全文获取类型
收费全文 | 225571篇 |
免费 | 2490篇 |
国内免费 | 709篇 |
专业分类
化学 | 107877篇 |
晶体学 | 3294篇 |
力学 | 7825篇 |
综合类 | 4篇 |
数学 | 18381篇 |
物理学 | 60253篇 |
无线电 | 31136篇 |
出版年
2016年 | 2511篇 |
2015年 | 1804篇 |
2014年 | 2642篇 |
2013年 | 8325篇 |
2012年 | 5589篇 |
2011年 | 6984篇 |
2010年 | 4954篇 |
2009年 | 5179篇 |
2008年 | 7048篇 |
2007年 | 7541篇 |
2006年 | 7324篇 |
2005年 | 6745篇 |
2004年 | 6179篇 |
2003年 | 5589篇 |
2002年 | 5500篇 |
2001年 | 7038篇 |
2000年 | 5593篇 |
1999年 | 4580篇 |
1998年 | 3804篇 |
1997年 | 3830篇 |
1996年 | 3739篇 |
1995年 | 3464篇 |
1994年 | 3331篇 |
1993年 | 3220篇 |
1992年 | 3717篇 |
1991年 | 3608篇 |
1990年 | 3422篇 |
1989年 | 3446篇 |
1988年 | 3300篇 |
1987年 | 2937篇 |
1986年 | 2757篇 |
1985年 | 3646篇 |
1984年 | 3658篇 |
1983年 | 3071篇 |
1982年 | 3211篇 |
1981年 | 3149篇 |
1980年 | 3013篇 |
1979年 | 3116篇 |
1978年 | 3355篇 |
1977年 | 3146篇 |
1976年 | 3110篇 |
1975年 | 2928篇 |
1974年 | 2874篇 |
1973年 | 2899篇 |
1972年 | 1900篇 |
1971年 | 1608篇 |
1968年 | 2040篇 |
1967年 | 2211篇 |
1966年 | 2018篇 |
1965年 | 1581篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
A new technique for mixing optical waves to generate microwave-frequency signals using an injection laser diode and a fibre-optic interferometer is reported. A beat signal is produced by interfering light from the laser with light from the same laser emitted earlier at a different frequency. The interferometer consists of an evanescent-field fibre coupler joined to a length of single-mode optical fibre. The laser is tuned by pulsed or bipolar current waveforms superimposed on a DC bias current. Feedback from an external cavity stabilises the laser frequency and reduces its linewidth. Efficient mixing at difference frequencies from 250 MHz to 2 GHz is obtained, with spectral widths of less than 1% of the centre frequency. 相似文献
132.
133.
A method using third order moments for estimating the regression coefficients as well as the latent state scores of the reduced-rank regression model when the latent variable(s) are non-normally distributed is presented in this paper. It is shown that the factor analysis type indeterminacy of the regression coefficient matrices is eliminated. A real life example of the proposed method is presented. Differences of this solution with the reduced-rank regression eigen solution are discussed. 相似文献
134.
Exact closed-form solutions are exhibited for the Hopf equation for stationary incompressible 3D Navier-Stokes flow, for the cases of homogeneous forced flow (including a solution with depleted nonlinearity) and inhomogeneous flow with arbitrary boundary conditions. This provides an exact method for computing two- and higher-point moments, given the mean flow. 相似文献
135.
The effect of secondary scattering that is produced by the induced current in an infinitely long, finitely conducting wire over a plane, finitely conducting earth from an incident electromagnetic pulse is investigated. This is accomplished by finding the field of a Hertzian dipole over the earth and identifying the part that is due to secondary scattering. Frequency-domain results for the overall induced current require that certain infinite integrals be numerically evaluated. Fourier inversion to the time domain in turn requires the numerical evaluation of another infinite integral. When the results are compared to those obtained when secondary scattering is ignored, it is found that secondary scattering slightly increases the current for the case of broadside incidence and reduces it for the case of grazing incidence by as much as two orders of magnitude. Peak currents are much smaller than previously thought. These general results hold for all of the earth parameters that were used in the study 相似文献
136.
P. David J. Hartfiel H. Janszen R. von Mutius J. Arvieux L. Farvacque B. Berthier B. Bonin J. C. Lugol 《Zeitschrift für Physik A Hadrons and Nuclei》1987,326(4):367-372
The excitation function of the fission probability P E E x) for238U has been measured in the reaction238U(α, α′ f) at 480 MeV bombarding energy. The reaction mechanism of this reaction is discussed for excitation energies belowB nf , the threshold for second chance fission, and aboveB nf up toE x =37 MeV. In comparing with results from fission induced by photons and by particle transfer reactions the (α, α′f) reaction gives too low values for the fission probabilityP f at excitation energies well aboveB nE . The role of the quasi-elastic knock-out process in this reaction is discussed. 相似文献
137.
H. Mazaheri 《分析论及其应用》2006,22(2):141-145
The concepts of quasi-Chebyshev and weakly-Chebyshev and σ-Chebyshev were defined [3 - 7], andas a counterpart to best approximation in normed linear spaces, best coapprozimation was introduced by Franchetti and Furi^[1]. In this research, we shall define τ-Chebyshev subspaces and τ-cochebyshev subspaces of a Banach space, in which the property τ is compact or weakly-compact, respectively. A set of necessary and sufficient theorems under which a subspace is τ-Chebyshev is defined. 相似文献
138.
J. H. Eberly 《Laser Physics》2006,16(6):921-926
We examine the application of Schmidt mode analysis to pure-state entanglement. Several examples permitting exact analytic calculation of Schmidt eigenvalues and eigenfunctions are included, as well as evaluation of the associated degree of entanglement. 相似文献
139.
Johan Håstad 《Random Structures and Algorithms》2006,29(4):466-474
We show that the operations of permuting columns and rows separately and independently mix a square matrix in constant time. © 2006 Wiley Periodicals, Inc. Random Struct. Alg., 2006 相似文献
140.
We study the asymptotic properties of the Bayes estimator in models differentiable in quadratic mean (DQM) in the case of
independent and identically distributed observations. The goal is to define weak assumptions on the model under which this
estimator is asymptotically efficient, regular, and asymptotically of minimal risk. The results of the paper are applied to
models based on a mixture distribution, the Cauchy distribution with location and scale parameter, and the Weibull distribution.
Bibliography: 10 titles.
Published in Zapiski Nauchnykh Seminarov POMI, Vol. 328, 2005, pp. 114–146. 相似文献