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31.
在CIMT’99展览会上,我们有幸与国内外一些著名的机床厂商探讨21世纪机床发展的趋势,并获得共识。现刊登部分厂家对21世纪机床发展的战略规划与思路。展望21世纪的全球化制造世界正以我们前所未料的速度发生着变化。发展中国家正日益开放市场,并从全球范围内广泛吸引投资,为制造业带来了更多的就业机会。这种新发展有利于提高许多国家的人民生活水平,而生活水平的提高又为新产品开拓了更广阔的前景。另外,电子商务也正对新客户、新竞争者的产生带来极其重要影响。然而,这些新兴市场具有高风险的待征,因此进入新兴市场的企业必须尽… 相似文献
32.
DA Brake G Strang JE Lineberger CH Fedor R Clare TA Banas T Miller 《Canadian Metallurgical Quarterly》1997,76(7):974-983
The immunogenicity of a tissue culture-derived vaccine generated from an Eimeria tenella-infected cell line in a serologically defined bird line, and the ability to confer protection against homologous challenge in young chicks was examined. The cell line, SB-CEV-1/F7, was infected with E. tenella sporozoites and the resulting 72-h postinfection cell-free supernatants were adjuvanted and used to immunize Leghorn chicks homozygous for the B19 haplotype. Peripheral blood and splenic lymphocytes from these immunized birds proliferated in vitro in response to both sporozoite and SB-CEV-1/F7 tissue culture-derived parasite antigens. In addition, splenic immune lymphocytes obtained from birds previously exposed to E. tenella in vivo responded to these tissue culture-derived parasite antigens in vitro. To evaluate the efficacy of the vaccine, B19B19 chicks were vaccinated s.c. with adjuvanted 72-h postinfection cell-free supernatants or an ammonium sulfate precipitate derivative thereof, orally boosted, and then subjected to homologous parasite challenge at 10 d of age. The level of protection (body weight gain, cecal lesions) was assessed 6 d after challenge. Performance results from four battery trials demonstrated that vaccinated birds were significantly protected against weight loss compared to unimmunized, challenged controls. In addition, in two of the four trials, vaccinated birds were significantly protected against lesions. These results provide strong evidence that tissue culture-derived parasite antigens obtained from the E. tenella-infected SB-CEV-1/F7 cell line are immunogenic in birds and can provide partial protection against E. tenella clinical coccidiosis. 相似文献
33.
Feodor F. Dragan Fedor V. Fomin Petr A. Golovach 《Journal of Computer and System Sciences》2011,77(6):1108-1119
A t-spanner of a graph G is a spanning subgraph S in which the distance between every pair of vertices is at most t times their distance in G. If S is required to be a tree then S is called a tree t-spanner of G. In 1998, Fekete and Kremer showed that on unweighted planar graphs deciding whether G admits a tree t-spanner is polynomial time solvable for t?3 and is NP-complete when t is part of the input. They also left as an open problem if the problem is polynomial time solvable for every fixed t?4. In this work we resolve the open question of Fekete and Kremer by proving much more general results:
•
The problem of finding a t-spanner of treewidth at most k in a given planar graph G is fixed parameter tractable parameterized by k and t. Moreover, for every fixed t and k, the running time of our algorithm is linear. •
Our technique allows to extend the result from planar graphs to much more general classes of graphs. An apex graph is a graph that can be made planar by the removal of a single vertex. We prove that the problem of finding a t-spanner of treewidth k is fixed parameter tractable on graphs that do not contain some fixed apex graph as a minor, i.e. on apex-minor-free graphs. The class of apex-minor-free graphs contains planar graphs and graphs of bounded genus. •
Finally, we show that the tractability border of the t-spanner problem cannot be extended beyond the class of apex-minor-free graphs and in this sense our results are tight. In particular, for every t?4, the problem of finding a tree t-spanner is NP-complete on K6-minor-free graphs.
34.
Fedor V. Fomin Pinar Heggernes Dieter Kratsch Charis Papadopoulos Yngve Villanger 《Algorithmica》2014,69(1):216-231
The Subset Feedback Vertex Set problem takes as input a pair (G,S), where G=(V,E) is a graph with weights on its vertices, and S?V. The task is to find a set of vertices of total minimum weight to be removed from G, such that in the remaining graph no cycle contains a vertex of S. We show that this problem can be solved in time O(1.8638 n ), where n=|V|. This is a consequence of the main result of this paper, namely that all minimal subset feedback vertex sets of a graph can be enumerated in time O(1.8638 n ). 相似文献
35.
Vitalii E. Konoplev Irina V. Pisareva Eugenii V. Vorontsov Fedor M. Dolgushin Andreas Franken John D. Kennedy Igor T. Chizhevsky 《Inorganic chemistry communications》2003,6(12):1454-1458
Reactions of [OsCl2(PPh3)3] 1 with the [NEt4]+ salts of the [6-Ph-nido-6-CB9H11]− anion 2 and the [arachno-6-CB9H14]− anion 3 each proceed selectively in methanol solution with the elimination of one boron-containing fragment from the starting cluster polyhedron, yielding together a short series of three novel 10-vertex hydridoosmamonocarbaboranes of isocloso {OsCB8} cluster geometry. 相似文献
36.
