全文获取类型
收费全文 | 3291篇 |
免费 | 384篇 |
国内免费 | 204篇 |
专业分类
化学 | 369篇 |
晶体学 | 5篇 |
力学 | 238篇 |
综合类 | 73篇 |
数学 | 1513篇 |
物理学 | 654篇 |
无线电 | 1027篇 |
出版年
2024年 | 9篇 |
2023年 | 35篇 |
2022年 | 76篇 |
2021年 | 92篇 |
2020年 | 81篇 |
2019年 | 71篇 |
2018年 | 86篇 |
2017年 | 105篇 |
2016年 | 139篇 |
2015年 | 113篇 |
2014年 | 235篇 |
2013年 | 260篇 |
2012年 | 199篇 |
2011年 | 201篇 |
2010年 | 154篇 |
2009年 | 183篇 |
2008年 | 197篇 |
2007年 | 174篇 |
2006年 | 173篇 |
2005年 | 154篇 |
2004年 | 139篇 |
2003年 | 132篇 |
2002年 | 103篇 |
2001年 | 104篇 |
2000年 | 86篇 |
1999年 | 81篇 |
1998年 | 67篇 |
1997年 | 67篇 |
1996年 | 47篇 |
1995年 | 35篇 |
1994年 | 35篇 |
1993年 | 30篇 |
1992年 | 29篇 |
1991年 | 21篇 |
1990年 | 28篇 |
1989年 | 15篇 |
1988年 | 16篇 |
1987年 | 12篇 |
1986年 | 4篇 |
1985年 | 9篇 |
1984年 | 15篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 12篇 |
1980年 | 14篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1976年 | 2篇 |
1973年 | 3篇 |
排序方式: 共有3879条查询结果,搜索用时 15 毫秒
31.
In the present paper, we study the rate of convergence in simultaneous approximation for the Bézier variant of the Baskakov-Beta operators by using the decomposition technique of functions of bounded variation. 相似文献
32.
B. G. Sidharth 《Foundations of Physics Letters》2006,19(6):611-617
In this note we consider a variable G cosmology which is consistent with observation and which had successfully predicted an ever expanding accelerating universe
with a small cosmological constant amongst other things. Three further tests are proposed in this note: First, the inexplicable
anomalous accelerations of the Pioneer spacecrafts can be explained. It is then shown that the observed shortening of the
orbital periods of binary pulsars is in good agreement with this model. Finally, more general changes in orbital parameters
are deduced, which may be observed in the future. 相似文献
33.
S. Juneja 《Queueing Systems》2007,57(2-3):115-127
Efficient estimation of tail probabilities involving heavy tailed random variables is amongst the most challenging problems
in Monte-Carlo simulation. In the last few years, applied probabilists have achieved considerable success in developing efficient
algorithms for some such simple but fundamental tail probabilities. Usually, unbiased importance sampling estimators of such
tail probabilities are developed and it is proved that these estimators are asymptotically efficient or even possess the desirable
bounded relative error property. In this paper, as an illustration, we consider a simple tail probability involving geometric
sums of heavy tailed random variables. This is useful in estimating the probability of large delays in M/G/1 queues. In this setting we develop an unbiased estimator whose relative error decreases to zero asymptotically. The key
idea is to decompose the probability of interest into a known dominant component and an unknown small component. Simulation
then focuses on estimating the latter ‘residual’ probability. Here we show that the existing conditioning methods or importance
sampling methods are not effective in estimating the residual probability while an appropriate combination of the two estimates
it with bounded relative error. As a further illustration of the proposed ideas, we apply them to develop an estimator for
the probability of large delays in stochastic activity networks that has an asymptotically zero relative error.
相似文献
34.
We discuss determination of jumps for functions with generalized bounded variation. The questions are motivated by A. Gelb
and E. Tadmor [1], F. Móricz [5] and [6] and Q. L. Shi and X. L. Shi [7]. Corollary 1 improves the results proved in B. I.
Golubov [2] and G. Kvernadze [3].
Supported by NSFC 10671062. 相似文献
35.
小球与均质自由杆的碰撞 总被引:2,自引:2,他引:0
讨论了小球与自由杆碰撞过程中所遵从的物理规律,明确了完全非弹性碰撞过程的能量特征,指出完全非弹性碰撞后两物体不一定粘在一起共同运动的事实. 相似文献
36.
Remco van der Hofstad Gerard Hooghiemstra Piet Van Mieghem 《Random Structures and Algorithms》2002,20(4):519-539
In this paper we study the covariance structure of the number of nodes k and l steps away from the root in random recursive trees. We give an analytic expression valid for all k, l and tree sizes N. The fraction of nodes k steps away from the root is a random probability distribution in k. The expression for the covariances allows us to show that the total variation distance between this (random) probability distribution and its mean converges in probability to zero. © 2002 Wiley Periodicals, Inc. Random Struct. Alg., 20: 519–539, 2002 相似文献
37.
On a nonlinear multigrid algorithm with primal relaxation for the image total variation minimisation
Digital image restoration has drawn much attention in the recent years and a lot of research has been done on effective variational partial differential equation models and their theoretical studies. However there remains an urgent need to develop fast and robust iterative solvers, as the underlying problem sizes are large. This paper proposes a fast multigrid method using primal relaxations. The basic primal relaxation is known to get stuck at a ‘local’ non-stationary minimum of the solution, which is usually believed to be ‘non-smooth’. Our idea is to utilize coarse level corrections, overcoming the deadlock of a basic primal relaxation scheme. A further refinement is to allow non-regular coarse levels to correct the solution, which helps to improve the multilevel method. Numerical experiments on both 1D and 2D images are presented. 相似文献
38.
39.
A series of 7-amino- and polyaminosterol analogues of squalamine and trodusquemine were synthesized involving a new stereoselective titanium reductive amination reaction in high chemical yields of up to 95% in numerous cases. These derivatives were evaluated for their in vitro antimicrobial properties against human pathogens. All the compounds present excellent activities against Gram-positive bacteria exhibiting similar results against Staphylococcus aureus and Streptococcus faecalis with minimum inhibitory concentrations (MICs) varying from 2.5 to 10 μg/mL. Numerous derivatives possess also MICs against Gram-negative Escherichia coli bacteria (MICs varying from 2.5 to 10 μg/mL) suggesting that nature of the amino group attached to the sterol moiety plays an important role on the activities of such products. 相似文献
40.
线性联合检测算法在TD-SCDMA系统中的性能分析与比较 总被引:18,自引:3,他引:15
时分、同步码分多址(TD-SCDMA)系统是基于时分双工(TDD)的块传输系统,它使用了联合检测这项关键技术来抵抗符号间干扰(ISI)和多址干扰(MAI)。以上行链路为例,本文分析了三种线性联合检测算法--匹配滤波块均衡器(MF-BLE)、迫零块均衡器(ZF-BLE)和最小均方误差块均衡器(MMSE-BLE)。从理论上,我们描述了这三种线性联合检测算法的基本原理,并对它们进行了比较(包括抗干扰性能和计算量)。随后,我们模拟了不同参数条件下(信道模型、码道数目、天线数目)这几种算法的性能,并与理论分析进行对照,得出一定结论。 相似文献