全文获取类型
收费全文 | 11858篇 |
免费 | 1901篇 |
国内免费 | 779篇 |
专业分类
化学 | 3366篇 |
晶体学 | 56篇 |
力学 | 1876篇 |
综合类 | 234篇 |
数学 | 5131篇 |
物理学 | 3875篇 |
出版年
2024年 | 17篇 |
2023年 | 171篇 |
2022年 | 225篇 |
2021年 | 395篇 |
2020年 | 332篇 |
2019年 | 279篇 |
2018年 | 295篇 |
2017年 | 511篇 |
2016年 | 584篇 |
2015年 | 524篇 |
2014年 | 647篇 |
2013年 | 904篇 |
2012年 | 621篇 |
2011年 | 653篇 |
2010年 | 594篇 |
2009年 | 679篇 |
2008年 | 718篇 |
2007年 | 743篇 |
2006年 | 595篇 |
2005年 | 487篇 |
2004年 | 502篇 |
2003年 | 517篇 |
2002年 | 426篇 |
2001年 | 323篇 |
2000年 | 363篇 |
1999年 | 331篇 |
1998年 | 288篇 |
1997年 | 208篇 |
1996年 | 205篇 |
1995年 | 182篇 |
1994年 | 153篇 |
1993年 | 132篇 |
1992年 | 92篇 |
1991年 | 94篇 |
1990年 | 83篇 |
1989年 | 59篇 |
1988年 | 70篇 |
1987年 | 54篇 |
1986年 | 43篇 |
1985年 | 72篇 |
1984年 | 38篇 |
1983年 | 28篇 |
1982年 | 38篇 |
1981年 | 28篇 |
1980年 | 48篇 |
1979年 | 44篇 |
1978年 | 27篇 |
1977年 | 35篇 |
1976年 | 36篇 |
1957年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
162.
年龄相关的种群扩散系统的最优分布控制 总被引:5,自引:1,他引:4
本文讨论年龄相关的种群扩散系统的最优分布控制 ,证明了最优分布控制的存在性 ,得到了分布控制为最优的充分必要条件及确定最优控制的最优性组 . 相似文献
163.
The educational system has gradually shifted from a face-to-face to an e-learning system, which has become prevalent in advanced countries with the advance of information technology, and connection of global networks. Accordingly, a growing demand is emerging for more reliable individual certification with technical precision in order to measure and record learning achievements and credentials of participants. The present system has a limit in terms of registration capacity, therefore, its accuracy has often been questioned. Against this background, an individual certification system is proposed particularly for access control in e-learning. Under our proposed system, a compact optical correlator for facial recognition is employed. This correlator was previously tested for of biometrics authentication accuracy and proved highly reliable, having recorded remarkably low error rates (below 1%). The recorded error rate is sufficiently robust that the system itself can be regarded as a valid and practical viable attestation system. 相似文献
164.
X. Y. You 《Heat and Mass Transfer》2003,40(1-2):75-81
Active feedback control for the onset of Rayleigh-Bénard Convection in temperature-dependent-viscosity liquids is investigated.
In this paper, three major problems are addressed: (1) The results of Tang-Bau control are improved by considering the effects
of temperature-dependent viscosity; (2) A more efficient two-plate control strategy is presented. A phenomenon of coalescence
of the unstable modes is observed as the controller gain is large enough; (3) A simple way to estimate the critical Rayleigh
number under the effects of temperature-dependent viscosity is described. Numerical results show that the effects of temperature-dependent
viscosity on the critical Rayleigh number should be taken into account in some cases and the onset of Rayleigh-Bénard convection
can be effectively delayed or advanced by the active feedback control strategies studied here. 相似文献
165.
针对带约束的凸多面体线性不确定模型,提出了一种新型鲁棒预测控制方法,它采用离散化的不确定模型构造最小-最大优化控制问题,并在其中直接引入状态反馈机制,与其他最小-最大预测控制方法相比,这种方法等效于增加了控制序列的长度,为优化问题增加了更多的自由度,从而扩大了可行域,作为最小化目标的是离散化不确定系统在整个预测时域上二次型成本函数的最大值,而不是各预测阶段应成本项的上界之和,从而减少了与最小-最大优化相关的方程个数,有利于降低计算复杂性,文中进一步证明了不确定系统的闭环稳定性取决于优化问题在初始时刻的可行性,并将优化问题转化为线性矩阵不等式形式。最后,以数值仿赵例子验证了方法的有效性。 相似文献
166.
El Kebir Boukas Peng Shi Sing Kiong Nguang 《Journal of Mathematical Analysis and Applications》2003,282(1):241-255
This paper studies the problem of stochastic stability and disturbance attenuation for a class of linear continuous-time uncertain systems with Markovian jumping parameters. The uncertainties are assumed to be nonlinear and state, control and external disturbance dependent. A sufficient condition is provided to solve the above problem. An H∞ controller is designed such that the resulting closed-loop system is stochastically stable and has a disturbance attenuation γ for all admissible uncertainties. It is shown that the control law is in terms of the solutions of a set of coupled Riccati inequalities. A numerical example is included to demonstrate the potential of the proposed technique. 相似文献
167.
工业污染排放中气──固两相流的光学测量 总被引:1,自引:0,他引:1
烟尘及各种工业性粉尘排放是典型的气-固两相系.迄今为止,国内外尚缺少有效的监测其排放的手段.本文基于光的散时原理,提出了一种非接触的光学方法,能对上述固体颗粒物的污染排放浓度及粒径大小进行实时、在线的监测。计算机模拟和实验研究表明,该方法具有广阔的工业应用前景。 相似文献
168.
Hans van den Berg Michel Mandjes Rudesindo Núñez-Queija 《Operations Research Letters》2007,35(3):297-307
We study a processor-sharing model in which users choose between a high- and a low-priority service, based on their utility functions and prices charged by the service provider. The latter aims at revenue maximization. The model is motivated by file transmissions in data networks with distributed congestion control. 相似文献
169.
Frank Rutz Martin Koch Shilpa Khare Martin Moneke Heike Richter Uwe Ewert 《International Journal of Infrared and Millimeter Waves》2006,27(4):547-556
We report on experiments that evaluate the potential of terahertz (THz) time-domain spectroscopy (TDS) for quality control
of polymeric compounds. We investigate specimens out of a polyethylene compound with silver-coated titanium dioxide nanospheres
and a glass-fiber reinforced epoxy composite. We further examine an industrial polymer product produced by injection molding.
Our data demonstrates that THz imaging is a powerful tool for contactless quality control in the polymer industry. 相似文献
170.
A class of long-range predictive adaptive fuzzy relational controllers is presented. The plant behavior is described over an extended time horizon by a fuzzy relational model which is identified based on input-output closed-loop observations of the plant variables. In this class of adaptive controllers the control law attempts to minimize a quadratic cost over an extended control horizon. When used with linear models, this approach has revealed a significant potential for overcoming the limitations of one-step ahead schemes, such as the stabilization of non-minimum phase plants. Here, a uniform framework is adopted for implementing both the fuzzy model and the fuzzy controller, namely distributed fuzzy relational structures gaining from their massive parallel processing features and from the learning capabilities typical of the connectivist approaches. Issues such as maintenance during the adaptation process of the meaning of linguistic terms used at both fuzzy systems interfaces are addressed, namely by introducing a new design methodology for on-line fuzzy systems interface adaptation. The examples presented reinforce the claim of the usefulness of this new approach. 相似文献