全文获取类型
收费全文 | 622篇 |
免费 | 4篇 |
国内免费 | 27篇 |
专业分类
系统科学 | 18篇 |
丛书文集 | 3篇 |
教育与普及 | 17篇 |
理论与方法论 | 2篇 |
现状及发展 | 115篇 |
研究方法 | 53篇 |
综合类 | 437篇 |
自然研究 | 8篇 |
出版年
2021年 | 4篇 |
2020年 | 4篇 |
2019年 | 8篇 |
2018年 | 21篇 |
2017年 | 14篇 |
2016年 | 7篇 |
2015年 | 7篇 |
2014年 | 9篇 |
2013年 | 3篇 |
2012年 | 16篇 |
2011年 | 28篇 |
2010年 | 11篇 |
2009年 | 21篇 |
2008年 | 22篇 |
2007年 | 34篇 |
2006年 | 25篇 |
2005年 | 27篇 |
2004年 | 52篇 |
2003年 | 30篇 |
2002年 | 23篇 |
2001年 | 68篇 |
2000年 | 58篇 |
1999年 | 38篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1992年 | 10篇 |
1991年 | 12篇 |
1990年 | 7篇 |
1989年 | 2篇 |
1988年 | 8篇 |
1987年 | 7篇 |
1986年 | 2篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1972年 | 2篇 |
1970年 | 4篇 |
1969年 | 3篇 |
1968年 | 2篇 |
1967年 | 2篇 |
1958年 | 2篇 |
1957年 | 4篇 |
1955年 | 1篇 |
1954年 | 1篇 |
1948年 | 1篇 |
排序方式: 共有653条查询结果,搜索用时 0 毫秒
621.
Extreme sexual dimorphism in a Miocene hominoid 总被引:2,自引:0,他引:2
Some Miocene hominoids may have been extremely sexually dimorphic for body size, inferred from the apparent dimorphism of dental and gnathic remains. But this has never been demonstrated convincingly for any fossil species because of small sample sizes, uncertainties about the number of species in most fossil samples, and the inability to reliably sex individual specimens. Here we demonstrate a case of extreme dental dimorphism, and presumed body-size dimorphism, in a Miocene hominoid sample in which these limitations have been overcome. Lufengpithecus lufengensis from the late Miocene site of Lufeng, China, was more dimorphic than the most dimorphic living hominoid, the orangutan, and may have been more dimorphic than any living anthropoid. 相似文献
622.
J. Q. Smith 《Journal of forecasting》1985,4(3):283-291
Diagnostic checks have become a standard tool for helping to assess the adequacy of a forecasting system since Box and Jenkins' (1970) ARIMA modelling technique became popular. However, most of the research has developed checks for normal or second-order stationary models. This paper gives various diagnostic checks that can be performed simply on nonnormal, non-standard models such as the class of multiprocess models (Harrison and Stevens, 1976), where residuals are definitely not normal. The performance to date of these models can then be objectively scrutinized on-line. Examples, including a generalized cusum technique, are given to illustrate the effectiveness of the techniques on specific series. 相似文献
623.
Crystal structures of insulin have been determined in various distinct forms, the relevance of which to receptor recognition has long been the subject of speculation. Recently the crystal structure of an inactive insulin analogue has been determined and, surprisingly, found to have a conformation identical to native insulin. On this basis Dodson and colleagues have suggested that the known insulin crystal structures reflect an inactive conformation, and that a change in conformation is required for activity--specifically, the carboxy terminal residues of the B-chain are proposed to separate from the amino terminal residues of the A-chain. Here we report the solution structure of an active insulin mutant, determined by two-dimensional NMR, which supports this hypothesis. In the mutant, the carboxy terminal beta-turn and beta-strand of the B-chain are destabilized and do not pack across the rest of the molecule. We suggest that analogous detachment of the carboxy terminal region of the B-chain occurs in native insulin on binding to its receptor. Our finding that partial unfolding of the B-chain exposes an alternative protein surface rationalizes the receptor-binding properties of a series of anomalous insulin analogues, including a mutant insulin associated with diabetes mellitus in man. 相似文献
624.
625.
626.
C P Huu I Sokan A Pham H Chanh G Pillat V T Nguyen 《Comptes rendus des séances de l'Académie des sciences. Série D, Sciences naturelles》1980,290(24):1549-1552
In the isolated Rabbit heart, high pressure, on the one hand, inhibits the noradrenaline release, and on the other hand, induces both the biosynthesis and release of PGs(PGE2,PGF2 alpha) from the auricles and mainly from the right auricle i.e. where the sympathetic nerve endings are the most numerous. Therefore, high pressure, by a negative feedback process, plays a great part in moderating the adrenergic neurotransmission responsible for the hypertension. These effects result from a supplementary pressure which acts on the walls of coronary arteries and arterioles and modifies their distensibility. It thus appears that in the walls of arteries and arterioles there are baroreceptors which are sensitive to the pressure variations in order to regulate adrenergic neurotransmission, by means of a reflex path still unknown. 相似文献
627.
628.
1 Results Molybdenum dioxide, with excellent chemical and physical properties, has been widely used in various fields[1]. As an anode material for lithium ion battery, it exhibits higher capacity than commercial carbonaceous materials, and proper morphology, structure and particle size are necessary for MoO2 to be employed as an anode material for lithium ion battery[2].We have successfully obtained tremella-like structure self-assembled with hexagonal MoO2 nanosheets via hydrothermal method using ethyl... 相似文献
629.
1 Introduction Immobilization of homogeneous catalysts onto polymer supports through covalent attachment has received wide attention because these materials offer advantage features of heterogeneous catalysis to homogeneous systems.The polymer-supported catalysts enhance their thermal stability,selectivity,recyclability and easy separation from reaction products leading to the operationally flexible[1-2].Such behaviour prompted us to know the effect of pore structures of polymer supporters on catalytic ... 相似文献
630.