首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   17932篇
  免费   121篇
  国内免费   119篇
系统科学   98篇
丛书文集   118篇
教育与普及   34篇
理论与方法论   49篇
现状及发展   8233篇
研究方法   851篇
综合类   8586篇
自然研究   203篇
  2012年   288篇
  2011年   469篇
  2010年   121篇
  2009年   117篇
  2008年   318篇
  2007年   380篇
  2006年   351篇
  2005年   340篇
  2004年   407篇
  2003年   298篇
  2002年   358篇
  2001年   624篇
  2000年   626篇
  1999年   447篇
  1992年   329篇
  1991年   288篇
  1990年   295篇
  1989年   269篇
  1988年   287篇
  1987年   308篇
  1986年   288篇
  1985年   373篇
  1984年   319篇
  1983年   252篇
  1982年   244篇
  1981年   270篇
  1980年   260篇
  1979年   605篇
  1978年   507篇
  1977年   401篇
  1976年   404篇
  1975年   398篇
  1974年   452篇
  1973年   381篇
  1972年   405篇
  1971年   494篇
  1970年   615篇
  1969年   443篇
  1968年   476篇
  1967年   478篇
  1966年   408篇
  1965年   311篇
  1964年   126篇
  1959年   134篇
  1958年   252篇
  1957年   168篇
  1956年   160篇
  1955年   132篇
  1954年   143篇
  1948年   110篇
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
1.
J. D. Trout has recently developed a new defense of scientific realism, a new version of the No Miracles Argument. I critically evaluate Trout's novel defense of realism. I argue that Trout's argument for scientific realism and the related explanation for the success of science are self-defeating. In the process of arguing against the traditional realist strategies for explaining the success of science, he inadvertently undermines his own argument.  相似文献   
2.
Areas of the Knysna estuarine bay in the Western Cape are dominated by three endemic South African truncatelloid microgastropods, temporarily known as ‘Hydrobiaknysnaensis (Krauss), ‘Assimineacapensis (Sowerby) and ‘Assimineaglobulus Connolly. Although first described 80–170 years ago and present in abundance (up to 100,000 m?2), they remain surrounded by confusion and still await taxonomic assignment, largely because they appear most atypical members of their groups by virtue of anatomy and/or biogeography and/or habitat. This study contributes in-life perspectives to morphological and phylogenetic analyses known to be on-going. At Knysna, they are syntopic: at least two occurring in >85% and all three in >40% of individual 0.0026 m2 samples from their region of dominance. Nevertheless, they tend to greater abundance in divergent microhabitats; ‘A.’ globulus dominating higher tidal levels, and ‘A.’ capensis and ‘Hydrobia’ lower ones; the former especially unvegetated sediment, the latter, if anything, seagrass. Interspecific feeding interactions appear unlikely to be responsible for these patterns, other evidence suggesting that all are maintained below carrying capacity. Field biology of ‘H.’ knysnaensis generally appears equivalent to that of northern-hemisphere intertidal hydrobiids and that of ‘A.’ globulus is typically assimineid, albeit at atypically low shore height. Unlike assimineids, however, ‘A.’ capensis is truly aquatic. The success of these truncatelloids in unusual circumstances may be consequent on the absence from South Africa of other microgastropod groups that fill their niches elsewhere in the southern hemisphere.  相似文献   
3.
This paper undertakes a comprehensive examination of 10 measures of core inflation and evaluates which measure produces the best forecast of headline inflation out‐of‐sample. We use the Personal Consumption Expenditure Price Index as our measure of inflation. We use two sets of components (17 and 50) of the Personal Consumption Expenditure Price Index to construct these core inflation measures and evaluate these measures at the three time horizons (6, 12 and 24 months) most relevant for monetary policy decisions. The best measure of core inflation for both sets of components and over all time horizons uses weights based on the first principal component of the disaggregated (component‐level) prices. Interestingly, the results vary by the number of components used; when more components are used the weights based on the persistence of each component is statistically equivalent to the weights generated by the first principal component. However, those forecasts using the persistence of 50 components are statistically worse than those generated using the first principal component of 17 components. The statistical superiority of the principal component method is due to the fact that it extracts (in the first principal component) the common source of variation in the component level prices that accurately describes trend inflation over the next 6–24 months.  相似文献   
4.
5.
6.
7.
8.
根据我国当前电力系统运行情况,对电网无功功率补偿的必要性及通用补偿方法及其装置进行了具体阐述。  相似文献   
9.
以具体工程为例,介绍了建筑物地基处理的设计、施工中遵循的原则,施工中遇到特殊情况时采取的处理方法和必要的检测方法,对几种桩基的设计要求、施工工艺及过程、质量控制等情况进行了简单阐述。  相似文献   
10.
通过1998年、2000年的试验研究,系统分析了顶凌覆盖技术在高寒区节水增温保温、增产增收的机理及其应用效果。提出该技术可以缓解低温干旱特别是春旱对农业生产的困扰,是一项可以抗旱保春播增产增收的实用技术,在高寒地区具有较高的推广价值。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号