= 40 1/A have been obtained by high-energy x-ray
diffraction. For the first time, it is possible to resolve the contributions of
Si-O, Al-O and Ca-O coordination polyhedra to the experimental atomic pair
distribution functions (PDF). It has been found that both Si and Al are
four-fold coordinated and so participate in a continuous tetrahedral network at
low values of x. The number of network breaking defects in the form of
non-bridging oxygens (NBO's) increases slowly with x until x=0.5 (NBO's ~ 10%
at x=0.5). By x=0.67 the network breaking defects become significant as
evidenced by the significant drop in the average coordination number of Si. By
contrast, Al-O tetrahedra remain free of NBO's and fully integrated in the
Al/Si-O network for all values of x. Calcium maintains a rather uniform
coordination sphere of approximately 5 oxygen atoms for all values of x. The
results suggest that not only Si/Al-O tetrahedra but Ca-O polyhedra, too, play
a role in determining the glassy structure
Lifetime measurement of excited low-spin states via the ) reaction
In this article a method for lifetime measurements in the sub-picosecond
regime via the Doppler-shift attenuation method (DSAM) following the inelastic
proton scattering reaction is presented. In a pioneering experiment we
extracted the lifetimes of 30 excited low-spin states of Ru, taking
advantage of the coincident detection of scattered protons and de-exciting
-rays as well as the large number of particle and -ray
detectors provided by the SONIC@HORUS setup at the University of Cologne. The
large amount of new experimental data shows that this technique is suited for
the measurement of lifetimes of excited low-spin states, especially for
isotopes with a low isotopic abundance, where ) or - in
case of investigating dipole excitations - ()
experiments are not feasible due to the lack of sufficient isotopically
enriched target material
Transition rates and nuclear structure changes in mirror nuclei 47Cr and 47V
Lifetime measurements in the mirror nuclei 47Cr and 47V were performed by
means of the Doppler-shift attenuation method using the multidetector array
EUROBALL, in conjunction with the ancillary detectors ISIS and the Neutron
Wall. The determined transition strengths in the yrast cascades are well
described by full pf shell model calculations.Comment: Latex2e, 11 pages, 3 figure
Parallel computing and molecular dynamics of biological membranes
In this talk I discuss the general question of the portability of Molecular
Dynamics codes for diffusive systems on parallel computers of the APE family.
The intrinsic single precision arithmetics of the today available APE platforms
does not seem to affect the numerical accuracy of the simulations, while the
absence of integer addressing from CPU to individual nodes puts strong
constraints on the possible programming strategies. Liquids can be very
satisfactorily simulated using the "systolic" method. For more complex systems,
like the biological ones at which we are ultimately interested in, the "domain
decomposition" approach is best suited to beat the quadratic growth of the
inter-molecular computational time with the number of elementary components of
the system. The promising perspectives of using this strategy for extensive
simulations of lipid bilayers are briefly reviewed.Comment: 4 pages LaTeX, 2 figures included, espcrc2.sty require
- …