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   40838篇
  免费   142篇
  国内免费   168篇
系统科学   228篇
丛书文集   545篇
教育与普及   76篇
理论与方法论   142篇
现状及发展   18474篇
研究方法   1662篇
综合类   19344篇
自然研究   677篇
  2013年   329篇
  2012年   592篇
  2011年   1214篇
  2008年   713篇
  2007年   842篇
  2006年   789篇
  2005年   851篇
  2004年   843篇
  2003年   742篇
  2002年   762篇
  2001年   1250篇
  2000年   1191篇
  1999年   814篇
  1994年   391篇
  1992年   781篇
  1991年   562篇
  1990年   622篇
  1989年   602篇
  1988年   567篇
  1987年   619篇
  1986年   659篇
  1985年   826篇
  1984年   618篇
  1983年   529篇
  1982年   491篇
  1981年   503篇
  1980年   522篇
  1979年   1324篇
  1978年   1026篇
  1977年   958篇
  1976年   783篇
  1975年   798篇
  1974年   1118篇
  1973年   972篇
  1972年   981篇
  1971年   1097篇
  1970年   1520篇
  1969年   1189篇
  1968年   1138篇
  1967年   1118篇
  1966年   1042篇
  1965年   772篇
  1964年   275篇
  1959年   388篇
  1958年   731篇
  1957年   488篇
  1956年   413篇
  1955年   371篇
  1954年   417篇
  1948年   290篇
排序方式: 共有10000条查询结果,搜索用时 375 毫秒
121.
1 Results Simultaneous micro-and nanostructuring was prepared on polyolefin surfaces by injection molding.The molds were made of electropolished aluminum foil where the micropatterns were structured with a custom made robot.Nanopatterns were subsequently created on the molds by oxidizing the aluminum surface electrochemically in polyprotic acid.The preparation technique allowed simultaneous control of the dimensions of the micro-and nanostructures.Structuring has a remarkable effect on the contact angle...  相似文献   
122.
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   
123.
124.
125.
Q Gu  F A Popp 《Experientia》1992,48(11-12):1069-1082
By considering an exciplex system consisting of collective molecules in interaction with both the 'pumping' fields and the biophoton fields, the two-level exciplex model and the three-level exciplex model are presented. They are useful for the investigation of the quasi-stationary behaviour of biophoton emission, and biophoton emission as a dynamic process in the presence of external perturbations. Our theoretical results predict a series of nonlinear effects, such as chaos, fractal behaviour, and non-equilibrium phase transition. These effects characterize the coherence nature of living systems. In our approaches, there are two important quantities f and x, which can be used to mark the working points of the two-level and three-level exciplex systems. All the influences of external perturbations on the exciplex systems, e.g. change of temperature, the addition of agents, exposure to light, etc., can be interpreted as shifts of the working points of the systems, leading to a diversity of nonlinear response of biophoton emission. In addition, the agreements of the theoretical results and the corresponding experimental observations on biophoton emission from biological systems in the presence of external perturbations are demonstrated.  相似文献   
126.
After a brief recall of the Sidelobe Canceler (SLC) working principle, including the derivation of a general formula for the Cancellation Ratio (CR), the effects of channel mismatching are investigated. In particular, curves providing CRvalues as a function of amplitude and phase channel mismatching, radar bandwidth, and jammer direction of arrival (JDOA) are provided for the cases of one and two auxiliary antennas. Subsequently, a time -space processor for performance restoration is analyzed in detail. In addition to the above mentioned quantities, the attainable CR value is expressed as a function of the space-time processor parameters. The contribution of the paper is related to the derivation of a number of mathematical equations of CR for several cases of practical interest for the radar engineer. In addition, several curves are presented to assist design of SLC systems.  相似文献   
127.
128.
M McClelland  A S Bhagwat 《Nature》1992,355(6361):595-596
  相似文献   
129.
130.
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   
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