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排序方式: 共有198条查询结果,搜索用时 15 毫秒
1.
H. D. Dahmen P. Manakos T. Mannel T. Ohl 《Zeitschrift fur Physik C Particles and Fields》1989,44(1):139-147
The photon two-point function is calculated using the exponential representation for the time-evolution operator taking into account terms up to second order ine in the exponent. The limiting case of vanishing electron mass is examined and it is shown that mass singularities cancel by means of unitarity. Near mass shell the causal Green function exhibits in the limit of vanishing electron mass the expected cut behaviour, off mass shell we recover the result of the usual perturbation theory. 相似文献
2.
Arne Barinka Stephan Dahlke Wolfgang Dahmen 《Advances in Computational Mathematics》2006,24(1-4):5-34
Recently adaptive wavelet methods have been developed which can be shown to exhibit an asymptotically optimal accuracy/work
balance for a wide class of variational problems including classical elliptic boundary value problems, boundary integral equations
as well as certain classes of noncoercive problems such as saddle point problems. A core ingredient of these schemes is the
approximate application of the involved operators in standard wavelet representation. Optimal computational complexity could
be shown under the assumption that the entries in properly compressed standard representations are known or computable in
average at unit cost. In this paper we propose concrete computational strategies and show under which circumstances this assumption
is justified in the context of elliptic boundary value problems.
Dedicated to Charles A. Micchelli on the occasion of his 60th birthday
Mathematics subject classifications (2000) 41A25, 41A46, 65F99, 65N12, 65N55.
This work has been supported in part by the Deutsche Forschungsgemeinschaft SFB 401, the first and third author are supported
in part by the European Community's Human Potential Programme under contract HPRN-CT-202-00286 (BREAKING COMPLEXITY). The
second author acknowledges the financial support provided through the European Union's Human Potential Programme, under contract
HPRN-CT-2002-00285 (HASSIP) and through DFG grant DA 360/4–1. 相似文献
3.
The central objective of this paper is to discuss linear independence of translates of discrete box splines which we introduced earlier as a device for the fast computation of multivariate splines. The results obtained here allow us to draw conclusions about the structure of such discrete splines which have, in particular, applications to counting the number of nonnegative integer solutions of linear diophantine equations. 相似文献
4.
H. M. Ortner H. H. Xu J. Dahmen K. Englert H. Opfermann W. Görtz 《Fresenius' Journal of Analytical Chemistry》1996,355(5-6):657-664
Surface characterization of new unused PTFE, PFA and PVDF labware has been carried out by light microscopy, scanning electron microscopy, profilometry and atomic force microscopy. It has been found that in spite of higher micro-roughness, PFA exhibits the lowest nano-roughness and hence seems best suited as vessel material for relatively mild pressureless chemical operations for sample preparation of ultrapure substances, as container material for storage of ultrapure liquids, and for transport of such liquids, e.g., from the producer in the chemical to the end user in the microelectronics industry. This suitability refers only to the surface quality of the investigated materials. PTFE-surfaces, due to the sintering process of production, exhibit the most unfavourable surface quality of the investigated fluorinated polymers. 相似文献
5.
The efficient esterification of various sulfonic acids and sulfonates using polymer-bound triazenes based on the triazene T2 linker is described. Esterification of enantiopure alpha-substituted sodium sulfonates was performed in the presence of an alkylating resin without racemization. Racemization is a serious drawback in the esterification route via sulfonyl chlorides because of intermediate sulfene formation. To demonstrate the versatility of this protocol, phosphoric and phosphinic acids have been converted into the corresponding esters as well. All products were obtained in good yield and excellent purities without any further purification steps. 相似文献
6.
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8.
U Dahmen C J D Hetherington V Radmilovic E Johnson S Q Xiao C P Luo 《Microscopy and microanalysis》2002,8(4):247-256
Twinning plays an important role in phase transformations and can have significant effects on microstructural evolution. Different roles of twinning in the development of microstructures during precipitation and phase transformations are reviewed and illustrated with examples from investigations by high-resolution electron microscopy, including the effect of multiple twinning on the development of Ge precipitates in Al-Ge and Ag-Ge alloys, the twin dissociation of grain boundaries in Au, the formation of hexagonal Si at twin intersections and the effect of twin boundaries on the equilibrium shape of Pb inclusions in Al. 相似文献
9.
[reaction: see text] The value of arylboranes as precursors for arylzinc reagents in asymmetric catalysis is demonstrated. Kinetic studies on the transmetalation reaction with zinc reagents rationalize the observed differences of three classes of arylboranes in catalytic applications. By using the stable and easily accessible triarylborane ammonia complexes, an array of chiral diarylmethanols in high yield and enantioselectivity was synthesized. 相似文献
10.
Andreas Stockhammer Klaus-Hermann Dahmen Tobias Gerfin Luigi M. Venanzi Volker Gramlich Walter Fetter 《Helvetica chimica acta》1991,74(5):989-992
The cluster compounds [Pt3(μ-CO)3(PR3)3] act as Lewis bases towards the metal halides of Group XI, MX, Group XII, MX2, and Group XIII, MX3, to form cluster compounds of the composition [{MXn}{Pt3(μ-CO)3(PR3)3}]. The X-ray crystal structure, NMR and IR data are given for the compound [{ZnI}{Pt3(μ-CO)3(PPh(i-Pr)2)3}]. 相似文献