全文获取类型
收费全文 | 25748篇 |
免费 | 63篇 |
国内免费 | 99篇 |
专业分类
系统科学 | 122篇 |
丛书文集 | 199篇 |
教育与普及 | 98篇 |
理论与方法论 | 114篇 |
现状及发展 | 10959篇 |
研究方法 | 998篇 |
综合类 | 13002篇 |
自然研究 | 418篇 |
出版年
2013年 | 155篇 |
2012年 | 328篇 |
2011年 | 758篇 |
2008年 | 398篇 |
2007年 | 447篇 |
2006年 | 437篇 |
2005年 | 456篇 |
2004年 | 537篇 |
2003年 | 466篇 |
2002年 | 451篇 |
2001年 | 674篇 |
2000年 | 719篇 |
1999年 | 471篇 |
1994年 | 344篇 |
1992年 | 420篇 |
1991年 | 337篇 |
1990年 | 400篇 |
1989年 | 390篇 |
1988年 | 399篇 |
1987年 | 424篇 |
1986年 | 415篇 |
1985年 | 526篇 |
1984年 | 378篇 |
1983年 | 369篇 |
1982年 | 298篇 |
1981年 | 295篇 |
1980年 | 347篇 |
1979年 | 755篇 |
1978年 | 619篇 |
1977年 | 586篇 |
1976年 | 492篇 |
1975年 | 565篇 |
1974年 | 726篇 |
1973年 | 644篇 |
1972年 | 628篇 |
1971年 | 816篇 |
1970年 | 988篇 |
1969年 | 777篇 |
1968年 | 761篇 |
1967年 | 711篇 |
1966年 | 693篇 |
1965年 | 487篇 |
1964年 | 206篇 |
1959年 | 255篇 |
1958年 | 482篇 |
1957年 | 343篇 |
1956年 | 307篇 |
1955年 | 280篇 |
1954年 | 260篇 |
1948年 | 277篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
12.
Although theoretical studies show that overcompensatory density-dependent mechanisms can potentially generate regular or chaotic fluctuations in animal numbers, the majority of realistic single-species models of invertebrate populations are not overcompensatory enough to cause sustained population cycles. The possibility that overcompensation may generate cycles or chaos in vertebrate populations has seldom been considered. Here we show that highly overcompensatng density-dependent mortality can generate recurrent population crashes consistent with those observed in a naturally limited population of Soay sheep. The observed interval of three or more years between crashes points to sharp 'focusing' of mortality over a narrow range of population density. 相似文献
13.
Astacin, a digestive zinc-endopeptidase from the crayfish Astacus astacus L., is the prototype for the 'astacin family', which includes mammalian metallo-endopeptidases and developmentally regulated proteins of man, fruitfly, frog and sea urchin. Here we report the X-ray crystal structure of astacin, which reveals a deep active-site cleft, with the zinc at its bottom ligated by three histidines, a water molecule and a more remote tyrosine. The third histidine (His 102) forms part of a consensus sequence, shared not only by the members of the astacin family, but also by otherwise sequentially unrelated proteinases, such as vertebrate collagenases. It may therefore represent the elusive 'third' zinc ligand in these enzymes. The amino terminus of astacin is buried forming an internal salt-bridge with Glu 103, adjacent to His 102. Astacin pro-forms extended at the N terminus, as observed for some 'latent' mammalian astacin homologues, did not exhibit this 'active' conformation, indicating an activation mechanism reminiscent of trypsin-like serine proteinases. 相似文献
14.
15.
16.
17.
H R Brenner 《Nature》1985,317(6038):572-573
18.
19.
1 Results This paper presents the development of platinum nanocomposites structures based on organically modified c.a.2 nm core platinum nanoparticles.The chemical modification of the 4-mercaptoaniline functionalized particles by various in coming molecules is evidenced and precisely quantified.The particles can be dissolved like molecules in various solvents depending on the features of the new crown and X-rays shows that the interparticle distance is affected by the crown modification.These platinum n... 相似文献
20.
This paper assesses the information content of two survey indicators for consumption developments in the near future for eight European countries in the period 1985–1998. Empirical work on this topic typically focuses on consumer confidence, the perceptions of buyers of consumption goods. This paper examines whether perceptions of sellers of consumption goods, measured by retail trade surveys, may also improve short‐term monitoring of consumption. We find that both consumer confidence and retailer confidence embody valuable information, when analysed in isolation. For France, Italy and Spain we conclude that adding retail confidence does not improve the indicator model once consumer confidence has been included. For the UK the reverse case is obtained. For the remaining four countries we show that combining consumer sentiment and retail trade confidence into a composite indicator leads to optimal results. Our results suggest that incorporating information from retail trade surveys may offer significant benefits for the analysis of short‐term prospects of consumption. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献