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   36743篇
  免费   110篇
  国内免费   212篇
系统科学   191篇
丛书文集   517篇
教育与普及   61篇
理论与方法论   128篇
现状及发展   17478篇
研究方法   1494篇
综合类   16691篇
自然研究   505篇
  2013年   294篇
  2012年   521篇
  2011年   1001篇
  2010年   214篇
  2008年   647篇
  2007年   713篇
  2006年   731篇
  2005年   676篇
  2004年   666篇
  2003年   632篇
  2002年   664篇
  2001年   1138篇
  2000年   1072篇
  1999年   752篇
  1992年   728篇
  1991年   533篇
  1990年   608篇
  1989年   605篇
  1988年   584篇
  1987年   662篇
  1986年   599篇
  1985年   756篇
  1984年   642篇
  1983年   484篇
  1982年   460篇
  1981年   490篇
  1980年   575篇
  1979年   1185篇
  1978年   1023篇
  1977年   926篇
  1976年   822篇
  1975年   818篇
  1974年   1041篇
  1973年   929篇
  1972年   955篇
  1971年   1064篇
  1970年   1372篇
  1969年   1003篇
  1968年   1067篇
  1967年   1025篇
  1966年   882篇
  1965年   623篇
  1964年   232篇
  1959年   318篇
  1958年   567篇
  1957年   368篇
  1956年   333篇
  1955年   313篇
  1954年   302篇
  1948年   210篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
A topoisomerase II inhibitor, novobiocin, and a deacetylase inhibitor, butyrate, synergistically transformed human liver cells into fibroblast-like cells. This morphological change was associated with an increased production of procollagen type III peptide and a simultaneous assembly of actin, tubulin, vimentin and cytokeratin. Novobiocin and butyrate had no marked effect on the phosphorylation state of cytokeratin proteins, but synergistically enhanced [3H]acetate uptake. From these results, it can be speculated that protein acetylation plays an important role in inducing the assembly of cytoskeletal proteins and the morphological transformation of human liver cells.  相似文献   
42.
We investigated the sound communication associated with mating inNezara viridula (L.) to clarify conflicting reports on the medium through which acoustic signals are borne. Recordings made from virgin pairs ofN. viridula during precopulatory and compulatory behaviour have revealed that signals transmitted as vibrations through the substrate are a more likely means of communication than signals transmitted by a radiation of sound pressure waves through the air (at least in Australian populations). We present evidence (additional to that already in the literature) that air-borne acoustics are an unlikely mechanism of sexual communication in this species. Specifically, the recorded pulse train patterns showed no consistency or repeatability in relation to different stages of mating behaviour. In contrast, frequency spectrograms of substrate-borne signals reveal repeatable patterns that do correspond with the stage of mating behaviour.  相似文献   
43.
Summary Using flurbiprofen, a chiral anti-inflammatory and analgesic 2-arylpropionic acid derivative, the enantiomers of which are not converted to each other (less than 5%) in rats or man, we obtained evidence that prostaglandin synthesis inhibition is primarily mediating the anti-inflammatory activity but prostaglandin synthesis independent mechanisms contribute to the analgesic effects. Thus, the S-form inhibited prostaglandin synthesis, inflammation and nociception in rats. The R-form had much less effect on prostaglandin synthesis and did not affect inflammation. It did, however, block nociception in rats almost as potently as the S-form. S-flurbiprofen, in contrast to the R-form, was clearly ulcerogenic in the gastrointestinal mucosa. These results indicate additional molecular mechanisms of analgesia and suggest the use of R-arylpropionic acids as analgesics.  相似文献   
44.
45.
The genetic engineering of production traits in domestic animals   总被引:1,自引:0,他引:1  
  相似文献   
46.
报道一种新颖的激光开槽、埋槽电极硅太阳电池的结构、工艺流程及其研制结果。在AM1.5,25℃,100mW/cm2的条件下,以面积为45cm3的36片硅片研制的硅太阳电池的输出参数的平均值为JSC=36.1mA/cm2,V∞=633mV,F.F.=0.798,η=18.23%,最后分析了种高性能硅太阳电池的设计特点。  相似文献   
47.
Summary A new bromotyrosine-derived alkaloid with antileukemic activity, purealidin A (5), has been isolated from the Okinawan marine spongePsammaplysilla purea and its chemical structure elucidated on the basis of the spectroscopic data.  相似文献   
48.
We develop in this paper an efficient way to select the best subset threshold autoregressive model. The proposed method uses a stochastic search idea. Differing from most conventional approaches, our method does not require us to fix the delay or the threshold parameters in advance. By adopting the Markov chain Monte Carlo techniques, we can identify the best subset model from a very large of number of possible models, and at the same time estimate the unknown parameters. A simulation experiment shows that the method is very effective. In its application to the US unemployment rate, the stochastic search method successfully selects lag one as the time delay and five best models from more than 4000 choices. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
49.
P Meyrand  J Simmers  M Moulins 《Nature》1991,351(6321):60-63
  相似文献   
50.
Analysis of 78 Huntington's disease (HD) chromosomes with multi-allele markers revealed 26 different haplotypes, suggesting a variety of independent HD mutations. The most frequent haplotype, accounting for about one third of disease chromosomes, suggests that the disease gene is between D4S182 and D4S180. However, the paucity of an expected class of chromosomes that can be related to this major haplotype by assuming single crossovers may reflect the operation of other mechanisms in creating haplotype diversity. Some of these mechanisms sustain alternative scenarios that do not require a multiple mutational origin for HD and/or its positioning between D4S182 and D4S180.  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