全文获取类型
收费全文 | 31535篇 |
免费 | 91篇 |
国内免费 | 135篇 |
专业分类
系统科学 | 317篇 |
丛书文集 | 558篇 |
教育与普及 | 78篇 |
理论与方法论 | 90篇 |
现状及发展 | 14242篇 |
研究方法 | 1284篇 |
综合类 | 14863篇 |
自然研究 | 329篇 |
出版年
2013年 | 184篇 |
2012年 | 423篇 |
2011年 | 768篇 |
2010年 | 189篇 |
2008年 | 536篇 |
2007年 | 642篇 |
2006年 | 587篇 |
2005年 | 595篇 |
2004年 | 541篇 |
2003年 | 607篇 |
2002年 | 511篇 |
2001年 | 1079篇 |
2000年 | 1069篇 |
1999年 | 626篇 |
1994年 | 366篇 |
1992年 | 613篇 |
1991年 | 455篇 |
1990年 | 554篇 |
1989年 | 495篇 |
1988年 | 452篇 |
1987年 | 518篇 |
1986年 | 529篇 |
1985年 | 639篇 |
1984年 | 466篇 |
1983年 | 429篇 |
1982年 | 397篇 |
1981年 | 428篇 |
1980年 | 440篇 |
1979年 | 1063篇 |
1978年 | 819篇 |
1977年 | 788篇 |
1976年 | 642篇 |
1975年 | 665篇 |
1974年 | 933篇 |
1973年 | 795篇 |
1972年 | 719篇 |
1971年 | 862篇 |
1970年 | 1135篇 |
1969年 | 945篇 |
1968年 | 888篇 |
1967年 | 878篇 |
1966年 | 775篇 |
1965年 | 569篇 |
1959年 | 325篇 |
1958年 | 505篇 |
1957年 | 340篇 |
1956年 | 313篇 |
1955年 | 292篇 |
1954年 | 288篇 |
1948年 | 199篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
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
52.
Low-molecular-weight GTP-binding proteins are strong candidates for regulators of membrane traffic. In yeast, mutations in the sec4 or ypt1 genes encoding small GTP-binding proteins inhibit constitutive membrane flow at the plasma membrane or Golgi complex, respectively. It has been suggested that membrane fusion-fission events are regulated by cycling of small GTP-binding proteins between a membrane-bound and free state, but although most of these small proteins are found in both soluble and tightly membrane-bound forms, there is no direct evidence to support such cycling. In rat brain a small GTP-binding protein, rab3A, is exclusively associated with synaptic vesicles, the secretory organelles of nerve terminals. Here we use isolated nerve terminals to study the fate of rab3A during synaptic vesicle exocytosis. We find that rab3A dissociates quantitatively from the vesicle membrane after Ca2(+)-dependent exocytosis and that this dissociation is partially reversible during recovery after stimulation. These results are direct evidence for an association-dissociation cycle of a small GTP-binding protein during traffic of its host membrane. 相似文献
53.
对目前的UBBE模型进行了适当的推广,使在解决估计问题时可以考虑那些可能是误差上界的数值,并在此基础上提出了一种方法,能够通过对估计精度和可靠性进行合理的权衡确定所需估计值,实际案例研究结果表明所提方法能够较好地解决估计精度和可靠性之间的矛盾。 相似文献
54.
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
55.
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
56.
57.
58.
M E MacDonald A Novelletto C Lin D Tagle G Barnes G Bates S Taylor B Allitto M Altherr R Myers 《Nature genetics》1992,1(2):99-103
Analysis of 78 Huntington's disease (HD) chromosomes with multi-allele markers revealed 26 different haplotypes, suggesting a variety of independent HD mutations. The most frequent haplotype, accounting for about one third of disease chromosomes, suggests that the disease gene is between D4S182 and D4S180. However, the paucity of an expected class of chromosomes that can be related to this major haplotype by assuming single crossovers may reflect the operation of other mechanisms in creating haplotype diversity. Some of these mechanisms sustain alternative scenarios that do not require a multiple mutational origin for HD and/or its positioning between D4S182 and D4S180. 相似文献
59.
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
60.
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