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   42289篇
  免费   91篇
  国内免费   122篇
系统科学   396篇
丛书文集   1023篇
教育与普及   110篇
理论与方法论   263篇
现状及发展   18890篇
研究方法   1618篇
综合类   19696篇
自然研究   506篇
  2013年   219篇
  2012年   545篇
  2011年   1116篇
  2010年   252篇
  2008年   713篇
  2007年   762篇
  2006年   767篇
  2005年   773篇
  2004年   710篇
  2003年   788篇
  2002年   724篇
  2001年   1294篇
  2000年   1225篇
  1999年   774篇
  1992年   741篇
  1991年   608篇
  1990年   646篇
  1989年   647篇
  1988年   645篇
  1987年   639篇
  1986年   656篇
  1985年   797篇
  1984年   628篇
  1983年   541篇
  1982年   471篇
  1981年   498篇
  1980年   601篇
  1979年   1306篇
  1978年   1123篇
  1977年   1122篇
  1976年   826篇
  1975年   875篇
  1974年   1311篇
  1973年   1056篇
  1972年   1077篇
  1971年   1329篇
  1970年   1768篇
  1969年   1388篇
  1968年   1272篇
  1967年   1333篇
  1966年   1134篇
  1965年   833篇
  1964年   220篇
  1959年   503篇
  1958年   731篇
  1957年   577篇
  1956年   486篇
  1955年   442篇
  1954年   483篇
  1948年   265篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Enzymatic control of cell division in micro-organisms   总被引:1,自引:0,他引:1  
NICKERSON WJ 《Nature》1948,162(4111):241-245
  相似文献   
82.
Image-based gait analysis as a means of biometric identification has attracted much research attention. Most of the existing methods focus on human identification, posture analysis and movement tracking. There have been few investigations on measuring the carried load based on the carrier's gait characteristics by automatic image processing. Nevertheless, this measurement is very useful in a number of applications, such as the study of the carried load on the postural development of children and adolescence. In this paper, we investigate how to automatically estimate the carried weight from a sequence of images. We present a method to extract human gait silhouette based on an observation that humans tend to minimize the energy during motion. We compute several angles of body leaning and determine the relationship of the carried weight, the leaning angles and the centroid location according to a human kinetic study. Our weight determination method has been verified successfully by experiments.  相似文献   
83.
84.
This paper introduces the conception of open complex giant system and the methodology for dealing with the system, with stress on its profound significance in development of science and technology. The authors conclude that the reductionism underlying the exact science is not suitable to open complex giant system, and the only feasible alternative is the meta-synthetic engineering from the qualitative to the quantitative.  相似文献   
85.
The effects of alcohol consumption, cigarette smoking and age on sister chromatid exchange (SCE) frequency in human lymphocytes were assessed by means of multiple linear regression. An increase in SCE rates was associated with alcohol consumption (p = 0.0001), smoking (p = 0.0231), and, to a small extent (p = 0.057), age. These three confounding factors explain 48% of the inter-personal variation in SCE rates among subjects studied.  相似文献   
86.
87.
During the past few years, several methods have been developed for the detection of specific nucleic acid sequences by in situ hybridization using non-radioactive labels such as fluorochromes, cytochemically detectable enzymes and electron-dense markers. These methods are preferable to autoradiography in terms of speed of performance and topological resolution. Their limited sensitivity, however, has so far restricted their use to the detection of repeated sequences. Here we report single gene detection with a procedure using 2-acetylaminofluorene (AAF)-modified probes, immunoperoxidase cytochemistry and reflection-contrast microscopy. We confirmed the autoradiographic data on the localization of the human thyroglobulin (Tg) gene to the distal end of the long arm of chromosome 8. A mixture of cosmid cHT2-derived subclones of the 3' part of the Tg gene, 22.3 kilobase pairs (kbp) in total, was used as a hybridization probe. This procedure can be used to map other unique sequences, if genomic clones are available from which clones with an appropriate amount of inserts can be isolated.  相似文献   
88.
89.
Acquired immunity and epidemiology of Schistosoma haematobium   总被引:1,自引:0,他引:1  
Human immune responses to schistosome infection have been characterized in detail. But there has been controversy over the relative importance of ecological factors (variation in exposure to infection) and immunological factors (acquired immunity) in determining the relationships between levels of infection and age typically found in areas where infection is endemic. Independent effects of exposure and age on the rates of reinfection with Schistosoma haematobium after chemotherapy have been demonstrated in the Gambia and Zimbabwe. This age effect could be the result of acquired immunity to infection. Indeed, allowing for variation in exposure and age, low rates of reinfection in the Gambia are correlated with high amounts of specific IgE antibodies--human IgE can kill S. mansoni schistosomulae in vitro. Further, animals can acquire immunologically mediated resistance to S. mansoni infection, although nonimmunological factors could also be involved. Acquisition of this immunity seems to be related to the cumulative effects of repeated infection and provides only partial protection. These characteristics are consistent with immuno-epidemiological data for both S. mansoni and S. haematobium infections of humans. We have now analysed age-prevalence data for human infection with S. haematobium, and find patterns of variation that are indeed consistent with the epidemiological effects of acquired immunity predicted by mathematical models.  相似文献   
90.
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