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611.
Gunnar Jeschke Wilfried Hoffbauer Martin Jansen 《Chemistry (Weinheim an der Bergstrasse, Germany)》1998,4(9):1755-1761
Unexpected splittings in solid-state MAS-NMR spectra can indicate high rather than low local symmetry. A quantitative analysis of the resulting lineshapes provides information about geometry from a different range of space and time averaging than that of X-ray crystallography, as is demonstrated here for tetraphosphorus decaoxide (spectra on the right). 相似文献
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We study locally Cohen-Macaulay space curves lying on normal surfaces. We prove some theorems on the behaviour of the cohomology functions and initial ideals of such space curves, which give a basic distinction between those curves and curves lying on non-normal surfaces.
614.
Marat Moustiakimov Mikael Kritikos Gunnar Westin 《Acta Crystallographica. Section C, Structural Chemistry》2001,57(5):515-516
The new tetranuclear alkoxide hexa‐μ2‐isopropoxy‐1:2κ4O;1:3κ4O;1:4κ4O‐hexaisopropoxy‐2κ2O,3κ2O,4κ2O‐trialuminium(III)neodymium(III), [Nd{Al(C3H7O)4}3], has a metal–oxygen NdAl3O12 core which consists of four metal atoms arranged in an approximately planar triangular geometry. The central Nd atom is six‐coordinated by O atoms and the Al atoms are four‐coordinated by O atoms. 相似文献
615.
Gunnar Andersson Lars Engebretsen Johan Hstad 《Journal of Algorithms in Cognition, Informatics and Logic》2001,39(2):162
We introduce a new method of constructing approximation algorithms for combinatorial optimization problems using semidefinite programming. It consists of expressing each combinatorial object in the original problem as a constellation of vectors in the semidefinite program. When we apply this technique to systems of linear equations mod p with at most two variables in each equation, we can show that the problem is approximable within (1 − κ(p))p, where κ(p) > 0 for all p. Using standard techniques, we also show that it is NP-hard to approximate the problem within a constant ratio, independent of p. 相似文献
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619.
Gunnar Brix Matthias E. Bellemann Hans-Joachim Zabel Peter Bachert Walter J. Lorenz 《Magnetic resonance imaging》1993,11(8)
A 19F MR chemical shift imaging (CSI) technique is presented which enables selective imaging of the antineoplastic drug 5-fluorouracil (5-FU) and its major catabolite α-fluoro-β-alanine (FBAL). The CSI sequence employs a chemical shift selective (CHESS) saturation pulse to suppress either the 5-FU or the FBAL resonance before the other component of the two-line 19F MR spectrum is measured. Because the transmitter frequency can always be set to the Larmor frequency of the 19F resonance to be imaged, this approach yields 5-FU and FBAL MR images free of chemical shift artifacts in read-out and slice-selection direction. In phantom experiments, selective 5-FU and FBAL images with a spatial resolution of 15 × 15 × 20 mm3 (4.5 ml) were obtained in 30 min from a model solution, whose drug and catabolite concentrations were similar to those estimated in the liver of tumor patients undergoing IV chemotherapy with 5-FU. The drug-specific MR imaging technique developed is, therefore, well-suited for the direct and noninvasive monitoring of the up-take and trapping of 5-FU in liver tumors in vivo. 相似文献
620.