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141.
Allan Carlsson Karl Håkansson Mathias Kvick Fredrik Lundell L. Daniel Söderberg 《Experiments in fluids》2011,51(4):987-996
Steerable filters are concluded to be useful in order to determine the orientation of fibers captured in digital images. The
fiber orientation is a key variable in the study of flowing fiber suspensions. Here, digital image analysis based on a filter
within the class of steerable filters is evaluated for suitability of finding the position and orientation of fibers suspended
in flowing suspensions. In sharp images with small noise levels, the steerable filter succeeds in determining the orientation
of artificially generated fibers with well-defined angles. The influence of reduced image quality on the orientation has been
quantified. The effect of unsharpness and noise is studied and the results show that the error in orientation is less than
1° for moderate levels. Images from two flow cases, one laminar shear flow and one turbulent, are also analyzed. The fiber
orientation distribution is determined in the flow-vorticity plane. For the laminar case a comparison is made to a robust,
but computationally more expensive, method involving convolutions with an oriented elliptic filter. A good agreement is found
when comparing the resulting fiber orientation distributions obtained with the two methods. For the turbulent case, it is
demonstrated that correct results are obtained and that the method can handle overlapping fibers. 相似文献
142.
Uwe Klingebiel Stefan Schmatz Eike Gellermann Christian Drost Mathias Noltemeyer 《Monatshefte für Chemie / Chemical Monthly》2001,40(5):1105-1124
This review describes the synthesis and isomerization reactions of cyclic silylhydrazines. Topics of discussion are the ring
expansion of the three-membered Si(SiN2) to the four-membered (SiN)2 ring by lithiation of the (SiN2) ring and by thermal silyl group insertion into the N*N bond, the ring expansion of a three-membered (SiN2) to a five-membered (CSi2N2) ring by SiCH2 insertion into the Si*N bond, the formation of isomeric four- and six-membered (SiN2)2 rings, the synthesis of five- and six-membered silylhydrazine rings, and the expansion of a five-membered (N2Si2N)N ring to the isomeric six-membered (SiNN)2 ring. The mechanisms of the isomerizations are explained by quantum chemical calculations, and the results are verified by
crystal structure determinations. 相似文献
143.
144.
Kalle Rnkk Mathias Berner Reija Jokela Arto Tolvanen 《Journal of heterocyclic chemistry》2000,37(5):1145-1149
The total syntheses of 11‐methoxydeethyleburnamonines ( 4 ) and ( 13 ) were carried out with use of 6‐methoxytryptophyl bromide ( 5 ) as starting material. Compound 5 was converted in three steps to trans‐ester 8 . Acid‐catalysed epimerization of 8 , lithium aluminum hydride reduction of the ester group, tosylation and substitution with cyanide gave the cis‐nitrile 12 . Acid‐induced cyclization of 12 yielded mainly (±)‐trans‐11‐methoxydeethyleburnamonine ( 13 ), whereas base‐induced cyclization gave (±)‐cis‐11‐methoxydeethyleburnamonine ( 4 ). 相似文献
145.
Mathias Mertens 《Czechoslovak Journal of Physics》2000,50(1):151-156
This paper uses the formulation of renormalization theory in a pure algebraic way providing the notion of Hopf algebras as introduced by A. Connes and D. Kreimer [Commun. Math. Phys. 199 (1998) 203]. First an introduction to the Hopf algebra of rooted trees will be given. The second section explores how renormalization is achieved using the Hopf algebra of rooted trees. So far the paper reviews the results of D. Kreimer as published in the paper cited above. The review will end with a sketch of a category-theoretical interpretation which is under thorough investigation at the time of writing. 相似文献
146.
Jin proved that whenever A and B are sets of positive upper density in Z, A+B is piecewise syndetic. Jin's theorem was subsequently generalized by Jin and Keisler to a certain family of abelian groups, which in particular contains Zd. Answering a question of Jin and Keisler, we show that this result can be extended to countable amenable groups. Moreover we establish that such sumsets (or — depending on the notation — “product sets”) are piecewise Bohr, a result which for G=Z was proved by Bergelson, Furstenberg and Weiss. In the case of an abelian group G, we show that a set is piecewise Bohr if and only if it contains a sumset of two sets of positive upper Banach density. 相似文献
147.
We consider a new class of estimators for volatility functionals in the setting of frequently observed Itō diffusions which are disturbed by i.i.d. noise. These statistics extend the approach of pre-averaging as a general method for the estimation of the integrated volatility in the presence of microstructure noise and are closely related to the original concept of bipower variation in the no-noise case. We show that this approach provides efficient estimators for a large class of integrated powers of volatility and prove the associated (stable) central limit theorems. In a more general Itō semimartingale framework this method can be used to define both estimators for the entire quadratic variation of the underlying process and jump-robust estimators which are consistent for various functionals of volatility. As a by-product we obtain a simple test for the presence of jumps in the underlying semimartingale. 相似文献
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