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   31718篇
  免费   3707篇
  国内免费   1470篇
系统科学   1278篇
丛书文集   1294篇
教育与普及   1331篇
理论与方法论   135篇
现状及发展   202篇
研究方法   119篇
综合类   32531篇
自然研究   5篇
  2025年   237篇
  2024年   511篇
  2023年   553篇
  2022年   662篇
  2021年   824篇
  2020年   481篇
  2019年   177篇
  2018年   289篇
  2017年   379篇
  2016年   374篇
  2015年   776篇
  2014年   1096篇
  2013年   993篇
  2012年   1175篇
  2011年   1274篇
  2010年   1245篇
  2009年   1421篇
  2008年   1504篇
  2007年   1569篇
  2006年   1240篇
  2005年   1286篇
  2004年   858篇
  2003年   746篇
  2002年   702篇
  2001年   857篇
  2000年   991篇
  1999年   1900篇
  1998年   1621篇
  1997年   1724篇
  1996年   1584篇
  1995年   1356篇
  1994年   1198篇
  1993年   1063篇
  1992年   896篇
  1991年   808篇
  1990年   715篇
  1989年   631篇
  1988年   518篇
  1987年   327篇
  1986年   182篇
  1985年   76篇
  1984年   16篇
  1983年   13篇
  1982年   6篇
  1981年   8篇
  1980年   6篇
  1979年   3篇
  1973年   7篇
  1970年   3篇
  1957年   3篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
立足西部高校的服装结构设计课程的现状,结合教学实践探索西部服装市场的需求特征,提出强化服装结构设计的师资专业素质,加强服装结构设计与其他学科的交叉渗透,结合西部的民族资源,通过教学手法的改进,从实践出发,提出以人体为核心的服装结构设计课程教学改革措施.  相似文献   
122.
Mt. Everest, the zenith of the world, plays an impor- tant role in the global climate change. During the lastdecade, some studies on glacier and snow have been undertaken in this region. The climate and environment changes in the past hundred years have b…  相似文献   
123.
液舱晃荡问题是近年来液货船设计中十分关注的问题之一,它涉及自由液面的大幅运动等一系列强非线性问题,因此给基于欧拉观点下空间拓扑网格的数值模拟带来了较大的难度. 文中基于拉格朗日观点下的有限点法(FPM),对不同充容率的液舱晃荡问题进行了数值模拟,并将数值计算结果与试验值进行对比验证. 结果表明:当激励频率与液舱固有频率一致时,舱内流体的晃荡较为剧烈;而当充容率不同时,舱壁对液体流动的抑制作用使得高充容液舱内的流体翻卷变形较为平缓. 通过有限点法有效且准确地模拟了不同充容率液舱在各激励频率下的水面晃荡情况,与试验现象基本吻合. 同时通过对舱壁监测点的压力计算得知,有限点法的数值结果与实测压力曲线基本保持一致. FPM针对不同工况下的二维液舱晃荡情况均给出了较为准确的数值模拟结果,有效验证了有限点法的可靠性.  相似文献   
124.
C-glycosides are important carbohydrate mimetics and have gained considerable attention because ot their attractive biological activities as pharmaceutical targets and their resistance to enzymatic degradation in vivo. Consequently, a large number of C-glycosides have been synthesized for their bioactivity investigation, and various synthetic methodologies have been developed. As a continuation of the synthesis of novel C-glycoside derivatives using exo-glycals as the precursor, we wish to present herein new approaches to the synthesis of C-aryl aminoglycoside via the 1,3-dipolar cycloaddition (Scheme 1 ) and C-disaccharides by Suzuki cross-coupling reaction (Scheme 2), and the synthesis of dihydropyrimidinonyl (DHPM) glycoside by the three component Biginelli reaction (Scheme 3).  相似文献   
125.
Genomic alterations in cultured human embryonic stem cells   总被引:22,自引:0,他引:22  
Cultured human embryonic stem cell (hESC) lines are an invaluable resource because they provide a uniform and stable genetic system for functional analyses and therapeutic applications. Nevertheless, these dividing cells, like other cells, probably undergo spontaneous mutation at a rate of 10(-9) per nucleotide. Because each mutant has only a few progeny, the overall biological properties of the cell culture are not altered unless a mutation provides a survival or growth advantage. Clonal evolution that leads to emergence of a dominant mutant genotype may potentially affect cellular phenotype as well. We assessed the genomic fidelity of paired early- and late-passage hESC lines in the course of tissue culture. Relative to early-passage lines, eight of nine late-passage hESC lines had one or more genomic alterations commonly observed in human cancers, including aberrations in copy number (45%), mitochondrial DNA sequence (22%) and gene promoter methylation (90%), although the latter was essentially restricted to 2 of 14 promoters examined. The observation that hESC lines maintained in vitro develop genetic and epigenetic alterations implies that periodic monitoring of these lines will be required before they are used in in vivo applications and that some late-passage hESC lines may be unusable for therapeutic purposes.  相似文献   
126.