Ghodrat Mahmoudi Maria G. Babashkina Waldemar Maniukiewicz Farhad Akbari Afkhami Bharath Babu Nunna Fedor I. Zubkov Aleksandra L. Ptaszek Dariusz W. Szczepanik Mariusz P. Mitoraj Damir A. Safin 《International journal of molecular sciences》2021,22(10)
In this work, we report solvent-induced complexation properties of a new N2S2 tetradentate bis-thiosemicarbazone ligand (H2LI), prepared by the condensation of 4-phenylthiosemicarbazide with bis-aldehyde, namely 2,2’-(ethane-1,2-diylbis(oxy)dibenzaldehyde, towards nickel(II). Using ethanol as a reaction medium allowed the isolation of a discrete mononuclear homoleptic complex [NiLI] (1), for which its crystal structure contains three independent molecules, namely 1-I, 1-II, and 1-III, in the asymmetric unit. The doubly deprotonated ligand LI in the structure of 1 is coordinated in a cis-manner through the azomethine nitrogen atoms and the thiocarbonyl sulfur atoms. The coordination geometry around metal centers in all the three crystallographically independent molecules of 1 is best described as the seesaw structure. Interestingly, using methanol as a reaction medium in the same synthesis allowed for the isolation of a discrete mononuclear homoleptic complex [Ni(LII)2] (2), where LII is a monodeprotonated ligand 2-(2-(2-(2-(dimethoxymethyl)phenoxy)ethoxy)benzylidene)-N-phenylhydrazine-1-carbothioamide (HLII). The ligand LII was formed in situ from the reaction of LI with methanol upon coordination to the metal center under synthetic conditions. In the structure of 2, two ligands LII are coordinated in a trans-manner through the azomethine nitrogen atom and the thiocarbonyl sulfur atom, also yielding a seesaw coordination geometry around the metal center. The charge and energy decomposition scheme ETS-NOCV allows for the conclusion that both structures are stabilized by a bunch of London dispersion-driven intermolecular interactions, including predominantly N–H∙∙∙S and N–H∙∙∙O hydrogen bonds in 1 and 2, respectively; they are further augmented by less typical C–H∙∙∙X (where X = S, N, O, π), CH∙∙∙HC, π∙∙∙π stacking and the most striking, attractive long-range intermolecular C–H∙∙∙Ni preagostic interactions. The latter are found to be determined by both stabilizing Coulomb forces and an exchange-correlation contribution as revealed by the IQA energy decomposition scheme. Interestingly, the analogous long-range C–H∙∙∙S interactions are characterized by a repulsive Coulomb contribution and the prevailing attractive exchange-correlation constituent. The electron density of the delocalized bonds (EDDB) method shows that the nickel(II) atom shares only ~0.8|e| due to the σ-conjugation with the adjacent in-plane atoms, demonstrating a very weak σ-metalloaromatic character. 相似文献
37.
38.
ABSTRACT: We report the experimental observation of the novel phenomenon of the resistivity decrease in porous PbTe layers during the pore formation process. Investigations were performed on the n-PbTe films with 2.3 micrometer thickness, which were near the point of the conductivity-type inversion at room temperature. Anodic electrochemical treatment for the porous layers with 41-52% porosity fabrication was performed using a KOH-based Norr electrolyte solution. For the porous lead telluride layers, the resistivity value at 300 K decreased 2.5-3 times. For the explanation of the observed phenomenon a physical model is proposed, which takes into account the Pb/Te ratio change during the anodic treatment. 相似文献
39.
Biopolymers: Programmable Biopolymers for Advancing Biomedical Applications of Fluorescent Nanodiamonds (Adv. Funct. Mater. 42/2015) 下载免费PDF全文
40.
BACKGROUND: Hairpin ribozymes (RNA enzymes) catalyze the same chemical reaction as ribonuclease A and yet RNAs do not usually have functional groups analogous to the catalytically essential histidine and lysine sidechains of protein ribonucleases. Some RNA enzymes appear to recruit metal ions to act as Lewis acids in charge stabilization and metal-bound hydroxide for general base catalysis, but it has been reported that the hairpin ribozyme functions in the presence of metal ion chelators. This led us to investigate whether the hairpin ribozyme exploits a metal-ion-independent catalytic strategy. RESULTS: Substitution of sulfur for nonbridging oxygens of the reactive phosphate of the hairpin ribozyme has small, stereospecific and metal-ion-independent effects on cleavage and ligation mediated by this ribozyme. Cobalt hexammine, an exchange-inert metal complex, supports full hairpin ribozyme activity, and the ribozyme's catalytic rate constants display only a shallow dependence on pH. CONCLUSIONS: Direct metal ion coordination to phosphate oxygens is not essential for hairpin ribozyme catalysis and metal-bound hydroxide does not serve as the general base in this catalysis. Several models might account for the unusual pH and metal ion independence: hairpin cleavage and ligation might be limited by a slow conformational change; a pH-independent or metal-cation-independent chemical step, such as breaking the 5' oxygen-phosphorus bond, might be rate determining; or finally, functional groups within the ribozyme might participate directly in catalytic chemistry. Whichever the case, the hairpin ribozyme appears to employ a unique strategy for RNA catalysis. 相似文献