全文获取类型
收费全文 | 12432篇 |
免费 | 2171篇 |
国内免费 | 608篇 |
专业分类
化学 | 1946篇 |
晶体学 | 45篇 |
力学 | 2609篇 |
综合类 | 224篇 |
数学 | 6336篇 |
物理学 | 4051篇 |
出版年
2024年 | 33篇 |
2023年 | 175篇 |
2022年 | 300篇 |
2021年 | 329篇 |
2020年 | 338篇 |
2019年 | 302篇 |
2018年 | 301篇 |
2017年 | 529篇 |
2016年 | 584篇 |
2015年 | 522篇 |
2014年 | 661篇 |
2013年 | 953篇 |
2012年 | 654篇 |
2011年 | 678篇 |
2010年 | 633篇 |
2009年 | 708篇 |
2008年 | 760篇 |
2007年 | 769篇 |
2006年 | 635篇 |
2005年 | 522篇 |
2004年 | 500篇 |
2003年 | 527篇 |
2002年 | 463篇 |
2001年 | 348篇 |
2000年 | 369篇 |
1999年 | 356篇 |
1998年 | 304篇 |
1997年 | 226篇 |
1996年 | 223篇 |
1995年 | 192篇 |
1994年 | 152篇 |
1993年 | 128篇 |
1992年 | 109篇 |
1991年 | 107篇 |
1990年 | 100篇 |
1989年 | 64篇 |
1988年 | 72篇 |
1987年 | 61篇 |
1986年 | 47篇 |
1985年 | 72篇 |
1984年 | 41篇 |
1983年 | 32篇 |
1982年 | 44篇 |
1981年 | 36篇 |
1980年 | 54篇 |
1979年 | 49篇 |
1978年 | 31篇 |
1977年 | 38篇 |
1976年 | 36篇 |
1957年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Solitary waves and their bifurcations of KdV like equation with higher order nonlinearity 总被引:3,自引:1,他引:2
We investigate the KdV like equation with higher order nonlinearity ut + a(1 +bun)unux + uxxx = 0with n ≥ 1, a, b ∈ R and α≠ 0. The bifurcations and explicit expressions of solitary wave solutions for theequation are discussed by using the bifurcation method and qualitative theory of dynamical systems. Thebifurcation diagrams, existence and number of the solitary waves are given. 相似文献
22.
William J. Reed 《Natural Resource Modeling》1989,3(4):463-480
It is assumed that the probability of destruction of a biological asset by natural hazards can be reduced through investment in protection. Specifically a model, in which the hazard rate depends on both the age of the asset and the accumulated invested protection capital, is assumed. The protection capital depreciates through time and its effectiveness in reducing the hazard rate is subject to diminishing returns. It is shown how the investment schedule to maximize the expected net present value of the asset can be determined using the methods of deterministic optimal control, with the survival probability regarded as a state variable. The optimal investment pattern involves “bang-bang-singular” control. A numerical scheme for determining jointly the optimal investment policy and the optimal harvest (or replacement) age is outlined and a numerical example involving forest fire protection is given. 相似文献
23.
An applied cell mapping method for optimal control problems 总被引:1,自引:0,他引:1
W. H. Zhu 《Journal of Optimization Theory and Applications》1989,60(3):485-500
From the application point of view, a series of modifications are proposed for the cell mapping method discussed in Ref. 1 for the optimal control analysis of dynamical systems. The cell order around the target set is rearranged. A set of common discriminate principles is used for the selection of the optimal one among competing control strategies of the same cost. Inequality constraints of the system are taken into account. The number of elements in the set of allowable time intervals is not prescribed, but left open. These modifications seem to make the cell mapping method more efficient for analyzing feedback systems and for obtaining their global optimal control information. The algorithms presented in this paper could broaden the application of the cell mapping approach of Ref. 1 to a wider class of engineering problems. 相似文献
24.
25.
26.
27.
In this paper, the special construction of a parallel robot, called spatial servopneumatic multi-axis test facility, will be discussed. The investigations include the following aspects: (i) the laboratory set-up of the robot, (ii) various results obtained in laboratory experiments, taking into account quite different control algorithms and command-input signals, (iii) a comparison of the laboratory experiments with the computer simulations of Part I of this paper, and ({vi}) a quality check of the results compared with the cost of the different controller realizations. The results of both the computer simulations and the laboratory experiments show: (i) The dynamic behavior of the parallel structure can be tremendously improved by using sophisticated nonlinear control algorithms. (ii) This improvement has to be paid by a drastically increased amount of work for deriving the model equations and control algorithms, and by augmented hardware cost of the sensing elements and controller electronics. (iii) Carefully developed model equations and identified model parameters provide theoretical models of the complex parallel structure that are very close to reality. This enables the design engineer to systematically investigate constructive alternatives of the design parameters, sensor and actuator concepts, and control strategies of the MAP prior to their hardware realization.This work has been supported by the German Science Foundation (DFG) under Contract No. Ha 1666/6-3. 相似文献
28.
We consider the optimal service control of a multiclass M/G/1 queueing system in which customers are served nonpreemptively and the system cost rate is additive across classes and increasing convex in the numbers present in each class. Following Whittle's approach to a class of restless bandit problems, we develop a Langrangian relaxation of the service control problem which serves to motivate the development of a class of index heuristics. The index for a particular customer class is characterised as a fair charge for service of that class. The paper develops these indices and reports an extensive numerical investigation which exhibits strong performance of the index heuristics for both discounted and average costs. 相似文献
29.
束晕-混沌的复杂性理论与控制方法及其应用前景 总被引:18,自引:0,他引:18
本文系统论述涉及强流加速器等强流离子束装置中产生的束晕-混沌的复杂性理论与控制方法及其应用前景。强流离子束在核材料生产与增殖、洁净核能、放射性废物嬗变、放射性药物生产、重离子聚变、高能物理、核科学与工程、国防与民用工业和医疗等许多方面都有极其重要的应用潜力和诱人的发展前景。尤其是,近年来强流加速器驱动的放射性洁净核能系统是国内外关注的热门课题,因为它比常规核电更安全、更干净、更便宜。但是,强流离子束形成的束晕-混沌的复杂性现象已引起了国内外广泛关注,需要加以抑制、控制和消除这类现象,解决这一难题已经成为强流离子束应用中的关键问题之一。目前不仅必须深入研究这类束晕-混沌的复杂特性及其产生的物理机制,而且需要研究如何实现对束晕-混沌的有效控制,并寻求和发展其新理论、新方法和新技术。这就向强流离子束物理和非线性-复杂性科学及其技术提出了一系列极富挑战性的新课题。本文结合国内外的研究概况,根据我们多年来的研究成果,特别是我们首创性地提出了一些束晕-混沌的有效控制方法,它们包括:非线性反馈控制法,小波反馈控制法,变结构控制法,延迟反馈控制法,参数自适应控制法等,进行重点的介绍。对上述课题当前的主要进展及相关问题进行系统的总结和比较全面综述的评论。最后,指出该领域今后的研究方向,以推动这个崭新领域的深入研究和应用发展。 相似文献
30.
Geng Sheng WANG 《数学学报(英文版)》2005,21(5):1005-1014
This work is concerned with Pontryagin's maximum principle of optimal control problems governed by some non-well-posed semilinear heat equations. A type of approach to the non-well-posed optimal control problem is given. 相似文献