全文获取类型
收费全文 | 38774篇 |
免费 | 357篇 |
国内免费 | 294篇 |
专业分类
化学 | 16290篇 |
晶体学 | 419篇 |
力学 | 1665篇 |
综合类 | 12篇 |
数学 | 9763篇 |
物理学 | 11276篇 |
出版年
2022年 | 158篇 |
2021年 | 191篇 |
2020年 | 210篇 |
2019年 | 281篇 |
2018年 | 1404篇 |
2017年 | 1683篇 |
2016年 | 1031篇 |
2015年 | 766篇 |
2014年 | 875篇 |
2013年 | 1456篇 |
2012年 | 3688篇 |
2011年 | 2955篇 |
2010年 | 2216篇 |
2009年 | 1942篇 |
2008年 | 1012篇 |
2007年 | 1047篇 |
2006年 | 929篇 |
2005年 | 4701篇 |
2004年 | 4129篇 |
2003年 | 2461篇 |
2002年 | 660篇 |
2001年 | 407篇 |
2000年 | 213篇 |
1999年 | 262篇 |
1998年 | 169篇 |
1997年 | 129篇 |
1996年 | 136篇 |
1995年 | 148篇 |
1994年 | 139篇 |
1993年 | 120篇 |
1992年 | 291篇 |
1991年 | 242篇 |
1990年 | 216篇 |
1989年 | 207篇 |
1988年 | 192篇 |
1987年 | 148篇 |
1986年 | 118篇 |
1985年 | 146篇 |
1984年 | 159篇 |
1983年 | 116篇 |
1982年 | 123篇 |
1981年 | 121篇 |
1980年 | 123篇 |
1979年 | 129篇 |
1978年 | 133篇 |
1977年 | 115篇 |
1976年 | 139篇 |
1975年 | 101篇 |
1974年 | 119篇 |
1973年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
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
52.
Synchronized whistlers recorded at Varanasi 总被引:1,自引:0,他引:1
Some interesting events of synchronized whistlers recorded at low latitude station Varanasi during magnetic storm period of
the year 1977 are presented. The dynamic spectrum analysis shows that the component whistlers are Eckersley whistlers having
dispersion 10 s1/2 and 30 s1/2. An attempt has been made to explain the dynamic spectra using lightning discharge generated from magnetospheric sources 相似文献
53.
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
54.
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
55.
56.
E. Beinrohr M. Németh P. Tsch?pel G. T?lg 《Fresenius' Journal of Analytical Chemistry》1992,344(3):93-99
Summary Flow-through electrochemical cells with porous working electrodes made of crushed reticulated vitreous carbon and plated with mercury were used for absolute analysis of trace amounts of lead by anodic stripping coulometry with collection (ASCWC) in a flow system. The role of mercury coating, flow rate and pH were investigated. The coulombic content of the collection peak corresponded to the theoretical values calculated by Faraday's law in a concentration range from about 10–9 to 10–6 mol/l. The relative error and the relative standard deviation was +0.15% and 0.8%, respectively for 2×10–6 mol/l analyte concentration. The absolute detection limit (3 s) was 0.1 ng of Pb, the linear response range 7×104.
One leave from: Department of Analytical Chemistry, Slovak Technical University, CS-812 37 Bratislava, Czechoslovakia 相似文献
57.
Celik Tarimci Anda? Karamustafa Kutalmi? Güven Mehmet Kabak Akgül Ye?ilada 《Analytical sciences》2003,19(9):1347-1348
The crystal structure of 1-[3-(3,4-dimethoxyphenyl)-2-propenoyl]pyrrolidine (C15H19NO3) (I) has been determined by X-ray analysis. It crystallizes orthorhombic space group Pbca with a = 24.295(3), b = 15.086(3), c = 7.552(3)A, V = 2768(1)A3, Z = 8, Dcalc = 1.254 g/cm3, mu = (Mo K(alpha)) = 0.87 cm(-1). The title compound has analgesic activity of cycloaliphatic amine part. The molecule is deviated from planar configuration. 相似文献
58.
Shyam K. Singh Oliver S. Fetzer John B. Hynes Thomas C. Williams 《Journal of heterocyclic chemistry》1991,28(5):1459-1461
Nitration of 2-amino-4-oxo-(3H)-5-trifluoromethylquinazoline is shown to occur exclusively at C6 as determined from an analysis of long range 1H and 19F scalar couplings to ring carbons. Nitration of 2-amino-4-oxo-(3H)-5-fluoroquinazoline is found to occur both at C6 and C8 as evident from an analysis of the 19F and 1H couplings of the ring protons. 相似文献
59.
We introduce and investigate the properties of Hochschild cohomology of algebras in an abelian monoidal category M. We show that the second Hochschild cohomology group of an algebra in M classifies extensions of A up to an equivalence. We characterize algebras of Hochschild dimension 0 (separable algebras), and of Hochschild dimension ≤1 (formally smooth algebras). Several particular cases and applications are included in the last section of the paper. 相似文献
60.
We prove that directly reducible lattices and selfdual subdirectly irreducible lattices of locally finite length are determined by their sublattice-lattices. As a corollary we obtain that splitting varieties are closed under the isomorphism of sublattice-lattices iff they are selfdual. A class of selfdual non-closed varieties is given too. 相似文献