全文获取类型
收费全文 | 4086篇 |
免费 | 72篇 |
国内免费 | 25篇 |
专业分类
化学 | 2580篇 |
晶体学 | 22篇 |
力学 | 118篇 |
数学 | 784篇 |
物理学 | 679篇 |
出版年
2021年 | 31篇 |
2020年 | 40篇 |
2019年 | 36篇 |
2018年 | 40篇 |
2017年 | 33篇 |
2016年 | 52篇 |
2015年 | 57篇 |
2014年 | 85篇 |
2013年 | 242篇 |
2012年 | 179篇 |
2011年 | 194篇 |
2010年 | 115篇 |
2009年 | 122篇 |
2008年 | 200篇 |
2007年 | 196篇 |
2006年 | 214篇 |
2005年 | 190篇 |
2004年 | 177篇 |
2003年 | 141篇 |
2002年 | 132篇 |
2001年 | 64篇 |
2000年 | 61篇 |
1999年 | 45篇 |
1998年 | 49篇 |
1997年 | 59篇 |
1996年 | 68篇 |
1995年 | 50篇 |
1994年 | 45篇 |
1993年 | 49篇 |
1992年 | 63篇 |
1991年 | 53篇 |
1990年 | 36篇 |
1989年 | 40篇 |
1988年 | 31篇 |
1987年 | 42篇 |
1986年 | 30篇 |
1985年 | 69篇 |
1984年 | 69篇 |
1983年 | 50篇 |
1982年 | 66篇 |
1981年 | 53篇 |
1980年 | 62篇 |
1979年 | 65篇 |
1978年 | 68篇 |
1977年 | 65篇 |
1976年 | 50篇 |
1975年 | 56篇 |
1974年 | 52篇 |
1973年 | 57篇 |
1971年 | 19篇 |
排序方式: 共有4183条查询结果,搜索用时 281 毫秒
21.
22.
23.
Guantao Chen Ronald J. Gould Michael S. Jacobson Richard H. Schelp 《Journal of Graph Theory》1994,18(7):663-672
For several years, the study of neighborhood unions of graphs has given rise to important structural consequences of graphs. In particular, neighborhood conditions that give rise to hamiltonian cycles have been considered in depth. In this paper we generalize these approaches to give a bound on the smallest number of cycles in G containing all the vertices of G. We show that if for all x, y ? V(G), |N(x) ∩ N(y)| ≧ 2n/5 + 1, then V(G) is coverable by at most two cycles. Several related results and extensions to t cycles are also given. 相似文献
24.
In a double groupoid S, we show that there is a canonical groupoid structure on the set of those squares of S for which the two source edges are identities; we call this the core groupoid of S. The target maps from the core groupoid to the groupoids of horizontal and vertical edges of S are now base-preserving morphisms whose kernels commute, and we call the diagram consisting of the core groupoid and these two morphisms the core diagram of S. If S is a double Lie groupoid, and each groupoid structure on S satisfies a natural double form of local triviality, we show that the core diagram determines S and, conversely, that a locally trivial double Lie groupoid may be constructed from an abstractly given core diagram satisfying some natural additional conditions.
In the algebraic case, the corresponding result includes the known equivalences between crossed modules, special double groupoids with special connection (Brown and Spencer), and cat1-groups (Loday). These cases correspond to core diagrams for which both target morphisms are (compatibly) split surjections. 相似文献
25.
26.
27.
The thermal decomposition of formaldehyde was investigated behind shock waves at temperatures between 1675 and 2080 K. Quantitative concentration time profiles of formaldehyde and formyl radicals were measured by means of sensitive 174 nm VUV absorption (CH2O) and 614 nm FM spectroscopy (HCO), respectively. The rate constant of the radical forming channel (1a), CH2O + M → HCO + H + M, of the unimolecular decomposition of formaldehyde in argon was measured at temperatures from 1675 to 2080 K at an average total pressure of 1.2 bar, k1a = 5.0 × 1015 exp(‐308 kJ mol?1/RT) cm3 mol?1 s?1. The pressure dependence, the rate of the competing molecular channel (1b), CH2O + M → H2 + CO + M, and the branching fraction β = k1a/(kA1a + k1b) was characterized by a two‐channel RRKM/master equation analysis. With channel (1b) being the main channel at low pressures, the branching fraction was found to switch from channel (1b) to channel (1a) at moderate pressures of 1–50 bar. Taking advantage of the results of two preceding publications, a decomposition mechanism with six reactions is recommended, which was validated by measured formyl radical profiles and numerous literature experimental observations. The mechanism is capable of a reliable prediction of almost all formaldehyde pyrolysis literature data, including CH2O, CO, and H atom measurements at temperatures of 1200–3200 K, with mixtures of 7 ppm to 5% formaldehyde, and pressures up to 15 bar. Some evidence was found for a self‐reaction of two CH2O molecules. At high initial CH2O mole fractions the reverse of reaction (6), CH2OH + HCO ? CH2O + CH2O becomes noticeable. The rate of the forward reaction was roughly measured to be k6 = 1.5 × 1013 cm3 mol?1 s?1. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 157–169 2004 相似文献
28.
Constants of Motion for Several One-Dimensional Systems and Problems Associated with Getting Their Hamiltonians 总被引:1,自引:1,他引:0
G. López L. A. Barrera Y. Garibo H. Hernández J. C. Salazar C. A. Vargas 《International Journal of Theoretical Physics》2004,43(10):2009-2021
The constants of motion of the following systems are deduced: a relativistic particle with linear dissipation; a no-relativistic particle with a time explicitly depending force; a no-relativistic particle with a constant force and time depending mass; and a relativistic particle under a conservative force with position depending mass. The Hamiltonian for these systems, which is determined by getting the velocity as a function of position and generalized linear momentum, can be found explicitly at first approximation for the first system. The Hamiltonians for the other systems are kept implicitly in their expressions for their constants of motion. 相似文献
29.
Ronald W. Clawson Jr. 《Tetrahedron letters》2007,48(34):6019-6021
An expedient synthesis of the alkaloid koniamborine, the only to date isolated naturally occurring pyrano[3,2-b]indole is presented. The key pyrano[3,2-b]indole forming step is a palladium-catalyzed reductive N-heteroannulation of 2-(4-methoxy-2-nitrophenyl)-(4H)-pyrane-4-one. 相似文献
30.
Dendritic cellls (DCs) comprise an essential component of the immune system. These cells, as antigen presenting cells (APCs)
to na?ve T cells, are crucial in the initiation of antigen specific immune responses. In the past years, several DC subsets
have been identified in different organs which exert different effects in order to elicit adaptive immune responses. Thus,
identification of such DC subsets has led to a better understanding of their distribution and function in the body. In this
review, several key properties of the immunobiology, immunopathogenesis and vaccine strategies using DCs will be discussed. 相似文献