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   56428篇
  免费   240篇
  国内免费   549篇
系统科学   1232篇
丛书文集   357篇
教育与普及   291篇
理论与方法论   516篇
现状及发展   32501篇
研究方法   912篇
综合类   19218篇
自然研究   2190篇
  2013年   803篇
  2012年   635篇
  2011年   2674篇
  2009年   617篇
  2008年   819篇
  2007年   924篇
  2006年   1033篇
  2005年   1197篇
  2004年   2292篇
  2003年   1894篇
  2002年   1594篇
  2001年   1448篇
  2000年   957篇
  1999年   981篇
  1998年   642篇
  1997年   774篇
  1996年   513篇
  1994年   687篇
  1993年   689篇
  1992年   917篇
  1991年   785篇
  1990年   851篇
  1989年   670篇
  1988年   627篇
  1987年   666篇
  1986年   744篇
  1985年   861篇
  1984年   785篇
  1983年   668篇
  1982年   790篇
  1981年   832篇
  1980年   940篇
  1979年   1431篇
  1978年   1285篇
  1977年   1287篇
  1976年   1089篇
  1975年   1088篇
  1974年   1020篇
  1973年   1247篇
  1972年   1323篇
  1971年   1366篇
  1970年   1480篇
  1969年   1273篇
  1968年   1172篇
  1967年   1123篇
  1966年   921篇
  1965年   732篇
  1964年   479篇
  1958年   564篇
  1957年   486篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
31.
水轮机水力稳定性的分析   总被引:2,自引:0,他引:2  
根据收集国内外水轮机水力稳定性资料 ,分析了引起水轮机水力不稳定性的因素 ,并结合实例分析可能引起的危害 ,最后提出了解决问题的方法和建议  相似文献   
32.
介绍了Authorware软件使用过程中的一些常见问题并给出了具体的解决方案 ;说明了在Au thorware软件使用过程中如何利用一些操作技巧  相似文献   
33.
文中就多层住宅生活、消防共用系统的技术经济问题进行了讨论。提出了采用紧急关闭水阀和合理选用变频泵等技术措施。为共用系统的功能发挥和运行可靠提供了保障。  相似文献   
34.
太阳能空调的可行性研究   总被引:3,自引:0,他引:3  
对太阳能空调系统进行了定性分析 ,从环保、节能角度阐述了发展太阳能空调的优势所在。提出了太阳能空调将具有广阔的市场前景。  相似文献   
35.
The autosomal recessive disorder Shwachman-Diamond syndrome, characterized by bone marrow failure and leukemia predisposition, is caused by deficiency of the highly conserved Shwachman-Bodian-Diamond syndrome (SBDS) protein. Here, we identify the function of the yeast SBDS ortholog Sdo1, showing that it is critical for the release and recycling of the nucleolar shuttling factor Tif6 from pre-60S ribosomes, a key step in 60S maturation and translational activation of ribosomes. Using genome-wide synthetic genetic array mapping, we identified multiple TIF6 gain-of-function alleles that suppressed the pre-60S nuclear export defects and cytoplasmic mislocalization of Tif6 observed in sdo1Delta cells. Sdo1 appears to function within a pathway containing elongation factor-like 1, and together they control translational activation of ribosomes. Thus, our data link defective late 60S ribosomal subunit maturation to an inherited bone marrow failure syndrome associated with leukemia predisposition.  相似文献   
36.
Resequencing genes provides the opportunity to assess the full spectrum of variants that influence complex traits. Here we report the first application of resequencing to a large population (n = 3,551) to examine the role of the adipokine ANGPTL4 in lipid metabolism. Nonsynonymous variants in ANGPTL4 were more prevalent in individuals with triglyceride levels in the lowest quartile than in individuals with levels in the highest quartile (P = 0.016). One variant (E40K), present in approximately 3% of European Americans, was associated with significantly lower plasma levels of triglyceride and higher levels of high-density lipoprotein cholesterol in European Americans from the Atherosclerosis Risk in Communities Study and in Danes from the Copenhagen City Heart Study. The ratio of nonsynonymous to synonymous variants was higher in European Americans than in African Americans (4:1 versus 1.3:1), suggesting population-specific relaxation of purifying selection. Thus, resequencing of ANGPTL4 in a multiethnic population allowed analysis of the phenotypic effects of both rare and common variants while taking advantage of genetic variation arising from ethnic differences in population history.  相似文献   
37.
In this paper, the collision problem of two moving objects is investigated. The objects are described by two algebraic sets (ellipses or circles in the paper). The collision problem discussed involves both static and dynamic case. The static case is that each object moves with known velocity. We use nonlinear programming to decide whether the objects collide. The dynamic case is that each object is controlled by a constraint external force which can be regulated online. For the dynamic case, the collision problem can be modelled as a Minmax problem which can be solved by using differential games. If collision occurs, the time and place of the first collision are given. The moving trajectories are provided in the paper.  相似文献   
38.
Summary A study was undertaken on the variations of the redox potential level produced by streptomycinin vitro andin vivo. We have been able to show that, owing to an oxidative effect, streptomycin produces an increase of the redox potential level. This oxidative effect varies in degree according to the condition of the patient.We also found that in the blood and in the spinal fluid of patients suffering from tubercular meningitis factors are present which inhibit the action of streptomycin.The results of our findings lead to the conclusion that the dose of streptomycin must be varied according to the condition of the patient if the constant level required for an efficient therapy is to be maintained in the blood and in the spinal fluid.  相似文献   
39.
40.
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