Under most microscopes, the fluorescent sig- nals emitted from biological structures, such as vesicles, as well as the signals from single molecules will appear as small puncta, which contribute to a Gaussian-like distri- bution. Accurate segmentation of these spots will funda- mentally affect our interpretation of a specific biological progress. Because of the complicated backgrounds in images, many algorithms fail to identify all of the inter- esting signals; the tremendous amount of time required for algorithms to process large datasets can also decrease their utility. Here, we introduce an excellent robust detection method based on the machine learning algorithm Ada- Boost, which outperforms threshold-based segmentation, wavelets, and FDA under most situations. We also provide a GPU/multi-core CPU implementation of this algorithm; this implementation accelerates the algorithm approxi- mately 10- and 7-fold acceleration compared with a single CPU implementation. The great reduction of time should make this method a promising candidate in the processing of large datasets. Furthermore, we demonstrate the use of our algorithm on true fluorescent micrographs, and the results show that machine learning-based detection meth- ods outperform the four other previously reported methods.  相似文献   
127.
0Introduction Near infrared(NIR)techniquesarebasedonsensitive,quantitativemeasurementsoffunctionalcontrastbe tweenhealthyanddiseasedtissue.Recently,researchersshowgreatinterestinmeasuringfunctionalpropertiesofbreasttis suesuchashemoglobinconcentrationoroxygensaturationbyNIRspectroscopyandNIRimaging[1,2].NIRlightcanpene trateseveralcentimetersintotissuebeforeitisattenuatedbe lowdetection.ThemainintrinsicmechanismsofNIRlightat tenuationintissuearethescatteringduetoindexofrefractionvariatio…  相似文献   
128.
Railway transportation plays an important role in modern society. As China's massive railway transportation network continues to grow in total mileage and operation density, the energy consumption of trains becomes a serious concern. For any given route, the geographic characteristics are known a priori, but the parameters(e.g., loading and marshaling) of trains vary from one trip to another. An extensive analysis of the train operation data suggests that the control gear operation of trains is the most important factor that affects the energy consumption. Such an observation determines that the problem of energy-efficient train driving has to be addressed by considering both the geographic information and the trip parameters. However, the problem is difficult to solve due to its high dimension, nonlinearity, complex constraints, and time-varying characteristics. Faced with these difficulties, we propose an energy-efficient train control framework based on a hierarchical ensemble learning approach. Through hierarchical refinement, we learn prediction models of speed and gear. The learned models can be used to derive optimized driving operations under real-time requirements. This study uses random forest and bagging – REPTree as classification algorithm and regression algorithm, respectively. We conduct an extensive study on the potential of bagging, decision trees, random forest, and feature selection to design an effective hierarchical ensemble learning framework. The proposed framework was testified through simulation. The average energy consumption of the proposed method is over 7% lower than that of human drivers.  相似文献   
129.
  总被引:2,自引:0,他引:2  
Superoxide dismutase (SOD) activity in the leaves of transgenic tobacco plants with betaine aldehyde dehydrogenase (BADH) gene was about 36% higher than that in the control plants (parent plants), activities of peroxidase (POD) and catalase (Cat) increased by about 62% and 88% respectively. Activities of ascorbate peroxidase (AsSPOD), dehydroascorbate redutase (DAsAR) and glutathione reductase (GR) in ascorbate-glutothion pathway located at chloroplasts increased by 67.7%, 47.9% and 38.8% respectively. These results indicated that the H2O2 produced by SOD catalyzing superoxide anion radicals (O-2) could be fully decomposed, and could not derive to form the strongest toxicant radicals ·OH. This is the first report to elucidate quantitatively that the activities of two kinds of antioxidative enzymes decomposed radicals and active oxygen were matched. Photoinhibition tolerant capacity of the transgenic tobacco plants was 35% higher than that in the parent plants. Increment of photoinhibition tolerant capacity in the transgenic tobacco plants might be due to increment of antioxidative enzymes activities, in turn being able to more effectively scavenge active oxygen and radicals, protect organization and function of chloroplasts. These results showed that the increment of antioxidative enzymes activities in the transgenic tobacco might be one of the reasons for the increment of resistance in the transgenic tobacco.  相似文献   
130.
<正> 汞属于极有害的环境毒物。由于五十年代日本的不幸事件和六十年代瑞典的鸟类灾难,水环境汞污染对健康的危害问题,引起了人们的普遍注意。1970年秋,在罗马举行的国际海洋污染对策会议上,把日本于1953  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