全文获取类型
收费全文 | 250501篇 |
免费 | 2760篇 |
国内免费 | 1117篇 |
专业分类
化学 | 118235篇 |
晶体学 | 3274篇 |
力学 | 14271篇 |
综合类 | 7篇 |
数学 | 49479篇 |
物理学 | 69112篇 |
出版年
2021年 | 1023篇 |
2020年 | 1147篇 |
2019年 | 1181篇 |
2018年 | 11374篇 |
2017年 | 11166篇 |
2016年 | 8302篇 |
2015年 | 2809篇 |
2014年 | 2840篇 |
2013年 | 8244篇 |
2012年 | 10110篇 |
2011年 | 18281篇 |
2010年 | 10424篇 |
2009年 | 10972篇 |
2008年 | 13823篇 |
2007年 | 15905篇 |
2006年 | 7402篇 |
2005年 | 8015篇 |
2004年 | 7513篇 |
2003年 | 7162篇 |
2002年 | 6102篇 |
2001年 | 6323篇 |
2000年 | 4882篇 |
1999年 | 3677篇 |
1998年 | 2837篇 |
1997年 | 2810篇 |
1996年 | 2833篇 |
1995年 | 2477篇 |
1994年 | 2309篇 |
1993年 | 2173篇 |
1992年 | 2442篇 |
1991年 | 2424篇 |
1990年 | 2112篇 |
1989年 | 2124篇 |
1988年 | 2151篇 |
1987年 | 2061篇 |
1986年 | 1973篇 |
1985年 | 2891篇 |
1984年 | 2882篇 |
1983年 | 2299篇 |
1982年 | 2513篇 |
1981年 | 2389篇 |
1980年 | 2345篇 |
1979年 | 2278篇 |
1978年 | 2340篇 |
1977年 | 2280篇 |
1976年 | 2207篇 |
1975年 | 2188篇 |
1974年 | 2076篇 |
1973年 | 2160篇 |
1972年 | 1234篇 |
排序方式: 共有10000条查询结果,搜索用时 426 毫秒
141.
A simple scheme is developed for treatment of vertical bed topography in shallow water flows. The effect of the vertical step on flows is modelled with the shallow water equations including local energy loss terms. The bed elevation is denoted with zb‐ for the left and zb+ for the right values at each grid point, hence exactly representing a discontinuity in the bed topography. The surface gradient method (SGM) is generalized to reconstruct water depths at cell interfaces involving a vertical step so that the fluxes at the cell interfaces can accurately be calculated with a Riemann solver. The scheme is verified by predicting a surge crossing a step, a tidal flow over a step and dam‐break flows on wet/dry beds. The results have shown good agreements compared with analytical solutions and available experimental data. The scheme is efficient, robust, and may be used for practical flow calculations. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
142.
A comprehensive study of the excited electronic states of HCl is reported. Using resonance enhanced multiphoton ionization ((2 + 1) REMPI) and time-of-flight mass spectrometry (TOFMS) over 120 band systems are analyzed. Supersonic jet techniques are used to prepare rotationally cold species for laser spectroscopy in the 77 000 to 96 000 cm−1 region. At least 50 new electronic states are characterized as well as several features only tentatively assigned previously. A long vibrational progression consisting of 29 vibrational levels of the deeply bound V1Σ+(0+) valence/ion-pair state is studied. We also extend the identification and analysis to high vibrational levels of low-lying Rydberg states. The assignments of [2Πi] Rydberg state complexes are evaluated in terms of spin-orbit coupling and united-atom calculations. In several band systems, the spectra exhibit anomalous rotational line intensities and broadened linewidths which are consistent with predissociation. Multiphoton ionization with mass spectrometry permits the investigation of isotope effects as well as the appearance of fragment species associated with repulsive potential curves. 相似文献
143.
In the following we find all of the three-dimensional flat spacetimes which are static and rotationally symmetric. 相似文献
144.
V. De Sabbata C. Sivaram H.-H. v. Borzeszkowski H.-J. Treder 《Annalen der Physik》1991,503(7):497-502
It is shown that in gravitational theories with torsion one is led to commutation rules corresponding to Landau-Peierls type uncertainty relations. 相似文献
145.
We consider a randomized version of the greedy algorithm for finding a large matching in a graph. We assume that the next edge is always randomly chosen from those remaining. We analyze the performance of this algorithm when the input graph is fixed. We show that there are graphs for which this Randomized Greedy Algorithm (RGA) usually only obtains a matching close in size to that guaranteed by worst-case analysis (i.e., half the size of the maximum). For some classes of sparse graphs (e.g., planar graphs and forests) we show that the RGA performs significantly better than the worst-case. Our main theorem concerns forests. We prove that the ratio to maximum here is at least 0.7690…, and that this bound is tight. 相似文献
146.
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
147.
148.
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
149.
Graham Smith 《The Journal of the Operational Research Society》1989,40(9):761-770
Courses which teach discrete-event simulation are based on many different simulation languages. The requirements for a language to support teaching simulation are discussed. In particular, it is recommended that such languages separate into distinct modules those aspects of simulation which are taught as separate topics. Implementation of the separation is discussed. The SEESIM language, developed as a teaching aid, is described, and examples of its use are given. Straightforward use of SEESIM can be learned quickly, yet the language provides facilities for a staged introduction to advanced concepts of simulation. 相似文献
150.
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