全文获取类型
收费全文 | 4143篇 |
免费 | 50篇 |
国内免费 | 22篇 |
专业分类
化学 | 2646篇 |
晶体学 | 20篇 |
力学 | 99篇 |
数学 | 771篇 |
物理学 | 679篇 |
出版年
2020年 | 31篇 |
2019年 | 27篇 |
2018年 | 34篇 |
2017年 | 37篇 |
2016年 | 38篇 |
2015年 | 37篇 |
2014年 | 65篇 |
2013年 | 215篇 |
2012年 | 165篇 |
2011年 | 200篇 |
2010年 | 106篇 |
2009年 | 101篇 |
2008年 | 201篇 |
2007年 | 202篇 |
2006年 | 214篇 |
2005年 | 197篇 |
2004年 | 192篇 |
2003年 | 157篇 |
2002年 | 143篇 |
2001年 | 65篇 |
2000年 | 54篇 |
1999年 | 42篇 |
1998年 | 47篇 |
1997年 | 63篇 |
1996年 | 75篇 |
1995年 | 50篇 |
1994年 | 52篇 |
1993年 | 54篇 |
1992年 | 61篇 |
1991年 | 52篇 |
1990年 | 39篇 |
1989年 | 42篇 |
1988年 | 37篇 |
1987年 | 43篇 |
1986年 | 31篇 |
1985年 | 75篇 |
1984年 | 76篇 |
1983年 | 50篇 |
1982年 | 67篇 |
1981年 | 61篇 |
1980年 | 67篇 |
1979年 | 77篇 |
1978年 | 81篇 |
1977年 | 66篇 |
1976年 | 58篇 |
1975年 | 59篇 |
1974年 | 56篇 |
1973年 | 63篇 |
1971年 | 20篇 |
1969年 | 21篇 |
排序方式: 共有4215条查询结果,搜索用时 15 毫秒
21.
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
22.
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
23.
24.
25.
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
26.
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
27.
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
28.
Colin J. Thompson Keith M. Briggs Peter Farrell Anthony Fleming Bruce Hocking Kenneth Joyner Vitas Anderson Andrew W. Wood 《Physica A》1995,220(3-4):471-484
A nonlinear Lorenz model describing interactions between charged particles and combined ac-dc electromagnetic fields is studied for various combinations of frequencies, field strengths and relative angle (θ) between the ac and dc magnetic fields. Strong directional effects on the magnitude and location of resonant particle motion are observed when θ is varied and the regular resonance windows in the aligned field (θ = 0) and linear version of the model studied previously by Durney etaal., break up to form irregular and less well pronounced regions of large and small particle displacements when nonlinearities are taken into account. The length of time takne to achieve resonant behaviour also becomes larger and more variable when nonlinearities are present. The possible relevance of these effects to interactions between electromagnetic fields and biological media is briefly discussed. 相似文献
29.
Ross W. Mair David G. Cory Sharon Peled Ching-Hua Tseng Samuel Patz Ronald L. Walsworth 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,135(2):478-486
Pulsed-field-gradient NMR techniques are demonstrated for measurements of time-dependent gas diffusion. The standard PGSE technique and variants, applied to a free gas mixture of thermally polarized xenon and O2, are found to provide a reproducible measure of the xenon diffusion coefficient (5.71 × 10−6m2s−1for 1 atm of pure xenon), in excellent agreement with previous, non-NMR measurements. The utility of pulsed-field-gradient NMR techniques is demonstrated by the first measurement of time-dependent (i.e., restricted) gas diffusion inside a porous medium (a random pack of glass beads), with results that agree well with theory. Two modified NMR pulse sequences derived from the PGSE technique (named the Pulsed Gradient Echo, or PGE, and the Pulsed Gradient Multiple Spin Echo, or PGMSE) are also applied to measurements of time dependent diffusion of laser polarized xenon gas, with results in good agreement with previous measurements on thermally polarized gas. The PGMSE technique is found to be superior to the PGE method, and to standard PGSE techniques and variants, for efficiently measuring laser polarized noble gas diffusion over a wide range of diffusion times. 相似文献
30.
Ronald I. Becker Isabella Lari Andrea Scozzari Giovanni Storchi 《Annals of Operations Research》2007,150(1):65-78
In this paper we consider the location of a path shaped facility on a grid graph. In the literature this problem was extensively
studied on particular classes of graphs as trees or series-parallel graphs. We consider here the problem of finding a path
which minimizes the sum of the (shortest) distances from it to the other vertices of the grid, where the path is also subject
to an additional constraint that takes the form either of the length of the path or of the cardinality. We study the complexity
of these problems and we find two polynomial time algorithms for two special cases, with time complexity of O(n) and O(nℓ) respectively, where n is the number of vertices of the grid and ℓ is the cardinality of the path to be located.
The literature about locating dimensional facilities distinguishes between the location of extensive facilities in continuous
spaces and network facility location. We will show that the problems presented here have a close connection with continuous
dimensional facility problems, so that the procedures provided can also be useful for solving some open problems of dimensional
facilities location in the continuous case. 相似文献