全文获取类型
收费全文 | 49845篇 |
免费 | 117篇 |
国内免费 | 206篇 |
专业分类
系统科学 | 269篇 |
丛书文集 | 1078篇 |
教育与普及 | 107篇 |
理论与方法论 | 281篇 |
现状及发展 | 22759篇 |
研究方法 | 1897篇 |
综合类 | 23085篇 |
自然研究 | 692篇 |
出版年
2013年 | 385篇 |
2012年 | 654篇 |
2011年 | 1380篇 |
2010年 | 285篇 |
2008年 | 860篇 |
2007年 | 915篇 |
2006年 | 942篇 |
2005年 | 919篇 |
2004年 | 873篇 |
2003年 | 860篇 |
2002年 | 882篇 |
2001年 | 1447篇 |
2000年 | 1336篇 |
1999年 | 915篇 |
1992年 | 884篇 |
1991年 | 704篇 |
1990年 | 760篇 |
1989年 | 780篇 |
1988年 | 751篇 |
1987年 | 806篇 |
1986年 | 776篇 |
1985年 | 944篇 |
1984年 | 744篇 |
1983年 | 637篇 |
1982年 | 568篇 |
1981年 | 580篇 |
1980年 | 744篇 |
1979年 | 1537篇 |
1978年 | 1331篇 |
1977年 | 1313篇 |
1976年 | 997篇 |
1975年 | 1065篇 |
1974年 | 1482篇 |
1973年 | 1291篇 |
1972年 | 1342篇 |
1971年 | 1576篇 |
1970年 | 2048篇 |
1969年 | 1546篇 |
1968年 | 1514篇 |
1967年 | 1540篇 |
1966年 | 1304篇 |
1965年 | 940篇 |
1964年 | 275篇 |
1959年 | 535篇 |
1958年 | 870篇 |
1957年 | 656篇 |
1956年 | 561篇 |
1955年 | 488篇 |
1954年 | 545篇 |
1948年 | 326篇 |
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
31.
In this paper two alternative loss criteria for the least squares Procrustes problem are studied. These alternative criteria
are based on the Huber function and on the more radical biweight function, which are designed to be resistant to outliers.
Using iterative majorization it is shown how a convergent reweighted least squares algorithm can be developed. In asimulation
study it turns out that the proposed methods perform well over a specific range of contamination. When a uniform dilation
factor is included, mixed results are obtained. The methods also yield a set of weights that can be used for diagnostic purposes. 相似文献
32.
Studies of intracellular traffic in yeast and mammalian systems have implicated members of the Rab family of small GTP-binding proteins as regulators of membrane fusion. We have used the patch clamp technique to measure exocytotic fusion events directly and investigate the role of GTP-binding proteins in regulating exocytosis in mast cells. Intracellular perfusion of mast cells with GTP-gamma S is sufficient to trigger complete exocytotic degranulation in the absence of other intracellular messengers. Here we show that GTP is a potent inhibitor of GTP-gamma S-induced degranulation, indicating that sustained activation of a GTP-binding protein is sufficient for membrane fusion. We have found that synthetic oligopeptides, corresponding to part of the effector domain of Rab3a, stimulate complete exocytotic degranulation, similar to that induced by GTP-gamma S. The response is selective for Rab3a sequence and is strictly dependent on Mg2+ and ATP. This suggests that sustained activation of a Rab3 protein causes exocytotic fusion. The peptide response can be accelerated by GDP-beta S, suggesting that Rab3a peptides compete with endogenous Rab3 proteins for a binding site on a target effector protein, which causes fusion on activation. 相似文献
33.
34.
35.
Vitamin B12 (methylcobalamin) was administered orally (3 mg/day) to 9 healthy subjects for 4 weeks. Nocturnal melatonin levels after exposure to bright light (ca. 2500 lx) were determined, as well as the levels of plasma melatonin over 24 h. The timing of sleep was also recorded. Vitamin B12 was given blind to the subjects and crossed over with placebo. We found that the 24-h melatonin rhythm was significantly phase-advanced (1.1 h) in the vitamin B12 trial as compared with that in the placebo trial. In addition, the 24-h mean of plasma melatonin level was much lower in the vitamin B12 trial than with the placebo. Furthermore, the nocturnal melatonin levels during bright light exposure were significantly lower in the vitamin B12 trial than with the placebo. On the other hand, vitamin B12 did not affect the timing of sleep. These findings raise the possibility that vitamin B12 phase-advances the human circadian rhythm by increasing the light sensitivity of the circadian clock. 相似文献
36.
Shiga toxin and some other protein toxins that act on targets in the cytosol have previously been shown to enter the trans-Golgi network. Transport by this route may be necessary for translocation of the toxin to the cytosol and for intoxication, but it is not known whether the enzymatically active part of the toxins actually enters the cytosol from the trans-Golgi network. It has been suggested that such toxins are transported in a retrograde manner to the endoplasmic reticulum and that translocation occurs in this organelle, but retrograde transport of endocytosed material beyond the trans-Golgi network has never been demonstrated. Here we show that in butyric acid-treated A431 cells endocytosed Shiga toxin is not only transported to the trans-Golgi network, but also to all Golgi stacks, to the endoplasmic reticulum and to the nuclear envelope. Furthermore, butyric acid sensitizes the cells to Shiga toxin, which is consistent with the possibility that retrograde transport is required for translocation of the toxin to the cytosol. 相似文献
37.
38.
39.
40.
1 Results The development of the concepts of nanotechnology has given an important impact on the design of new polymer based materials which are in most cases characterized by a multiphase morphology. When at least one phase has nanometric dimension(s) the system can be considered as a nanocomposite where the interface is not only determining for the adhesion but also may play a role in some bulk properties. Indeed in nanostructured multiphase solids the interface is significant as a bulk component. The... 相似文献