全文获取类型
收费全文 | 53776篇 |
免费 | 1082篇 |
国内免费 | 411篇 |
专业分类
化学 | 28565篇 |
晶体学 | 470篇 |
力学 | 1848篇 |
综合类 | 12篇 |
数学 | 12118篇 |
物理学 | 12256篇 |
出版年
2023年 | 178篇 |
2022年 | 265篇 |
2021年 | 380篇 |
2020年 | 500篇 |
2019年 | 478篇 |
2018年 | 1486篇 |
2017年 | 1702篇 |
2016年 | 1419篇 |
2015年 | 1142篇 |
2014年 | 1187篇 |
2013年 | 1837篇 |
2012年 | 4660篇 |
2011年 | 4034篇 |
2010年 | 2775篇 |
2009年 | 2357篇 |
2008年 | 2004篇 |
2007年 | 1972篇 |
2006年 | 1949篇 |
2005年 | 5653篇 |
2004年 | 4965篇 |
2003年 | 3111篇 |
2002年 | 1351篇 |
2001年 | 711篇 |
2000年 | 452篇 |
1999年 | 535篇 |
1998年 | 466篇 |
1997年 | 402篇 |
1996年 | 390篇 |
1995年 | 330篇 |
1994年 | 361篇 |
1993年 | 319篇 |
1992年 | 427篇 |
1991年 | 342篇 |
1990年 | 351篇 |
1989年 | 291篇 |
1988年 | 293篇 |
1987年 | 228篇 |
1986年 | 202篇 |
1985年 | 308篇 |
1984年 | 285篇 |
1983年 | 209篇 |
1982年 | 218篇 |
1981年 | 203篇 |
1980年 | 194篇 |
1979年 | 193篇 |
1978年 | 219篇 |
1977年 | 198篇 |
1976年 | 215篇 |
1974年 | 195篇 |
1973年 | 173篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
91.
The Nambu-bracket quantization of the hydrogen atom is worked out as an illustration of the general method. The dynamics of topological open branes is controlled classically by Nambu brackets. Such branes then may be quantized through the consistent quantization of the underlying Nambu brackets: properly defined, the quantum Nambu-brackets comprise an associative structure, although the naive derivation property is mooted through operator entwinement. For superintegrable systems, such as the hydrogen atom, the results coincide with those furnished by Hamiltonian quantization - but the method is not limited to Hamiltonian systems. 相似文献
92.
Ian R. Butler Alfonso G. Callabero Jennifer R. Amey Dafydd A. Thomas Thomas Gelbrich 《Tetrahedron letters》2004,45(3):467-472
The synthesis of a range of ferrocene-substituted ethynylanthracenes and 1,1′-(bis-(2-ethenylanthraquinoyl)ferrocene has been achieved. The synthesis relies on the production of ferrocenylanthraquinones as key precursors. The products were obtained in the reactions of ferrocenylanthraquinones with phenylethynyllithium or trimethylsilylethynyllithium followed by reduction with tin chloride. The key products such as 1- and 2-ferrocenyl(9,10-bis-phenylethynyl)anthracenes have been characterised by X-ray single crystal diffraction. 相似文献
93.
The reaction of a new fluorinated unsymmetrical allenylazine with dimethyl acetylenedicarboxylate and phenylacetylene affords the combined intra-intermolecular criss-cross cycloaddition products, 2,3-disubstituted-1,10-diazatricyclo[5.2.1.04,10]deca-2,6-diene derivatives. The products contain three fused five-membered rings with two nitrogen atoms within an unsaturated heterocyclic system. The structures were assigned using 2D NMR correlations and in the case of the phenylacetylene adduct by X-ray structure analysis. 相似文献
94.
Ewa Stró?yna 《Journal of Differential Equations》2003,193(1):239-259
We solve completely the problem of classification of germs of complex planar vector fields with nilpotent singularity with respect to formal orbital equivalence. 相似文献
95.
Jacques Rouzaud MatthewD. Jones Robert Raja BrianF.G. Johnson John MeurigThomas MelindaJ. Duer 《Helvetica chimica acta》2003,86(5):1753-1759
A set of new, air‐stable, RhI‐based heterogeneous asymmetric hydrogenation catalysts have been synthesised, characterised, and tested. Individual members of this new family all exhibit good enantioselectivity. 相似文献
96.
Parameters of Gaussian multivariate models are often estimated using the maximum likelihood approach. In spite of its merits, this methodology is not practical when the sample size is very large, as, for example, in the case of massive georeferenced data sets. In this paper, we study the asymptotic properties of the estimators that minimize three alternatives to the likelihood function, designed to increase the computational efficiency. This is achieved by applying the information sandwich technique to expansions of the pseudo-likelihood functions as quadratic forms of independent normal random variables. Theoretical calculations are given for a first-order autoregressive time series and then extended to a two-dimensional autoregressive process on a lattice. We compare the efficiency of the three estimators to that of the maximum likelihood estimator as well as among themselves, using numerical calculations of the theoretical results and simulations. 相似文献
97.
In this paper, by using elementary analysis, we establish some new Lyapunov-type inequalities for nonlinear systems of differential equations, special cases of which contain the well-known equations such as Emden-Fowler-type and half-linear equations. The inequalities obtained here can be used as handy tools in the study of qualitative behaviour of solutions of the associated equations. 相似文献
98.
A procedure for the generation of slanted gas-filled icicles by freezing, using a domestic refrigerator, is described. The freezing vessel was a plastic ice-cube tray, which was filled both with tap and deionized water and was frozen successively from the outer to the inner compartments of the tray. Icicles having slanted elevations grew out of the surface of the deionized water of the innermost compartments. The erection angle of the icicles to the horizontal lay between 30° and 60°, for the three longest and thinnest specimens it was almost exactly 30°. All icicles have gas inclusions. Their shape varies between an irregular distribution of circular bubbles and a nearly uninterrupted axial gas channel together with dendrite-like, radially distorted bubbles. If a cold (-18°C) specimen comes into contact with warm and humid room-air, then hoarfrost is observed at the bottom and the top of the icicle, while the area in between remains transparent. 相似文献
99.
100.
We give a number of characterizations of bodies of constant width in arbitrary dimension. As an application, we describe a way to construct a body of constant width in dimension n, one of its (n – 1)‐dimensional projection being given. We give a number of examples, like a four‐dimensional body of constant width whose 3D‐projection is the classical Meissner's body. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献