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   40696篇
  免费   191篇
  国内免费   329篇
系统科学   490篇
丛书文集   793篇
教育与普及   105篇
理论与方法论   136篇
现状及发展   16638篇
研究方法   1588篇
综合类   20557篇
自然研究   909篇
  2013年   475篇
  2012年   775篇
  2011年   1712篇
  2010年   498篇
  2009年   451篇
  2008年   894篇
  2007年   1082篇
  2006年   1030篇
  2005年   1001篇
  2004年   901篇
  2003年   709篇
  2002年   743篇
  2001年   1156篇
  2000年   1177篇
  1999年   819篇
  1992年   723篇
  1991年   548篇
  1990年   580篇
  1989年   542篇
  1988年   538篇
  1987年   542篇
  1986年   555篇
  1985年   714篇
  1984年   552篇
  1983年   431篇
  1982年   398篇
  1981年   428篇
  1980年   508篇
  1979年   1170篇
  1978年   900篇
  1977年   920篇
  1976年   700篇
  1975年   747篇
  1974年   1084篇
  1973年   911篇
  1972年   985篇
  1971年   1088篇
  1970年   1409篇
  1969年   1150篇
  1968年   1030篇
  1967年   1077篇
  1966年   1002篇
  1965年   739篇
  1959年   387篇
  1958年   715篇
  1957年   508篇
  1956年   399篇
  1955年   375篇
  1954年   405篇
  1948年   257篇
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
1.
This is a contribution towards a history and philosophy of modeling in its early stages in electromagnetism. In 1873, James Clerk Maxwell (1831–1879) hinted at the methodology of modeling at the end of his Treatise on Electricity and Magnetism. We focus on Maxwell's impact on physicists who immediately followed him, specifically Oliver Lodge (1851–1940) and George Francis FitzGerald (1851–1901). We begin with the role that the scientific concept of model played in the late nineteenth century, as assessed by Ludwig Boltzmann (1844–1906). We then discuss the role of hypothesis as a methodology, the appeal to (dynamical) illustration, and the way Maxwell applied model and working model in his studies of electromagnetism. We show that for Maxwell these key terms were kept distinct, but Lodge did not maintain these distinctions and, in this regard, FitzGerald followed Lodge. Notwithstanding Lodge's influence, Fitzgerald modified Maxwell's theory based on the mechanical model he designed, thereby implicitly taking the first step towards modeling. This methodology consists in drawing consequences from the (mechanical) model to the (electrodynamic) theory and modifying the latter in light of the functioning of the former. At the core of our argument is the thesis that it was a methodological novelty to move from the concept of model to the methodology of modeling. The introduction of modeling as a new methodology into physics in the late nineteenth century was a major event which deserves proper recognition.  相似文献   
2.
通过张量分析,给出一个具有一般形式的非负张量谱半径的上下界估计不等式,在特别情况下改进了相关非负张量谱半径的估计不等式.  相似文献   
3.
4.
5.
6.
We measure and predict states of Activation and Happiness using a body sensing application connected to smartwatches. Through the sensors of commercially available smartwatches we collect individual mood states and correlate them with body sensing data such as acceleration, heart rate, light level data, and location, through the GPS sensor built into the smartphone connected to the smartwatch. We polled users on the smartwatch for seven weeks four times per day asking for their mood state. We found that both Happiness and Activation are negatively correlated with heart beats and with the levels of light. People tend to be happier when they are moving more intensely and are feeling less activated during weekends. We also found that people with a lower Conscientiousness and Neuroticism and higher Agreeableness tend to be happy more frequently. In addition, more Activation can be predicted by lower Openness to experience and higher Agreeableness and Conscientiousness. Lastly, we find that tracking people’s geographical coordinates might play an important role in predicting Happiness and Activation. The methodology we propose is a first step towards building an automated mood tracking system, to be used for better teamwork and in combination with social network analysis studies.  相似文献   
7.
Clutch size (CS) is a well-studied life history characteristic, and optimal egg size theory predicts that CS should correlate positively with reproductive investment. Turtles are good subjects for studies of reproductive strategies because few turtle species exhibit parental care; therefore quantifying their short-term reproductive investment is mostly limited to measuring egg size and number. Clutch size is usually reported as an average value for each turtle population, and where CS variation is noted, it is usually attributable to variation in adult body size. In two long-term studies of ecologically dissimilar species we detected a dramatic increase in CS in a common snapping turtle (Chelydra serpentina) population and in a diamondback terrapin (Malaclemys terrapin) population. It is unknown whether these changes are due to either variation in adult body size or resource availability; but the temporal patterns we observed have apparently never been reported previously. These trends remain unexplained and should be explored further.  相似文献   
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.
系统地总结和介绍了我国高性能计算机,包括“银河”系列巨型机、“曙光”系列巨型机、“神威”系列巨型机、“深腾”系列巨型机以及“深超”系列巨型机的开发研制以及应用情况。  相似文献   
10.
对矿井索车优化设计中的几个关键性问题进行了探讨,包括驱动轮和尾轮的设计、钢丝绳直径的选择计算、托绳轮间距的确定、绳卡的设计、张紧力和张紧行程的确定、互锁问题的解决等。  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