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The authors show that any k-Osserman Lorentzian algebraic curvaturetensor has constant sectional curvature, and give an elementaryproof that any local 2-point homogeneous Lorentzian manifoldhas constant sectional curvature. They also show that a SzabóLorentzian covariant derivative algebraic curvature tensor vanishes.2000 Mathematics Subject Classification 53B20. 相似文献
23.
V. Dřínek M. Urbanová Z. Bastl I. Gregora V. Vorlíček J. Šubrt J. Pola 《Applied Physics A: Materials Science & Processing》1998,66(5):503-509
2 laser radiation in the gas phase results in the deposition of carbonaceous films composed of a graphitic and monohydrogenated
carbon, in which H is dominantly bonded to sp3 sites. The films incorporate oxygen in the topmost layers. The technique is suitable for the preparation of thin C-based
films at low substrate temperatures.
Received: 23 July 1997/Accepted: 18 November 1997 相似文献
24.
Tolić-Nørrelykke IM Munteanu EL Thon G Oddershede L Berg-Sørensen K 《Physical review letters》2004,93(7):078102
The viscoelastic properties of the cytoplasm of living yeast cells were investigated by studying the motion of lipid granules naturally occurring in the cytoplasm. A large frequency range of observation was obtained by a combination of video-based and laser-based tracking methods. At time scales from 10(-4) to 10(2) s, the granules typically perform subdiffusive motion with characteristics different from previous measurements in living cells. This subdiffusive behavior is thought to be due to the presence of polymer networks and membranous structures in the cytoplasm. Consistent with this hypothesis, we observe that the motion becomes less subdiffusive upon actin disruption. 相似文献
25.
Iva Rezi? 《Monatshefte für Chemie / Chemical Monthly》2011,172(2):1219-1225
Abstract
The formulation of a detergent product is a complicated task which depends on many economical, ecological, and practical parameters. The goal is to achieve a product which will have optimal washing properties and significantly decrease the surface tension. Therefore, prediction of the minimal surface tension of a mixture of four different surfactants can be very beneficial for the industry. In this work this task was performed by using Stat-Ease Design Expert software. This program was successfully applied for prediction of the minimal surface tension and optimization of the composition of different surfactant mixtures. Predicted data were compared with experimental results, and very good correlation was achieved. For further application, more complex optimization procedures could be performed by adding additional parameters to the Design Expert software. Therefore, this program is extremely useful for the laundry industry and could be successfully applied in many other, more complex tasks. 相似文献26.
We report on the elaboration of macroporous antimony film electrodes. The strategy to create macroporous electrodes is based on the replication of colloidal crystal templates. These electrodes of controlled porosity show an increased internal electroactive area and a significantly improved electrochemical performance. The application of this novel electrochemical sensor for the sensitive quantification of traces of heavy metals has been demonstrated in combination with differential pulse anodic stripping voltammetry in model solutions. 相似文献
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28.
Iva Franjić 《Mathematische Nachrichten》2015,288(17-18):2016-2027
This paper aims to contribute to the exploration of quadrature formulae by proving that the error of a quadrature formula has the Schur‐convexity property. The emphasis is on the quadrature formulae with the maximum degree of precision. The Schur‐convexity of the error has an interesting implication – the monotonicity of the error. Namely, it turns out that the absolute value of the error is always smaller on a smaller interval (of the two which share the same midpoint). 相似文献
29.
Nikolina Berova Stefan Eojadziev Nevenka Bresciani-Pahor Petko M. Ivanova Biserka Koji-Prodic Rositza Rakovska
Iva Rui-Toro Günther Snatzke 《Helvetica chimica acta》1990,73(7):1853-1860
The absolute configuration of the 2-(phenylmethyl)pyridine derivatives 1 – 9 had been established by X-ray diffraction and chemical correlation. Their CD spectra have been studied in different solvents for the free and protonated forms. It has now been found that, from the sign of the strong CD couplet between 270 and 220 nm, which was observable for all these compounds besides 7 and 9 , their absolute configuration can be determined much quicker. 相似文献
30.
Structure of Framework Aluminum Lewis Sites and Perturbed Aluminum Atoms in Zeolites as Determined by 27Al{1H} REDOR (3Q) MAS NMR Spectroscopy and DFT/Molecular Mechanics 下载免费PDF全文
Dr. Jiří Brus Libor Kobera Dr. Wolfgang Schoefberger Dr. Martina Urbanová Petr Klein Dr. Petr Sazama Dr. Edyta Tabor Dr. Stepan Sklenak Dr. Anna V. Fishchuk Dr. Jiří Dědeček 《Angewandte Chemie (International ed. in English)》2015,54(2):541-545
Zeolites are highly important heterogeneous catalysts. Besides Brønsted SiOHAl acid sites, also framework AlFR Lewis acid sites are often found in their H‐forms. The formation of AlFR Lewis sites in zeolites is a key issue regarding their selectivity in acid‐catalyzed reactions. The local structures of AlFR Lewis sites in dehydrated zeolites and their precursors—“perturbed” AlFR atoms in hydrated zeolites—were studied by high‐resolution MAS NMR and FTIR spectroscopy and DFT/MM calculations. Perturbed framework Al atoms correspond to (SiO)3AlOH groups and are characterized by a broad 27Al NMR resonance (δi=59–62 ppm, CQ=5 MHz, and η=0.3–0.4) with a shoulder at 40 ppm in the 27Al MAS NMR spectrum. Dehydroxylation of (SiO)3AlOH occurs at mild temperatures and leads to the formation of AlFR Lewis sites tricoordinated to the zeolite framework. Al atoms of these (SiO)3Al Lewis sites exhibit an extremely broad 27Al NMR resonance (δi≈67 ppm, CQ≈20 MHz, and η≈0.1). 相似文献