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   29473篇
  免费   53篇
  国内免费   113篇
系统科学   181篇
丛书文集   453篇
教育与普及   75篇
理论与方法论   132篇
现状及发展   12635篇
研究方法   1357篇
综合类   14289篇
自然研究   517篇
  2013年   198篇
  2012年   482篇
  2011年   985篇
  2010年   194篇
  2009年   148篇
  2008年   553篇
  2007年   578篇
  2006年   606篇
  2005年   617篇
  2004年   627篇
  2003年   541篇
  2002年   520篇
  2001年   897篇
  2000年   854篇
  1999年   599篇
  1992年   515篇
  1991年   423篇
  1990年   450篇
  1989年   412篇
  1988年   420篇
  1987年   450篇
  1986年   439篇
  1985年   561篇
  1984年   457篇
  1983年   348篇
  1982年   321篇
  1981年   317篇
  1980年   396篇
  1979年   894篇
  1978年   692篇
  1977年   676篇
  1976年   552篇
  1975年   626篇
  1974年   839篇
  1973年   754篇
  1972年   763篇
  1971年   890篇
  1970年   1163篇
  1969年   865篇
  1968年   798篇
  1967年   864篇
  1966年   731篇
  1965年   516篇
  1959年   312篇
  1958年   534篇
  1957年   376篇
  1956年   339篇
  1955年   274篇
  1954年   312篇
  1948年   252篇
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
991.
992.
993.
994.
1 Introduction Metal, metal oxide and metal compound nanoparticles (NPs) received considerable attention due to their unique properties: catalytic, magnetic, optical, electronic, etc. We believe that for different applications, there are preferable morphologies of NP-stabilizing medium composites. For example, small (1-3 nm) nanoparticles formed in micro/mesoporous hypercrosslinked polystyrene demonstrate excellent catalytic properties in various hydrogenation and oxidation reactions due to high surface...  相似文献   
995.
The locations and properties of common deletion variants in the human genome are largely unknown. We describe a systematic method for using dense SNP genotype data to discover deletions and its application to data from the International HapMap Consortium to characterize and catalogue segregating deletion variants across the human genome. We identified 541 deletion variants (94% novel) ranging from 1 kb to 745 kb in size; 278 of these variants were observed in multiple, unrelated individuals, 120 in the homozygous state. The coding exons of ten expressed genes were found to be commonly deleted, including multiple genes with roles in sex steroid metabolism, olfaction and drug response. These common deletion polymorphisms typically represent ancestral mutations that are in linkage disequilibrium with nearby SNPs, meaning that their association to disease can often be evaluated in the course of SNP-based whole-genome association studies.  相似文献   
996.
997.
J E Brown  J E Lisman 《Nature》1975,258(5532):252-254
  相似文献   
998.
YIN HC 《Nature》1948,162(4128):928
  相似文献   
999.
Thoracic aortic aneurysms and dissections are a main feature of connective tissue disorders, such as Marfan syndrome and Loeys-Dietz syndrome. We delineated a new syndrome presenting with aneurysms, dissections and tortuosity throughout the arterial tree in association with mild craniofacial features and skeletal and cutaneous anomalies. In contrast with other aneurysm syndromes, most of these affected individuals presented with early-onset osteoarthritis. We mapped the genetic locus to chromosome 15q22.2-24.2 and show that the disease is caused by mutations in SMAD3. This gene encodes a member of the TGF-β pathway that is essential for TGF-β signal transmission. SMAD3 mutations lead to increased aortic expression of several key players in the TGF-β pathway, including SMAD3. Molecular diagnosis will allow early and reliable identification of cases and relatives at risk for major cardiovascular complications. Our findings endorse the TGF-β pathway as the primary pharmacological target for the development of new treatments for aortic aneurysms and osteoarthritis.  相似文献   
1000.
The weighted linear choice model is one of the most popular models in the social sciences. In this model the utility of a choice object is represented as a weighted sum of attribute-level desirabilities, where the weights are attribute importances. In many empirical contexts the choice objects are such that individuals are highly correlated in terms of their desirability ordering of levels within attribute (e.g., price levels, durability levels, etc.) but may differ appreciably in terms of their evaluations of each attribute's importance.In this paper we address the problem of how dissimilar two individuals may be, in a rank correlation sense, given that they agree completely on the desirability ordering of levels within attributes, but may disagree considerably regarding the importance they attach to the attributes themselves. The problem has interesting implications regarding the potential value of clustering individuals' utility functions for market segmentation or other such purposes.The authors would like to thank the editor and three anonymous reviewers for their excellent comments on an earlier draft of the paper.  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