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   24957篇
  免费   70篇
  国内免费   115篇
系统科学   125篇
丛书文集   207篇
教育与普及   52篇
理论与方法论   102篇
现状及发展   11081篇
研究方法   1149篇
综合类   12024篇
自然研究   402篇
  2013年   138篇
  2012年   409篇
  2011年   783篇
  2010年   171篇
  2008年   477篇
  2007年   505篇
  2006年   517篇
  2005年   512篇
  2004年   484篇
  2003年   446篇
  2002年   474篇
  2001年   774篇
  2000年   747篇
  1999年   524篇
  1992年   454篇
  1991年   351篇
  1990年   402篇
  1989年   381篇
  1988年   386篇
  1987年   402篇
  1986年   375篇
  1985年   507篇
  1984年   422篇
  1983年   306篇
  1982年   326篇
  1981年   306篇
  1980年   337篇
  1979年   786篇
  1978年   625篇
  1977年   526篇
  1976年   543篇
  1975年   562篇
  1974年   611篇
  1973年   552篇
  1972年   581篇
  1971年   702篇
  1970年   882篇
  1969年   679篇
  1968年   731篇
  1967年   689篇
  1966年   602篇
  1965年   442篇
  1964年   175篇
  1959年   246篇
  1958年   443篇
  1957年   282篇
  1956年   267篇
  1955年   226篇
  1954年   256篇
  1948年   194篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
In this paper, we introduce a new class of generalized convex function, namely, a-pseudounivex function, by combining the concepts of pseudo-univex and α-invex functions. Further, we establish some relationships between vector variational-like inequality problems and vector optimization problems under the assumptions of α-pseudo-univex functions. Results obtained in this paper present a refinement and improvement of previously known results.  相似文献   
102.
103.
104.
Conotoxins of the O-superfamily affecting voltage-gated sodium channels   总被引:1,自引:1,他引:0  
The venoms of predatory cone snails harbor a rich repertoire of peptide toxins that are valuable research tools, but recently have also proven to be useful drugs. Among the conotoxins with several disulfide bridges, the O-superfamily toxins are characterized by a conserved cysteine knot pattern: C-C-CC-C-C. While ω-conotoxins and κ-conotoxins block Ca2+ and K+ channels, respectively, the closely related δ- and μO-conotoxins affect voltage-gated Na+ channels (Nav channels). δ-conotoxins mainly remove the fast inactivation of Nav channels and, thus, functionally resemble long-chain scorpion α-toxins. μO-conotoxins are functionally similar to μ-conotoxins, since they inhibit the ion flow through Nav channels. Recent results from functional and structural assays have gained insight into the underlying molecular mechanisms. Both types of toxins are voltage-sensor toxins interfering with the voltage-sensor elements of Nav channels. Received 27 December 2006; received after revision 30 January 2007; accepted 19 February 2007  相似文献   
105.
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   
106.
107.
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   
108.
Karyotypes of seven species of bats from Thailand (Chiroptera,Mammalia)   总被引:2,自引:0,他引:2  
Summary Karyotypes ofMyotis siligorensis, Myotis mystacinus, Pipistrellus pulveratus, Tylonycteris robustula, Miniopterus schreibersi fuliginosus, Hipposideros fulvus andAselliscus stoliczkanus from Thailand are investigated.Acknowledgment. The authors are very grateful to Dr N. Ratanawarabhan, Mrs S. Sittilert, P. Noonpakdee and S. Kuanchalern of Thailand Institute of Scientific and Technological Research for their valuable advice and assistance during our field survey in Thailand.  相似文献   
109.
The way that we perceive and interact with objects depends on our previous experience with them. For example, a bird expert is more likely to recognize a bird as a sparrow, a sandpiper or a cockatiel than a non-expert. Neurons in the inferior temporal cortex have been shown to be important in the representation of visual objects; however, it is unknown which object features are represented and how these representations are affected by categorization training. Here we show that feature selectivity in the macaque inferior temporal cortex is shaped by categorization of objects on the basis of their visual features. Specifically, we recorded from single neurons while monkeys performed a categorization task with two sets of parametric stimuli. Each stimulus set consisted of four varying features, but only two of the four were important for the categorization task (diagnostic features). We found enhanced neuronal representation of the diagnostic features relative to the non-diagnostic ones. These findings demonstrate that stimulus features important for categorization are instantiated in the activity of single units (neurons) in the primate inferior temporal cortex.  相似文献   
110.
On going trend of miniaturization in electronic rel at ed parts, which is an average of two times in every 5~7 years introduce grindin g challenges. In grinding process, the surface waviness control of thin parts is an ardent task due to its warpage, induced by the high specific grinding energy (2~10 J/mm 3). Therefore, coolant is often used to avoid thermal damage, obtai n better surface integrity and to prolong wheel life. However coolant, the incomp ressibility media introduce high forces at the gri...  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