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排序方式: 共有531条查询结果,搜索用时 31 毫秒
1.
Manuel Fernández-López Eduardo García-Río Demir N. Kupeli 《Annals of Global Analysis and Geometry》2002,21(1):1-13
A characterization of time functions on a spacetime is made by using theMöbius equation. It is shown that a time function characterized in this wayyields past timelike geodesic incompleteness and local Lorentzian warpedproduct decomposition of spacetime, provided that the stress-energy tensoris a fluid. Also, by imposing additional assumptions on the stress-energytensor and global analytic structure of the spacetime, more restrictivedecompositions closer to Robertson–Walker spacetimes are obtained. 相似文献
2.
本文对广义风险过程中的渐近方差作了非参数估计,得出并证明了两个定理,为广义风险过程中破产概率的区间估计作了理论准备. 相似文献
3.
We discuss role of partially gravitating scalar fields, scalar fields whose energy–momentum tensors vanish for a subset of dimensions, in dynamical compactification of a given set of dimensions. We show that the resulting spacetime exhibits a factorizable geometry consisting of usual four-dimensional spacetime with full Poincaré invariance times a manifold of extra dimensions whose size and shape are determined by the scalar field dynamics. Depending on the strength of its coupling to the curvature scalar, the vacuum expectation value (VEV) of the scalar field may or may not vanish. When its VEV is zero the higher-dimensional spacetime is completely flat and there is no compactification effect at all. On the other hand, when its VEV is nonzero the extra dimensions get spontaneously compactified. The compactification process is such that a bulk cosmological constant is utilized for curving the extra dimensions. 相似文献
4.
5.
A new enantioselective synthesis of furan‐2‐yl amines and amino acids is described, in which the key step is the oxazaborolidine‐catalyzed enantioselective reduction of O‐benzyl (E)‐ and (Z)‐furan‐2‐yl ketone oximes to the corresponding chiral amines. The chirality of the furan‐2‐yl amines is fully controlled by the appropriate choice of the geometrical isomer of the O‐benzyl oxime. Oxidation of the furan ring furnished amino acids in high yields. 相似文献
6.
8,8"-Biflavonoidsareantw0rtantclass0fbiflavonoids.Asasystematicresearchofthe8,8"-biflav0noids,wehavepreviouslyreportedthestudyonthereactionof2'-hydroxychaloneswithl2-H2SO4-DMSOsystem.'InthispaPer,thesynthesisof5,5",7,7',-tetTameth0xy-8,8"-biflavonela2,4,4",7,7"-tetramethoxy-8,8"-biflavone1b',4',4",,5,5,',7,7"-hexamethoxy-8,8"-biflav0ne1c4and4,4"-dibenZyl0xy-5,5",7,7"-tetramothoxy-8,8"-biflavoneldisreported.ThesyntheticmethodisshownintheSchemebelow.2-Hydroxyacet0phenonescondensedwithsubsti… 相似文献
7.
New, sterically demanding 1,3-dialkyl-3,4,5,6-tetrahydropyrimidinium salts (2) as NHC precursors have been synthesized and characterized. These salts, in combination with palladium acetate, provided active catalysts for the cross-coupling of aryl chlorides and bromides under mild conditions. The catalytic system was applied to the Heck, Suzuki and benzaldehyde (Kumada) coupling reactions. Catalyst activity was found to be influenced by the presence of a methoxy group on the ring of the p-position of benzyl substituent of the ligand precursor. 相似文献
8.
Ayhan S Demir Idris Mecitoglu Cihangir Tanyeli Volkan Gülbeyaz 《Tetrahedron: Asymmetry》1996,7(12):3359-3364
New catalysts have been prepared from (S)- and (R)- proline and the asymmetric borane reduction of prochiral ketones using these catalysts has been studied. The secondary alcohols were obtained in 76–95% yield with 57– 96% enantiomeric excesses. 相似文献
9.
高效液相色谱氮杂冠醚键合固定相的合成及性能研究 总被引:2,自引:0,他引:2
我们曾用γ-氯丙基键合硅胶(CPS),在氢化钠(NaH)作用下合成了3-(氮杂-18-冠-6)丙基键合固定相(BCP)[1].该固定相对碱基、核苷酸、硝基苯酚等有较好的分离选择性,但由于NaH对硅胶基质腐蚀作用较大,导致BCP柱效及渗透性较低.本文采... 相似文献
10.
A one-step synthesis of 5-alkoxypyrrole-3-phosphonates is presented starting from suitable α-cyanomethyl-β-ketophosphonates. The key step in the synthesis involves a one-pot addition and heteroannulation sequence. The zinc perchlorate-catalyzed addition of alcohols to the nitrile carbon of α-cyanomethyl-β-ketophosphonates followed by annulation furnished 5-alkoxypyrrole-3-phosphonates. The addition-annulation process is carried out in the presence of water and 4,5-dihydro-5-oxo-1H-pyrrole-3-phosphonates (pyrrolinones) are obtained in good yields. 相似文献