全文获取类型
收费全文 | 29034篇 |
免费 | 391篇 |
国内免费 | 524篇 |
专业分类
系统科学 | 679篇 |
丛书文集 | 798篇 |
教育与普及 | 302篇 |
理论与方法论 | 117篇 |
现状及发展 | 8159篇 |
研究方法 | 788篇 |
综合类 | 18955篇 |
自然研究 | 151篇 |
出版年
2015年 | 264篇 |
2014年 | 339篇 |
2013年 | 333篇 |
2012年 | 580篇 |
2011年 | 822篇 |
2010年 | 452篇 |
2009年 | 602篇 |
2008年 | 804篇 |
2007年 | 933篇 |
2006年 | 715篇 |
2005年 | 742篇 |
2004年 | 675篇 |
2003年 | 640篇 |
2002年 | 591篇 |
2001年 | 1008篇 |
2000年 | 1045篇 |
1999年 | 1037篇 |
1998年 | 590篇 |
1997年 | 655篇 |
1996年 | 565篇 |
1995年 | 502篇 |
1994年 | 491篇 |
1993年 | 432篇 |
1992年 | 677篇 |
1991年 | 581篇 |
1990年 | 565篇 |
1989年 | 505篇 |
1988年 | 476篇 |
1987年 | 407篇 |
1986年 | 366篇 |
1985年 | 368篇 |
1984年 | 245篇 |
1981年 | 239篇 |
1980年 | 263篇 |
1979年 | 570篇 |
1978年 | 466篇 |
1977年 | 471篇 |
1976年 | 354篇 |
1975年 | 376篇 |
1974年 | 586篇 |
1973年 | 455篇 |
1972年 | 415篇 |
1971年 | 515篇 |
1970年 | 662篇 |
1969年 | 581篇 |
1968年 | 495篇 |
1967年 | 532篇 |
1966年 | 441篇 |
1965年 | 330篇 |
1958年 | 296篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
81.
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
82.
研究了三聚甲醛合成过程进料浓度、催化剂用量等对副产物(特别是甲酸)生成行为的影响。结果表明,该体系副产物的生成强烈地依赖于反应时间和催化剂硫酸的用量,而与原料浓度的波动无关。在反应-精馏过程的反应器中添加惰性高沸有机物,可有效地缩短反应物的停留时间,提高三聚甲酸的选择性,大大降低快速反应-精馏过程对反应器的设计要求。 相似文献
83.
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
84.
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
85.
Zhu Canyan Wang Yiming Yang Huibao & Liu Jiasheng School of Electronics Information Engineering Soochow Univ. Suzhou P.R.China 《系统工程与电子技术(英文版)》2006,17(4):875-882
1. INTRODUCTION As the development of technologies of digital signal processing and integrated circuits, the structure of wireless communication system has been improved. Many schemes for software defined radio (SDR) systems have been developed since 1995, and they can be divided into five tiers of solutions according to their developing history and their definitions[1]. The five tiers are conventional hardware-implemented scheme; software-controlled radio scheme; SDR, that implements m… 相似文献
86.
87.
88.
萃取浮选光度法测定微量钼的研究及应用 总被引:2,自引:0,他引:2
研究了用甲苯萃取浮选Mo(Ⅴ)—SCN~-—Kφ三元络合物的各种条件。在硫酸介质中,紫红色络合物最大吸收波长位于585nm,摩尔吸光系数为2.8×10~5l·mol~(-1)·cm~(-1),钼量在0—10μg/25ml范围内符合比耳定律。大量常见元素均不干扰,40倍的钨(Ⅵ)亦不干扰钼的测定,方法已应用于钨矿中微量钼的测定,其结果令人满意。 相似文献
89.
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
90.
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