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   56170篇
  免费   7979篇
  国内免费   8088篇
化学   28589篇
晶体学   732篇
力学   2118篇
综合类   853篇
数学   4852篇
物理学   14047篇
无线电   21046篇
  2024年   151篇
  2023年   920篇
  2022年   1451篇
  2021年   1779篇
  2020年   1653篇
  2019年   1676篇
  2018年   1528篇
  2017年   1709篇
  2016年   1970篇
  2015年   2314篇
  2014年   2787篇
  2013年   3688篇
  2012年   4224篇
  2011年   4464篇
  2010年   3747篇
  2009年   3796篇
  2008年   4229篇
  2007年   3984篇
  2006年   3689篇
  2005年   3248篇
  2004年   2560篇
  2003年   1955篇
  2002年   1910篇
  2001年   1765篇
  2000年   1643篇
  1999年   1222篇
  1998年   911篇
  1997年   813篇
  1996年   815篇
  1995年   691篇
  1994年   655篇
  1993年   600篇
  1992年   558篇
  1991年   455篇
  1990年   413篇
  1989年   320篇
  1988年   289篇
  1987年   210篇
  1986年   216篇
  1985年   208篇
  1984年   147篇
  1983年   110篇
  1982年   100篇
  1981年   74篇
  1980年   68篇
  1979年   81篇
  1978年   55篇
  1976年   46篇
  1974年   48篇
  1973年   48篇
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
81.
Color image retrieval based on hidden Markov models   总被引:1,自引:0,他引:1  
In this correspondence, a new approach to retrieving images from a color image database is proposed. Each image in the database is represented by a two-dimensional pseudo-hidden Markov model (2-D PHMM), which characterizes the chromatic and spatial information about the image. In addition, a flexible pictorial querying method is used, by which users can paint the rough content of the desired images in a query picture. Image matching is achieved by comparing the query picture with each 2-D PHMM in the database. Experimental results show that the proposed approach is indeed effective.  相似文献   
82.
Identifies the basic technological components of multimedia communications. Among these technologies, multimedia processing and transmission already benefit heavily from VLSI advances. in fact, these two technologies could not have matured without special-purpose VLSI chips. We have examined basic processing required in these technologies and some VLSI architectures. The focus has been on standard-compliant VLSI chips, because the eventual goal of communications is to allow everyone to reach others without constraints. There are still a number of evolving standards, which means that we might witness yet another wave of VLSI chips for multimedia communications  相似文献   
83.
Summary A supersonic jet spectrum of 9,10-dichloroanthracene is measured by stimulated-emission-pumping fluorescence dip spectrometry and conventional fluorescence spectrometry. The performance obtained is compared for these spectrometric methods, providing same information concerned with the energy level of the ground state. The former is more preferential for measurement of a high-resolution spectrum, since the spectral resolution is determined by the linewidth of the dumping laser. On the other hand, the latter is more preferential for measurement with better sensitivity at the expense of the spectral resolution, since the fluorescence throughput can be improved by increasing the slit width of the monochromator.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday  相似文献   
84.
张明  于文  张君  张远仪  王文魁 《物理学报》1996,45(10):1724-1728
利用原位X射线衍射技术得到非晶调制多层膜Nb/Si中的互扩散系数与退火温度的关系,调制周期L=3.2nm的非晶调制多层膜是用粒子溅射方法制备的.温度范围为423—523K的有效互扩散系数通过原位测量多层膜的一级调制峰强度与退火温度之间的关系而得到.利用缺陷陷阱延迟扩散机制解释了所得到的扩散系数与退火温度的关系.建立了可以解释较小前置系数的模型 关键词:  相似文献   
85.
The theory of tree-growing (RECPAM approach) is developed for outcome variables which are distributed as the canonical exponential family. The general RECPAM approach (consisting of three steps: recursive partition, pruning and amalgamation), is reviewed. This is seen as constructing a partition with maximal information content about a parameter to be predicted, followed by simplification by the elimination of ‘negligible’ information. The measure of information is defined for an exponential family outcome as a deviance difference, and appropriate modifications of pruning and amalgamation rules are discussed. It is further shown how the proposed approach makes it possible to develop tree-growing for situations usually treated by generalized linear models (GLIM). In particular, Poisson and logistic regression can be tree-structured. Moreover, censored survival data can be treated, as in GLIM, by observing a formal equivalence of the likelihood under random censoring and an appropriate Poisson model. Three examples are given of application to Poisson, binary and censored survival data.  相似文献   
86.
The spontaneous magnetization of the Ising model on a 4–8 lattice with six different coupling constants and two different magnetic moments is studied. A formula for the spontaneous magnetization is proposed. The result agrees with the exact low-temperature series expansions up to the 12th order.  相似文献   
87.
Let V be a hypersurface with an isolated singularity at the origin in ? n+1. It is a natural question to ask when V is defined by weighted homogeneous polynomial or homogeneous polynomial up to biholomorphic change of coordinates. In 1971, a beautiful theorem of Saito gives a necessary and sufficient condition for V to be defined by a weighted homogeneous polynomial. For a two-dimensional isolated hypersurface signularity V, Xu and Yau found a coordinate free characterization for V to be defined by a homogeneous polynomial. Recently Lin and Yau gave necessary and sufficient conditions for a 3-dimensional isolated hypersurface singularity with geometric genus bigger than zero to be defined by a homogeneous polynomial. The purpose of this paper is to prove that Lin-Yau’s theorem remains true for singularities with geometric genus equal to zero.  相似文献   
88.
The singularly perturbed boundary value problem for nonlinear higher order ordinary differential equation involving two small parameters has been considered. Under appropriate assumptions, for the three cases:ε/μ2→0(μ→0),μ2/ε→0 (ε→0) andε=μ2, the uniformly valid asymptotic solution is obtained by using the expansion method of two small parameters and the theory of differential inequality.  相似文献   
89.
一个图的最小填充问题是寻求边数最少的弦母图,一个图的树宽问题是寻求团数最小的弦母图,这两个问题分别在稀疏矩阵计算及图的算法设计中有非常重要的作用.一个k-树G的补图G称为k-补树.本文给出了k-补树G的最小填充数f(G) 及树宽TW(G).  相似文献   
90.
介绍了原子力显微镜在生物物理领域的最新应用:蛋白质去折叠、DNA拉伸、生物膜受力,通过分析实验得到的力谱,可以获取关于蛋白质、DNA、生物膜结构信息.原子力显微镜不仅能反映测量体系的力学性质,由于其具有独特的时间、空间分辨及实时成像,因而能提供更多信息.  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