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   2002篇
  免费   181篇
  国内免费   116篇
化学   277篇
晶体学   9篇
力学   109篇
综合类   58篇
数学   1197篇
物理学   649篇
  2024年   5篇
  2023年   32篇
  2022年   53篇
  2021年   49篇
  2020年   52篇
  2019年   58篇
  2018年   59篇
  2017年   87篇
  2016年   97篇
  2015年   60篇
  2014年   148篇
  2013年   144篇
  2012年   96篇
  2011年   101篇
  2010年   99篇
  2009年   127篇
  2008年   124篇
  2007年   117篇
  2006年   102篇
  2005年   90篇
  2004年   69篇
  2003年   65篇
  2002年   68篇
  2001年   54篇
  2000年   63篇
  1999年   58篇
  1998年   47篇
  1997年   29篇
  1996年   21篇
  1995年   11篇
  1994年   19篇
  1993年   16篇
  1992年   7篇
  1991年   15篇
  1990年   8篇
  1989年   3篇
  1988年   8篇
  1987年   2篇
  1986年   6篇
  1985年   8篇
  1984年   2篇
  1983年   2篇
  1982年   8篇
  1981年   3篇
  1980年   2篇
  1979年   1篇
  1976年   1篇
  1973年   1篇
  1972年   1篇
  1936年   1篇
排序方式: 共有2299条查询结果,搜索用时 31 毫秒
81.
During the transmission of infrared radiation, the atmospheric transmittance could be a complex parameter due to the absorbing and scattering of atmosphere, as well as the influences from the environment and transmission distance. With the help of a spectral bisection method, a new assessing formula and solution is raised by calculating the operating distance of infrared system. In the small segments, MODTRAN can be used to figure out the percentage of penetration, which is called by advanced program, so as to get the infrared radiation in those segments. The calculated data of the segments were summed up and used to calculate the operating distance of the infrared system. Compared with the conventional calculation methods that the transmittance was used as a constant or a average, The calculation precise of the operating distance is highly increased by this method the results of all small segments by comparing with the traditional methods. The whole computing process becomes more clear and effective by taking the influences from visibility of atmosphere, altitude, targets zenith angle and spatial frequency into consideration, and by building an instant monitoring system of the operating distance. The final computing result and real effecting distance are based on the general simulation for penetration rate and the tendency of operating distance in all conditions.  相似文献   
82.
We study a two-layer neural network made of N and M(N) neurons, producing a two-way association search for a family of p(N) patterns, where each pattern is a pair of two independent sub-categories of information having respectively N and M(N) components. In terms of the ratio =lim N M(N)/N, we study the retrieval capability of this network and show that there exists, at least, three regimes of association for which we determine the evolution of the threshold c () of the storage capacity =lim N p(N)/N.  相似文献   
83.
The encoding process of finding the best-matched codeword (winner) for a certain input vector in image vector quantization (VQ) is computationally very expensive due to a lot of k-dimensional Euclidean distance computations. In order to speed up the VQ encoding process, it is beneficial to firstly estimate how large the Euclidean distance is between the input vector and a candidate codeword by using appropriate low dimensional features of a vector instead of an immediate Euclidean distance computation. If the estimated Euclidean distance is large enough, it implies that the current candidate codeword could not be a winner so that it can be rejected safely and thus avoid actual Euclidean distance computation. Sum (1-D), L2 norm (1-D) and partial sums (2-D) of a vector are used together as the appropriate features in this paper because they are the first three simplest features. Then, four estimations of Euclidean distance between the input vector and a codeword are connected to each other by the Cauchy–Schwarz inequality to realize codeword rejection. For typical standard images with very different details (Lena, F-16, Pepper and Baboon), the final remaining must-do actual Euclidean distance computations can be eliminated obviously and the total computational cost including all overhead can also be reduced obviously compared to the state-of-the-art EEENNS method meanwhile keeping a full search (FS) equivalent PSNR.  相似文献   
84.
Dependency distance, measured by the linear distance between two syntactically related words in a sentence, is generally held as an important index of memory burden and an indicator of syntactic difficulty. Since this constraint of memory is common for all human beings, there may well be a universal preference for dependency distance minimization (DDM) for the sake of reducing memory burden. This human-driven language universal is supported by big data analyses of various corpora that consistently report shorter overall dependency distance in natural languages than in artificial random languages and long-tailed distributions featuring a majority of short dependencies and a minority of long ones. Human languages, as complex systems, seem to have evolved to come up with diverse syntactic patterns under the universal pressure for dependency distance minimization. However, there always exist a small number of long-distance dependencies in natural languages, which may reflect some other biological or functional constraints. Language system may adapt itself to these sporadic long-distance dependencies. It is these universal constraints that have shaped such a rich diversity of syntactic patterns in human languages.  相似文献   
85.
86.
87.
Let BE be a bounded symmetric domain realized as the unit open ball of JB*-triples.The authors will characterize the bounded weighted composition operator from the Bloch space B(BE) to weighted Hardy space Hv in terms of Kobayashi distance.The authors also give a sufficient condition for the compactness,and also give the upper bound of its essential norm.As a corollary,they show that the boundedness and compactness are equivalent for composition operator fromB(B...  相似文献   
88.
Bubble behaviors near a boundary in an ultrasonic field are the fundamental forms of acoustic cavitation and of substantial importance in various applications, such as industry cleaning, chemical engineering and food processing. The effects of two important factors that strongly affect the dynamics of a single acoustic cavitation bubble, namely, the initial bubble radius and the standoff distance, were investigated in this work. The temporal evolution of the bubble was recorded using high speed microphotography. Meanwhile, the time of bubble collapse and the characteristics of the liquid jets were analyzed. The results demonstrate that the intensity of the acoustic cavitation, which is characterized by the time of bubble collapse and the liquid jet speed, reaches the optimum level under suitable values of the initial bubble radius and the normalized standoff distance. As the initial bubble radius and the normalized standoff distance increase or decrease from the optimal values, the time of the bubble collapse increases, and the first liquid jet’s speed decreases substantially, whereas the speeds of the second and third liquid jets exhibit no substantial changes. These results on bubble dynamics in an ultrasonic field are important for identifying or correcting the mechanisms of acoustic cavitation and for facilitating its optimization and application.  相似文献   
89.
研究了4度循环图,构造出其任意两点之间的四条内部点不交路,并且给出其宽直径的一个较好的上界。  相似文献   
90.
This article represents the electrical studies of an interdigitated highly skewed N(4-n-pentyloxybenzylidene) 4-n-alkylaniline (5O.16) compound. Interestingly the compound is partially bent like and unsymmetrical in alkyl chain length. Dielectric and impedance spectroscopy studies indicate the coupling between the liquid crystal and the electrical field. Studies were carried out as a function of temperature as well as frequency. Semicircular nature of the Cole–Cole plots indicates the reorientation of the molecule with the applied field. Using the experimental data and the theoretically fitted results the effective equivalent model circuit was designed. Which used to explain the behavior of the compound under the external electric effect and the influence of the electrodes with different variable represent the resistor circuit. However, the effect of the conductivity of temperature and frequency are also reported.  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