全文获取类型
收费全文 | 91179篇 |
免费 | 609篇 |
国内免费 | 432篇 |
专业分类
化学 | 31756篇 |
晶体学 | 918篇 |
力学 | 7193篇 |
数学 | 34234篇 |
物理学 | 18119篇 |
出版年
2022年 | 194篇 |
2021年 | 221篇 |
2020年 | 225篇 |
2019年 | 245篇 |
2018年 | 10571篇 |
2017年 | 10368篇 |
2016年 | 6337篇 |
2015年 | 1111篇 |
2014年 | 620篇 |
2013年 | 1157篇 |
2012年 | 4361篇 |
2011年 | 11122篇 |
2010年 | 6052篇 |
2009年 | 6427篇 |
2008年 | 7126篇 |
2007年 | 9216篇 |
2006年 | 692篇 |
2005年 | 1699篇 |
2004年 | 1947篇 |
2003年 | 2297篇 |
2002年 | 1352篇 |
2001年 | 503篇 |
2000年 | 522篇 |
1999年 | 325篇 |
1998年 | 328篇 |
1997年 | 307篇 |
1996年 | 401篇 |
1995年 | 271篇 |
1994年 | 224篇 |
1993年 | 284篇 |
1992年 | 240篇 |
1991年 | 254篇 |
1990年 | 193篇 |
1989年 | 198篇 |
1988年 | 212篇 |
1987年 | 270篇 |
1986年 | 240篇 |
1985年 | 276篇 |
1984年 | 252篇 |
1983年 | 203篇 |
1982年 | 237篇 |
1981年 | 241篇 |
1980年 | 238篇 |
1979年 | 247篇 |
1978年 | 225篇 |
1977年 | 201篇 |
1976年 | 176篇 |
1975年 | 192篇 |
1974年 | 154篇 |
1973年 | 158篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
111.
In cricket, a rain-affected pitch can make batting more difficult than normal. Several other conditions such as poor light or an initially lively pitch, may also result in difficulties for the batsmen. In this note, we refer to all of them as ‘sticky wickets’. On sticky wickets, lower order batsmen are often sent into ‘hold the fort’ until conditions improve. In this paper, a stochastic dynamic programming model is used to examine the appropriateness of this policy. The model suggests that the tactic is often optimal when the sticky wicket persists until the end of the day's play, but not often when the sticky wicket is transitory. In some circumstances, it is worthwhile, on a normal wicket near the end of the day, to send in a lower order batsman to hold the fort (a night watchman): when the wicket is sticky, this tactic is even more worthwhile. 相似文献
112.
113.
114.
Ying Zhang Gang Hu Shigang Chen H.A. Cerdeira 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(3):381-384
A method of controlling global stochasticity in Hamiltonian systems by applying nonlinear perturbation is proposed. With the
well-known standard map we demonstrate that this control method can convert global stochasticity into regular motion in a
wide chaotic region for arbitrary initial condition, in which the control signal remains very weak after a few kicks. The
system in which chaos has been controlled approximates to the original Hamiltonian system, and this approach appears robust
against small external noise. The mechanism underlying this high control efficiency is intuitively explained.
Received 15 January 2002 Published online 6 June 2002 相似文献
115.
Direct measurement of fluid velocity gradients at a wall by PIV image processing with stereo reconstruction 总被引:1,自引:0,他引:1
Velocity gradient is typically estimated in Particle Image Velocimetry (PIV) by differentiating a measured velocity field,
which amplifies noise in the measured velocities. If gradients near a boundary are sought, such noise is usually greater than
in bulk fluid, because of small tracer displacement, uncertainty in the effective positions of velocity vectors, intense deformation
of tracer patterns, and laser reflection. We consider here a modified form of the Particle Image Distortion (PID) method todirectly calculate velocity gradients at a fixed wall, and refer it as “PIV/IG” (“Interface Gradiometry”). Results from synthetic
2D PIV images suggest our method achieves higher SNR and accuracy than velocity differentiation. Also, we have developed a
procedure to reconstruct three-dimensional velocity gradient at a fixed wall the two non-zero components from PIV/IG data
obtained in stereo views; these equations simplify considerably thanks to the no-slip condition. Experimental data from the
bottom wall of turbulent open channel flow appear to suffer from a form of pixel locking. As with standard PIV, this underlines
the importance of adequate tracer diameter in the images, sufficient seeding density, and of dynamic range of the camera sensor. 相似文献
116.
117.
Joseph Shipman 《Mathematical Intelligencer》2007,29(4):9-14
Theorems 2 and 3 establish the minimum algebraic conditions necessary for a field to be algebraically closed, and they can
therefore be said to “optimize” the Fundamental Theorem of Algebra. But each specific“degree implication” is a first-order
consequence of the axioms for fields, and could have been discovered two centuries ago; the existence of these finitary relationships
appears to have been unsuspected by practically everyone, with one important exception. 相似文献
118.
Ling-yan Tang Song-he Song 《应用数学学报(英文版)》2007,23(2):303-310
In this paper,we use Daubechies scaling functions as test functions for the Galerkin method,and discuss Wavelet-Galerkin solutions for the Hamilton-Jacobi equations.It can be proved that the schemesare TVD schemes.Numerical tests indicate that the schemes are suitable for the Hamilton-Jacobi equations.Furthermore,they have high-order accuracy in smooth regions and good resolution of singularities. 相似文献
119.
Luca Lambertini Raimondello Orsini 《Central European Journal of Operations Research》2007,15(1):105-117
We reconsider the role of network externalities in a dynamic spatial monopoly where the firm must invest in order to accumulate
capacity, while consumers may have either linear or quadratic preferences. We (1) characterise saddle point equilibria, (2)
prove that the extent of market coverage is increasing in the network effect and (3) unlike the existing static literature
on the same problem, the monopolist may not make introductory price offers. Then, we briefly deal with the socially optimal
solution, showing that, in general, a planner would serve more consumers than a profit-seeking monopolist.
相似文献
120.
Summary High-purity indium was analysed by spark source mass spectrometry, using electrical and photoplate detection. For the calibration of the differences in elemental sensitivity, a standard sample was prepared in which 10 impurities were determined by graphite furnace atomic absorption spectrometry. In this way accuracies of less than 40% were obtained for relatively homogeneous elements at ppm and sub-ppm level. About 40 elements could be determined with detection limits of 10 to 30 ppb. Two pattern recognition methods, principal component analysis and clustering analysis, were applied to obtain information on trace element distribution, which indicated that a number of elements were strongly spatially correlated in the analysed sample.
On leave from: Department of Chemistry, Nanjing Normal University, Nanjing, People's Republic of China 相似文献
Chemische Analyse und Verteilungsbestimmung von Spurenelementen in Indium-Matrix durch Funkenquellen-Massenspektrometrie
On leave from: Department of Chemistry, Nanjing Normal University, Nanjing, People's Republic of China 相似文献