首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   537篇
  免费   6篇
  国内免费   3篇
化学   283篇
力学   14篇
数学   166篇
物理学   83篇
  2022年   5篇
  2021年   22篇
  2020年   9篇
  2019年   7篇
  2018年   5篇
  2017年   14篇
  2016年   16篇
  2015年   15篇
  2014年   22篇
  2013年   52篇
  2012年   41篇
  2011年   41篇
  2010年   33篇
  2009年   33篇
  2008年   19篇
  2007年   26篇
  2006年   18篇
  2005年   22篇
  2004年   26篇
  2003年   14篇
  2002年   8篇
  2001年   7篇
  2000年   5篇
  1999年   6篇
  1998年   2篇
  1997年   7篇
  1996年   5篇
  1995年   6篇
  1994年   5篇
  1993年   3篇
  1992年   5篇
  1991年   3篇
  1990年   7篇
  1989年   7篇
  1988年   3篇
  1987年   4篇
  1986年   3篇
  1985年   2篇
  1984年   1篇
  1983年   2篇
  1982年   3篇
  1981年   4篇
  1980年   4篇
  1976年   2篇
  1972年   1篇
  1935年   1篇
排序方式: 共有546条查询结果,搜索用时 15 毫秒
121.
Conclusions The racemic form of the AI component of the red San Jose scale pheromone and its analog were synthesized on the basis of available citronellol and geraniol derivatives.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1143–1146, May, 1989.  相似文献   
122.
A graph X is said to be distance-balanced if for any edge uv of X, the number of vertices closer to u than to v is equal to the number of vertices closer to v than to u. A graph X is said to be strongly distance-balanced if for any edge uv of X and any integer k, the number of vertices at distance k from u and at distance k+1 from v is equal to the number of vertices at distance k+1 from u and at distance k from v. Exploring the connection between symmetry properties of graphs and the metric property of being (strongly) distance-balanced is the main theme of this article. That a vertex-transitive graph is necessarily strongly distance-balanced and thus also distance-balanced is an easy observation. With only a slight relaxation of the transitivity condition, the situation changes drastically: there are infinite families of semisymmetric graphs (that is, graphs which are edge-transitive, but not vertex-transitive) which are distance-balanced, but there are also infinite families of semisymmetric graphs which are not distance-balanced. Results on the distance-balanced property in product graphs prove helpful in obtaining these constructions. Finally, a complete classification of strongly distance-balanced graphs is given for the following infinite families of generalized Petersen graphs: GP(n,2), GP(5k+1,k), GP(3k±3,k), and GP(2k+2,k).  相似文献   
123.
On Cubic Graphs Admitting an Edge-Transitive Solvable Group   总被引:2,自引:2,他引:0  
Using covering graph techniques, a structural result about connected cubic simple graphs admitting an edge-transitive solvable group of automorphisms is proved. This implies, among other, that every such graph can be obtained from either the 3-dipole Dip3 or the complete graph K 4, by a sequence of elementary-abelian covers. Another consequence of the main structural result is that the action of an arc-transitive solvable group on a connected cubic simple graph is at most 3-arc-transitive. As an application, a new infinite family of semisymmetric cubic graphs, arising as regular elementary abelian covering projections of K 3,3, is constructed.  相似文献   
124.
Systematic modification of three structurally different minerals (zeolite, mica, and vermiculite) was carried out with the aim of determining the modification mechanism and exposing the hydrophobic surface that can be used as a sorbent for many organic compounds. Mechanism of modification with cationic surfactant depends strongly on the mineral type. In order to identify the influence of aluminosilicates structural differences on the modification process, adsorption experiments with organic matter and water vapor, supplemented with the DTA/TG analysis, were performed. The cation exchange capacity (CEC) value was 1454?>?560?>?28 meq kg?1 for zeolite (clinoptilolite), vermiculite, and mica (muscovite), respectively. Despite its CEC value, vermiculite adsorbed three times the amount of organic matter than did clinoptilolite due to the porous structure of zeolite, which acted to limit the adsorption only on the external exchangeable cations. If the loading amount is equal to the CEC or the external cation exchange capacity for clinoptilolite (ECEC?≈?10% CEC), the monolayer will form while mineral surface will have hydrophobic character. Only one active center exists at the surface of the clinoptilolite that was identified by DTA curves with a sharp and defined peak around 300 °C and by the mass loss at the TG diagrams. Two significant and equal active centers were observed in vermiculite, one for the exchange of the surface cations and the other for the interlayer cations and H2O molecules. Muscovite CEC is negligible, and due to the absence of any other functional groups, the modification of this mineral was impossible.  相似文献   
125.
