全文获取类型
收费全文 | 194426篇 |
免费 | 2805篇 |
国内免费 | 846篇 |
专业分类
化学 | 108738篇 |
晶体学 | 2596篇 |
力学 | 8052篇 |
综合类 | 7篇 |
数学 | 20785篇 |
物理学 | 57899篇 |
出版年
2021年 | 1321篇 |
2020年 | 1559篇 |
2019年 | 1553篇 |
2018年 | 1446篇 |
2017年 | 1421篇 |
2016年 | 3024篇 |
2015年 | 2467篇 |
2014年 | 3183篇 |
2013年 | 8817篇 |
2012年 | 7708篇 |
2011年 | 9555篇 |
2010年 | 5834篇 |
2009年 | 5845篇 |
2008年 | 8657篇 |
2007年 | 8550篇 |
2006年 | 8260篇 |
2005年 | 7691篇 |
2004年 | 6855篇 |
2003年 | 5797篇 |
2002年 | 5751篇 |
2001年 | 6406篇 |
2000年 | 4899篇 |
1999年 | 3803篇 |
1998年 | 2943篇 |
1997年 | 2922篇 |
1996年 | 2878篇 |
1995年 | 2534篇 |
1994年 | 2449篇 |
1993年 | 2298篇 |
1992年 | 2579篇 |
1991年 | 2472篇 |
1990年 | 2218篇 |
1989年 | 2181篇 |
1988年 | 2200篇 |
1987年 | 2114篇 |
1986年 | 2009篇 |
1985年 | 3008篇 |
1984年 | 2973篇 |
1983年 | 2357篇 |
1982年 | 2566篇 |
1981年 | 2433篇 |
1980年 | 2393篇 |
1979年 | 2325篇 |
1978年 | 2418篇 |
1977年 | 2358篇 |
1976年 | 2256篇 |
1975年 | 2232篇 |
1974年 | 2166篇 |
1973年 | 2218篇 |
1972年 | 1274篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
A. J. Maroulis P. D. Akrivos C. P. Hadjiantoniou-Maroulis 《Journal of heterocyclic chemistry》1993,30(4):913-916
Oxidation of the title compounds yields, besides the reported isoimides 3 and/or the amides 4 , also the imides 5 . The observed product dichotomy is considered as the result of an intramolecular nucleophilic attack on the aroyl group, of the pressumed zwitterionic intermediate 2 , by O or N present in the ambident N-aroylimine site of 2 . The results of AM1 calculations agree with the product studies and both permit the formulation of a set of rules correlating structure and selectivity. 相似文献
23.
24.
25.
We examine two criteria for balance of a gain graph, one based on binary cycles and one on circles. The graphs for which each criterion is valid depend on the set of allowed gain groups. The binary cycle test is invalid, except for forests, if any possible gain group has an element of odd order. Assuming all groups are allowed, or all abelian groups, or merely the cyclic group of order 3, we characterize, both constructively and by forbidden minors, the graphs for which the circle test is valid. It turns out that these three classes of groups have the same set of forbidden minors. The exact reason for the importance of the ternary cyclic group is not clear. © 2005 Wiley Periodicals, Inc. J Graph Theory 相似文献
26.
Hugh Thomas 《Order》2006,23(2-3):249-269
In this paper, we study lattices that posess both the properties of being extremal (in the sense of Markowsky) and of being left modular (in the sense of Blass and Sagan). We call such lattices trim and show that they posess some additional appealing properties, analogous to those of a distributive lattice. For example, trimness is preserved under taking intervals and suitable sublattices. Trim lattices satisfy a weakened form of modularity. The order complex of a trim lattice is contractible or homotopic to a sphere; the latter holds exactly if the maximum element of the lattice is a join of atoms. Any distributive lattice is trim, but trim lattices need not be graded. The main example of ungraded trim lattices are the Tamari lattices and generalizations of them. We show that the Cambrian lattices in types A and B defined by Reading are trim; we conjecture that all Cambrian lattices are trim. 相似文献
27.
J. C. Fabris S. V. B. Gonçalves P. E. de Souza 《General Relativity and Gravitation》2002,34(12):2111-2126
The mass power spectrum for a Universe dominated by the Chaplygin gas is evaluated numerically from scales of the order of the Hubble horizon to 100 Mpc. The results are compared with a pure baryonic Universe and a cosmological constant model. In all three cases, the spectrum increases with k, the wavenumber of the perturbations. The slope of the spectrum is higher for the baryonic model and smaller for the cosmological constant model, the Chaplygin gas interpolating these two models. The results are analyzed in terms of the sound velocity of the Chaplygin gas and the moment the Universe begins to accelerate. 相似文献
28.
Allen C. HirshfeldPeter Henselder 《Annals of Physics》2002,302(1):59-77
Deformation quantization, which achieves the passage from classical mechanics to quantum mechanics by the replacement of the pointwise multiplication of functions on phase space by the star product, is a powerful tool for treating systems involving bosonic degrees of freedom, both in quantum mechanics and in quantum field theory. In the present paper we show how these methods may be naturally extended to systems involving fermions. In particular we show how supersymmetric quantum mechanics can be formulated in this approach and consider examples involving both non-relativistic and relativistic systems. 相似文献
29.
The Wheeler–DeWitt equation of arbitrary Hartle–Hawking factor ordering for several minisuperspace universe models, such as the pure gravity Friedmann–Robertson–Walker and Taub ones, is mapped onto the dynamics of corresponding classical oscillators. The latter ones are studied by the classical Ermakov invariant method, which is a natural approach in this context. For the more realistic case of a minimally coupled massive scalar field, one can study, within the same type of approach, the corresponding squeezing features as a possible means of describing cosmological evolution. Finally, we comment on the analogy with the accelerator physics. 相似文献
30.