全文获取类型
收费全文 | 35410篇 |
免费 | 293篇 |
国内免费 | 295篇 |
专业分类
化学 | 12637篇 |
晶体学 | 305篇 |
力学 | 1523篇 |
综合类 | 12篇 |
数学 | 9394篇 |
物理学 | 9537篇 |
无线电 | 2590篇 |
出版年
2022年 | 71篇 |
2021年 | 122篇 |
2020年 | 135篇 |
2019年 | 122篇 |
2018年 | 1317篇 |
2017年 | 1577篇 |
2016年 | 900篇 |
2015年 | 717篇 |
2014年 | 643篇 |
2013年 | 1020篇 |
2012年 | 3513篇 |
2011年 | 2712篇 |
2010年 | 2011篇 |
2009年 | 1741篇 |
2008年 | 855篇 |
2007年 | 871篇 |
2006年 | 891篇 |
2005年 | 5000篇 |
2004年 | 4108篇 |
2003年 | 2455篇 |
2002年 | 632篇 |
2001年 | 418篇 |
2000年 | 193篇 |
1999年 | 245篇 |
1998年 | 178篇 |
1997年 | 121篇 |
1996年 | 110篇 |
1995年 | 102篇 |
1994年 | 102篇 |
1993年 | 88篇 |
1992年 | 228篇 |
1991年 | 197篇 |
1990年 | 214篇 |
1989年 | 167篇 |
1988年 | 167篇 |
1987年 | 137篇 |
1986年 | 104篇 |
1985年 | 120篇 |
1984年 | 98篇 |
1983年 | 99篇 |
1982年 | 83篇 |
1981年 | 69篇 |
1980年 | 77篇 |
1979年 | 81篇 |
1978年 | 77篇 |
1976年 | 135篇 |
1975年 | 83篇 |
1974年 | 90篇 |
1973年 | 101篇 |
1972年 | 69篇 |
排序方式: 共有10000条查询结果,搜索用时 612 毫秒
61.
Exploiting information theory for adaptive mobility and resource management in future cellular networks 总被引:2,自引:0,他引:2
We utilize tools from information theory to develop adaptive algorithms for two key problems in cellular networks: location tracking and resource management. The use of information theory is motivated by the fundamental observation that overheads in many aspects of mobile computing can be traced to the randomness or uncertainty in an individual user's movement behavior. We present a model-independent information-theoretic approach for estimating and managing this uncertainty, and relate it to the entropy or information content of the user's movement process. Information-theoretic mobility management algorithms are very simple, yet reduce overhead by /spl sim/80 percent in simulated scenarios by optimally adapting to each individual's movement. These algorithms also allow for flexible tradeoff between location update and paging costs. Simulation results demonstrate how an information-theory-motivated resource provisioning strategy can meet QoS bounds with very small wastage of resources, thus dramatically reducing the overall blocking rate. 相似文献
62.
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
63.
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
64.
65.
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
66.
We demonstrate the phenomenon of Talbot self-image plane shift by tuning the wavelength of the broadband light source. A superluminescent diode (SLD) is used as a broad-band light source and an acousto-optic tunable filter (AOTF) as wavelength-scanning device. A periodic grating is illuminated by the wavelength tuned light of SLD using AOTF and Talbot self-image plane is shifted continuously in the longitudinal direction without mechanically moving the grating. The wavelength-scanning Talbot effect is then implemented for the measurement of arbitrary step-height of discontinuous objects with extended range. The main advantages of the proposed system are non-mechanical scanning, high stability because of it’s common-path geometry and compactness. Since the measurement of the phase is not required the system is free from phase ambiguity problem and therefore, the range of measurement is large as compare to interferometric techniques. 相似文献
67.
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
68.
69.
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
70.
C. B?uerle Yu M. Bunkov S. N. Fisher H. Godfrin G. R. Pickett 《Czechoslovak Journal of Physics》1996,46(Z1):5-6
During studies of the impact of single neutrons on superfluid3He at around 100μK we have measured the thermal energy released in the superfluid following the nuclear reaction, n+3He→p+3H+764keV. However, we find that the measured energy deposited in the quasiparticle system shortly after this reaction is substantially
less than what we expect. We believe that this energy deficit represents that energy which has gone into the creation of vortices,
in analogy to the creation of cosmic strings in the early universe via the Kibble mechanism. Furthermore, from the magnitude
of the deficit, we can infer the vortex density which compares favorably with that predicted in Zurek’s scenairo. 相似文献