全文获取类型
收费全文 | 32328篇 |
免费 | 92篇 |
国内免费 | 180篇 |
专业分类
系统科学 | 165篇 |
丛书文集 | 540篇 |
教育与普及 | 45篇 |
理论与方法论 | 116篇 |
现状及发展 | 15316篇 |
研究方法 | 1373篇 |
综合类 | 14599篇 |
自然研究 | 446篇 |
出版年
2013年 | 284篇 |
2012年 | 447篇 |
2011年 | 894篇 |
2010年 | 190篇 |
2008年 | 583篇 |
2007年 | 644篇 |
2006年 | 640篇 |
2005年 | 593篇 |
2004年 | 602篇 |
2003年 | 547篇 |
2002年 | 575篇 |
2001年 | 1000篇 |
2000年 | 960篇 |
1999年 | 655篇 |
1992年 | 644篇 |
1991年 | 459篇 |
1990年 | 529篇 |
1989年 | 521篇 |
1988年 | 486篇 |
1987年 | 589篇 |
1986年 | 514篇 |
1985年 | 672篇 |
1984年 | 532篇 |
1983年 | 419篇 |
1982年 | 386篇 |
1981年 | 405篇 |
1980年 | 505篇 |
1979年 | 973篇 |
1978年 | 849篇 |
1977年 | 830篇 |
1976年 | 691篇 |
1975年 | 723篇 |
1974年 | 955篇 |
1973年 | 834篇 |
1972年 | 858篇 |
1971年 | 956篇 |
1970年 | 1253篇 |
1969年 | 919篇 |
1968年 | 920篇 |
1967年 | 905篇 |
1966年 | 775篇 |
1965年 | 529篇 |
1964年 | 197篇 |
1959年 | 302篇 |
1958年 | 505篇 |
1957年 | 352篇 |
1956年 | 300篇 |
1955年 | 285篇 |
1954年 | 282篇 |
1948年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
73.
74.
75.
76.
77.
78.
1 Results The photosynthetic bacterial reaction center (RC) is a membrane protein complex.The RC is composed of three protein subunits and redox components such as bacteriochlorophylls, bacteriopheophytins,and quinones.The RC performs the photochemical electron transfer from the bacteriochlorophyll dimer through a series of electron donor and acceptor molecules to a secondary quinone,QB.QB accepts electrons from a primary quinone,QA,in two sequential electron transfer reactions.The second electron trans... 相似文献
79.
The aim of this paper is to compare the forecasting performance of competing threshold models, in order to capture the asymmetric effect in the volatility. We focus on examining the relative out‐of‐sample forecasting ability of the SETAR‐Threshold GARCH (SETAR‐TGARCH) and the SETAR‐Threshold Stochastic Volatility (SETAR‐THSV) models compared to the GARCH model and Stochastic Volatility (SV) model. However, the main problem in evaluating the predictive ability of volatility models is that the ‘true’ underlying volatility process is not observable and thus a proxy must be defined for the unobservable volatility. For the class of nonlinear state space models (SETAR‐THSV and SV), a modified version of the SIR algorithm has been used to estimate the unknown parameters. The forecasting performance of competing models has been compared for two return time series: IBEX 35 and S&P 500. We explore whether the increase in the complexity of the model implies that its forecasting ability improves. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
80.
On the face of it, the directors of new large scientific projects have an impossible task. They have to make technical decisions about sciences in which they have never made a research contribution—sciences in which they have no contributory expertise. Furthermore, these decisions must be accepted and respected by the scientists who are making research contributions. The problem is discussed in two interviews conducted with two directors of large scientific projects. The paradox is resolved for the managers by their use of interactional and referred expertise. The same analysis might be applicable to management in general. An Appendix, co-authored with Jeff Shrager, compares the notion of referred expertise with contributory expertise. 相似文献