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   48283篇
  免费   308篇
  国内免费   636篇
系统科学   1272篇
丛书文集   205篇
教育与普及   326篇
理论与方法论   521篇
现状及发展   30175篇
研究方法   246篇
综合类   14292篇
自然研究   2190篇
  2014年   459篇
  2013年   880篇
  2012年   522篇
  2011年   2521篇
  2010年   464篇
  2009年   707篇
  2008年   719篇
  2007年   777篇
  2006年   850篇
  2005年   1016篇
  2004年   2151篇
  2003年   1746篇
  2002年   1411篇
  2001年   1126篇
  2000年   632篇
  1999年   805篇
  1998年   657篇
  1997年   782篇
  1996年   539篇
  1994年   688篇
  1993年   700篇
  1992年   726篇
  1991年   651篇
  1990年   705篇
  1989年   528篇
  1988年   517篇
  1987年   507篇
  1986年   581篇
  1985年   713篇
  1984年   663篇
  1983年   567篇
  1982年   734篇
  1981年   774篇
  1980年   841篇
  1979年   1150篇
  1978年   1078篇
  1977年   1091篇
  1976年   953篇
  1975年   915篇
  1974年   697篇
  1973年   1020篇
  1972年   1091篇
  1971年   1049篇
  1970年   1000篇
  1969年   988篇
  1968年   959篇
  1967年   803篇
  1966年   663篇
  1965年   565篇
  1964年   452篇
排序方式: 共有10000条查询结果,搜索用时 859 毫秒
11.
Archive for History of Exact Sciences - We show that Dedekind, in his proof of the principle of definition by mathematical recursion, used implicitly both the concept of an inductive cone from an...  相似文献   
12.
Classical dendritic cells (cDCs) play a pivotal role in the early events that tip the immune response toward persistence or viral control. In vitro studies indicate that HIV infection induces the dysregulation of cDCs through binding of the LILRB2 inhibitory receptor to its MHC-I ligands and the strength of this interaction was proposed to drive disease progression. However, the dynamics of the LILRB2/MHC-I inhibitory axis in cDCs during early immune responses against HIV are yet unknown. Here, we show that early HIV-1 infection induces a strong and simultaneous increase of LILRB2 and MHC-I expression on the surface of blood cDCs. We further characterized the early dynamics of LILRB2 and MHC-I expression by showing that SIVmac251 infection of macaques promotes coordinated up-regulation of LILRB2 and MHC-I on cDCs and monocytes/macrophages, from blood and lymph nodes. Orientation towards the LILRB2/MHC-I inhibitory axis starts from the first days of infection and is transiently induced in the entire cDC population in acute phase. Analysis of the factors involved indicates that HIV-1 replication, TLR7/8 triggering, and treatment by IL-10 or type I IFNs increase LILRB2 expression. Finally, enhancement of the LILRB2/MHC-I inhibitory axis is specific to HIV-1 and SIVmac251 infections, as expression of LILRB2 on cDCs decreased in naturally controlled chikungunya virus infection of macaques. Altogether, our data reveal a unique up-regulation of LILRB2 and its MHC-I ligands on cDCs in the early phase of SIV/HIV infection, which may account for immune dysregulation at a critical stage of the anti-viral response.  相似文献   
13.
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   
14.
15.
16.
17.
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   
18.
19.
If the general arguments concerning theinvolvement of variation and selection inexplanations of ``fit'' are valid, then variationand selection explanations should beappropriate, or at least potentiallyappropriate, outside the paradigm historisticdomains of biology and knowledge. In thisdiscussion, I wish to indicate some potentialroles for variation and selection infoundational physics – specifically inquantum field theory. I will not be attemptingany full coherent ontology for quantum fieldtheory – none currently exists, and none islikely for at least the short term future. Instead, I wish to engage in some partiallyspeculative interpretations of some interestingresults in this area with the aim ofdemonstrating that variation and selectionnotions might play a role even here. Ifvariation and selection can survive in even asinhospitable and non-paradigmatic a terrain asfoundational physics, then it can surviveanywhere.  相似文献   
20.
自主机器人人工进化方法研究   总被引:1,自引:0,他引:1  
在介绍人工进化研究状况的基础上 ,着重阐述了进化机器人学的关键问题 :基本概念、进化技术、进化计算和实验研究方法等 ,探索了进化机器人学未来的研究趋势及所面临的挑战 .  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