Stevanović  Dragan 《Order》2022,39(1):77-94

The k-th spectral moment Mk(G) of the adjacency matrix of a graph G represents the number of closed walks of length k in G. We study here the partial order ? of graphs, defined by G ? H if Mk(G) ≤ Mk(H) for all k ≥?0, and are interested in the question when is ? a linear order within a specified set of graphs? Our main result is that ? is a linear order on each set of starlike trees with constant number of vertices. Recall that a connected graph G is a starlike tree if it has a vertex u such that the components of G ? u are paths, called the branches of G. It turns out that the ? ordering of starlike trees with constant number of vertices coincides with the shortlex order of sorted sequence of their branch lengths.

  相似文献   
126.
Cytochrome c oxidase (CcO) is the terminal transmembrane enzyme of the respiratory electron transport chain in aerobic cells. It catalyzes the reduction of oxygen to water and utilizes the free energy of the reduction reaction for proton pumping, a process which results in a membrane electrochemical proton gradient. Although the structure of the enzyme has been solved for several organisms, the molecular mechanism of proton pumping and proton exit pathways remain unknown. In our previous work, the continuum electrostatic calculations were employed to evaluate the electrostatic potential, energies, and protonation state of bovine cytochrome c oxidase for different redox states of the enzyme. A possible mechanism of oxygen reduction and proton pumping via His291 was proposed. In this paper, using electrostatic calculations, we examine the proton exit pathways in the enzyme. By monitoring the changes of the protonation states, proton affinities, and energies of electrostatic interactions between the titratable groups in different redox states of CcO, we identified the clusters of strongly interacting residues. Using these data, we detected four possible proton exit points on the periplasmic side of the membrane (Lys171B/Asp173B, His24B/Asp25B, Asp51, and Asp300). We then were able to trace the proton exit pathways and to evaluate the energy profiles along the paths. On the basis of energetic considerations and the conservation of the residues in a protein sequence, the most likely exit pathway is one via the Lys171B/Asp173B site. The obtained results are fully consistent with our His291 model of proton pumping, and provide a rationale for the absence of proton leaking in CcO between the pumping strokes.  相似文献   
127.
By the following simple formula (1) $$\forall x \exists y (x = xyy, y = xyx)$$ We characterize semigroups from the title. Considering a local property of their ?-classes we get bands and Boolean groups as extreme cases of semigroups with that property. We also provide a construction showing that ?-classes can be sufficiently complicated (at least as Abelian groups are). Then we permute right-hand sides of identities in (1) getting Boolean semigroups (x3=x) and so-called anti-inverse semigroups. Finally we show that Boolean semigroups are a proper subclass of the intersection of anti-inverse semigroups and unions of dihedral groups.  相似文献   
128.
The macromolecular chain conformational state during the amination of chloromethylated polystyrene (CMPS) with two aliphatic amines, namely methyl(2-hydroxyethyl)amine (MHEA) and N,N-dimethyl(2-hydroxypropyl)amine (DM2HPA), has been studied. Viscosimetric and light scattering measurements were performed during reactions in binary solvent mixtures. The observed kinetic deviations have been related with the conformational transformations of the macromolecular chain.  相似文献   
129.
Ionic thermocurrents in SrCl2-K crystals have been investigated for the first time. Two maxima of thermally stimulated depolarization currents have been detected at T=112°K and T=210–240°K. The low-temperature maximum with a reorientation energy =0.25 eV and a frequency factor o=7.1·109 Hz is due to relaxation of impurity-vacancy dipoles. Its intensity grows linearly with the concentration of the dopand. The high-temperature maximum is due to the space charge. Its activation energy =0.46 eV coincides with the migration energy of anion vacancies associated with the potassium ions.Translated from Izvestiya Vysshikh Zavedenii, Fizika, No. 9, pp. 80–83, September, 1982.  相似文献   
130.
The reaction of chloromethylated polystyrene with tris(2-hydroxyethyl)amine in N,N-dimethylformamide is described for the conditions to prepare soluble reaction products. The groups of the quaternary ammonium salt, which appear in the first stage of the reaction, transpose to the amino-ether groups. The reaction was followed by elementary analysis, IR and 1H-NMR spectra, and viscosimetric measurements for nondialyzed and dialyzed samples. The presence of the tertiary amine groups on obtained polymers was also shown by titration. The polymers from the reaction of chloromethylated polystyrene with tris(2-hydroxyethyl)amine reacted easily with benzyl chloride.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号