全文获取类型
收费全文 | 1961篇 |
免费 | 9篇 |
国内免费 | 18篇 |
专业分类
系统科学 | 33篇 |
丛书文集 | 2篇 |
教育与普及 | 12篇 |
理论与方法论 | 49篇 |
现状及发展 | 235篇 |
研究方法 | 311篇 |
综合类 | 1235篇 |
自然研究 | 111篇 |
出版年
2021年 | 7篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 15篇 |
2017年 | 20篇 |
2016年 | 22篇 |
2015年 | 15篇 |
2014年 | 19篇 |
2013年 | 27篇 |
2012年 | 181篇 |
2011年 | 300篇 |
2010年 | 72篇 |
2009年 | 14篇 |
2008年 | 137篇 |
2007年 | 202篇 |
2006年 | 167篇 |
2005年 | 181篇 |
2004年 | 160篇 |
2003年 | 150篇 |
2002年 | 164篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 12篇 |
1998年 | 2篇 |
1997年 | 5篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1985年 | 4篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1971年 | 3篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1968年 | 5篇 |
1967年 | 2篇 |
1966年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有1988条查询结果,搜索用时 8 毫秒
281.
282.
283.
284.
The ‘international’ electrical units, initially defined by the International Electrical Congress of Chicago in 1893, represented a major step forward in international electrical standardisation. Yet they were flawed both theoretically and technically, were adopted inconsistently in different countries and were soon subject to criticism and revision. This paper addresses the extent to which the international units—notwithstanding their flaws—were in fact adequate for the needs of engineering, commerce and science at the time, and concludes that the practical position was actually very satisfactory. It also addresses how the international electrical units came in to being, in the context of alternative philosophies of the nature of the standardisation process—that it can be essentially central or local. The central influence on the international units was the series of international electrical congresses of the late nineteenth century, which I conclude were a highly unsatisfactory mechanism, responsible for many of the weaknesses in the resulting standards. In contrast, I argue that it was local influences, the networks of scientists and standards laboratories that defined and maintained the standards, which gave them their strength. 相似文献
285.
Michael Funke 《Journal of forecasting》1992,11(2):111-125
The purpose of the paper is to investigate the accuracy of forecasts derived from univariate and multivariate time-series models. An iterative method to adjust for impact assessment in univariate ARIMA models is discussed and illustrated for the German unemployment rate. Finally, we also examine the pros and cons of the impact assessment model in comparison with VAR models. 相似文献
286.
本书是根据作者在Hertfordshire大学与Bergamo大学为本科生和研究生教授非线性优化方面的讲稿编撰而成的,中心内容是非线性最优化的数值方法。 相似文献
287.
288.
289.
290.
Stefansson H Helgason A Thorleifsson G Steinthorsdottir V Masson G Barnard J Baker A Jonasdottir A Ingason A Gudnadottir VG Desnica N Hicks A Gylfason A Gudbjartsson DF Jonsdottir GM Sainz J Agnarsson K Birgisdottir B Ghosh S Olafsdottir A Cazier JB Kristjansson K Frigge ML Thorgeirsson TE Gulcher JR Kong A Stefansson K 《Nature genetics》2005,37(2):129-137
A refined physical map of chromosome 17q21.31 uncovered a 900-kb inversion polymorphism. Chromosomes with the inverted segment in different orientations represent two distinct lineages, H1 and H2, that have diverged for as much as 3 million years and show no evidence of having recombined. The H2 lineage is rare in Africans, almost absent in East Asians but found at a frequency of 20% in Europeans, in whom the haplotype structure is indicative of a history of positive selection. Here we show that the H2 lineage is undergoing positive selection in the Icelandic population, such that carrier females have more children and have higher recombination rates than noncarriers. 相似文献