全文获取类型
收费全文 | 17254篇 |
免费 | 42篇 |
国内免费 | 53篇 |
专业分类
系统科学 | 253篇 |
丛书文集 | 444篇 |
教育与普及 | 36篇 |
理论与方法论 | 50篇 |
现状及发展 | 7964篇 |
研究方法 | 726篇 |
综合类 | 7728篇 |
自然研究 | 148篇 |
出版年
2012年 | 203篇 |
2011年 | 408篇 |
2010年 | 97篇 |
2009年 | 95篇 |
2008年 | 271篇 |
2007年 | 345篇 |
2006年 | 290篇 |
2005年 | 301篇 |
2004年 | 271篇 |
2003年 | 328篇 |
2002年 | 260篇 |
2001年 | 609篇 |
2000年 | 609篇 |
1999年 | 344篇 |
1992年 | 328篇 |
1991年 | 253篇 |
1990年 | 298篇 |
1989年 | 271篇 |
1988年 | 262篇 |
1987年 | 276篇 |
1986年 | 284篇 |
1985年 | 339篇 |
1984年 | 242篇 |
1983年 | 219篇 |
1982年 | 200篇 |
1981年 | 238篇 |
1980年 | 261篇 |
1979年 | 568篇 |
1978年 | 464篇 |
1977年 | 469篇 |
1976年 | 352篇 |
1975年 | 374篇 |
1974年 | 583篇 |
1973年 | 455篇 |
1972年 | 413篇 |
1971年 | 511篇 |
1970年 | 660篇 |
1969年 | 575篇 |
1968年 | 496篇 |
1967年 | 528篇 |
1966年 | 439篇 |
1965年 | 332篇 |
1964年 | 86篇 |
1959年 | 198篇 |
1958年 | 293篇 |
1957年 | 192篇 |
1956年 | 172篇 |
1955年 | 167篇 |
1954年 | 159篇 |
1948年 | 88篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
Non-Minimum Phase Nonlinear System Predictive Control Based on Local Recurrent Neural Networks 总被引:3,自引:0,他引:3
Abstract: After a recursive multi-step-ahead predictor for nonlinear systems based on local recurrent neural networks isintroduced, an intelligent PID controller is adopted to correct the errors including identified model errors and accumulatederrors produced in the recursive process. Characterized by predictive control, this method can achieve a good controlaccuracy and has good robustness. A simulation study shows that this control algorithm is very effective. 相似文献
23.
Dynamic Characteristics of Inter-Satellite Links in LEO Networks 总被引:1,自引:0,他引:1
To establish an efficient inter-satellite link (ISL) in an LEO network, the effect of geometric characteristics of ISL on the ISLs and the devices on the LEO satellite should be examined. Because of the continuous movement of the LEO satellite, the time-varying behaviours of the ISL's geometric charactersistics continuously change with the changes of the satellite's position on the orbit. These dynamic geometric characteristics of the ISLs are important for ISL's performance analyzing and the design of the devices on the LEO satellite. This paper describes dynamic geometric characteristics of ISL, analyzes the impact of these regulations on the tracking system of the satellite's antenna and the power adjusting system of the satellite's transmitter, with the Iridium system as an example. 相似文献
24.
Qiangfu Zhao Zhong Wang Youan KeDept. of Electronic Eng. Beijing Institute of Technology P.O.Box Beijing China 《系统工程与电子技术(英文版)》1991,(2)
It has long been realized that the problem of radar imaging is a special case of image reconstruction in which the data are incomplete and noisy. In other fields, iterative reconstruction algorithms have been used successfully to improve the image quality. This paper studies the application of iterative algorithms in radar imaging. A discrete model is first derived, and the iterative algorithms are then adapted to radar imaging. Although such algorithms are usually time consuming, this paper shows that, if the algorithms are appropriately simplified, it is possible to realize them even in real time. The efficiency of iterative algorithms is shown through computer simulations. 相似文献
25.
Support vector machine (SVM), as a novel approach in pattern recognition, has demonstrated a success in face detection and face recognition. In this paper, a face recognition approach based on the SVM classifier with the nearest neighbor classifier (NNC) is proposed. The principal component analysis (PCA) is used to reduce the dimension and extract features. Then one-against-all stratedy is used to train the SVM classifiers. At the testing stage, we propose an al- 相似文献
26.
We develop in this paper an efficient way to select the best subset threshold autoregressive model. The proposed method uses a stochastic search idea. Differing from most conventional approaches, our method does not require us to fix the delay or the threshold parameters in advance. By adopting the Markov chain Monte Carlo techniques, we can identify the best subset model from a very large of number of possible models, and at the same time estimate the unknown parameters. A simulation experiment shows that the method is very effective. In its application to the US unemployment rate, the stochastic search method successfully selects lag one as the time delay and five best models from more than 4000 choices. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
27.
28.
29.
T lymphocytes recognize antigen in the form of peptides that associate with specific alleles of class I or class II major histocompatibility (MHC) molecules. By contrast with the clear MHC allele-specific binding of peptides to purified class II molecules purified solubilized class I molecules either bind relatively poorly or show degenerate specificity. Using photo-affinity labelling, we demonstrate here the specific interaction of peptides with cell-associated MHC class I molecules and show that this involves metabolically active processes. 相似文献
30.
European blackcaps,Sylvia atricapilla, with one breeding season per year, have a single-peaked annual testes cycle. However, African conspecifics from the Cape Verde Islands with two breeding seasons per annum demonstrate a two-peaked cycle. Both population-specific cycles reflect differences in the respective endogenous circannual rhythms. Experimental hybridization of birds of the two populations resulted in an intermediate pattern of testes cycle, thus demonstrating that there are genetic components for some temporal aspects in an avian reproductive cycle. Another characteristic of the African birds, their extremely rapid juvenile development and early sexual maturity (at an age of 5–6 months) proved largely to be a photoperiodic (short-day) effect in birds hatched in autumn. The same effect could also be induced in European conspecifics exposed to correspondingly short day-lengths. 相似文献