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   49851篇
  免费   112篇
  国内免费   207篇
系统科学   269篇
丛书文集   1078篇
教育与普及   107篇
理论与方法论   281篇
现状及发展   22759篇
研究方法   1897篇
综合类   23087篇
自然研究   692篇
  2013年   385篇
  2012年   654篇
  2011年   1380篇
  2010年   285篇
  2008年   860篇
  2007年   915篇
  2006年   942篇
  2005年   919篇
  2004年   873篇
  2003年   860篇
  2002年   882篇
  2001年   1447篇
  2000年   1336篇
  1999年   914篇
  1992年   884篇
  1991年   704篇
  1990年   760篇
  1989年   780篇
  1988年   751篇
  1987年   806篇
  1986年   776篇
  1985年   944篇
  1984年   744篇
  1983年   637篇
  1982年   568篇
  1981年   580篇
  1980年   744篇
  1979年   1537篇
  1978年   1331篇
  1977年   1313篇
  1976年   997篇
  1975年   1065篇
  1974年   1482篇
  1973年   1291篇
  1972年   1342篇
  1971年   1576篇
  1970年   2048篇
  1969年   1546篇
  1968年   1514篇
  1967年   1540篇
  1966年   1304篇
  1965年   940篇
  1964年   275篇
  1959年   535篇
  1958年   870篇
  1957年   656篇
  1956年   561篇
  1955年   488篇
  1954年   545篇
  1948年   326篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
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   
32.
1.0MeV208Pb离子在非晶Si中的投影射程RP和射程偏差ΔRP作为注量和温度二者的函数用背散射法进行测定.注量的变化范围为5×1013~7×1014cm-2.注入是在室温和t=-120℃下完成的.由由实验所确定的投影射程,射程偏差与注量或温度无关,并且分别等于295和72.2nm.与TRIM86的计算结果相比较,发现RP的偏离为18%,而ΔRP的偏离为36%.RP和ΔRP二者与注量及温度的无关性,排除了所观察到的与TRIM的矛盾是由于注入期间辐射增强扩散或离子束混合效应而引起的解释。  相似文献   
33.
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   
34.
35.
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   
36.
H Namdari  E J Bottone 《Experientia》1991,47(5):434-436
Aeromonas caviae isolated from stools of diarrheic formula-fed infants and environmental sources produce acetic acid when grown in glucose broth, which is bactericidal (suicide phenomenon). A. caviae grows anaerobically in a minimal medium or under permissive conditions such as the intestinal tract of formula-fed infants. These isolates adhered to HEp-2 cells and produced a cytotoxic and a cytotonic enterotoxin which underscore their enteropathogenicity.  相似文献   
37.
This article presents a case example involving the renaming of an organizational change process from BPR to outsourcing. The paper discusses the important ramifications of such a name change in that the recognition of outsourcing allowed an organizational commitment to counseling and termination advice. The paper discusses two ways of looking at the name change, one from a critical theory perspective and one from a realist perspective. It demonstrates how theory can provide useful yet markedly different interpretations of such organizational events. Critical theory operates from within what can be termed the transitive epistemological dimension, whereas critical realism tends to emphasize the importance of ontological issues. Each has important things to say about the situation and improves our understanding overall.  相似文献   
38.
P Villa  J Courtin 《Nature》1991,351(6328):613-614
  相似文献   
39.
P Meyrand  J Simmers  M Moulins 《Nature》1991,351(6321):60-63
  相似文献   
40.
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