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   44941篇
  免费   3029篇
  国内免费   2452篇
化学   22209篇
晶体学   570篇
力学   2271篇
综合类   230篇
数学   10647篇
物理学   14495篇
  2024年   74篇
  2023年   340篇
  2022年   636篇
  2021年   698篇
  2020年   821篇
  2019年   818篇
  2018年   1809篇
  2017年   2044篇
  2016年   1566篇
  2015年   1540篇
  2014年   1596篇
  2013年   2021篇
  2012年   4671篇
  2011年   3832篇
  2010年   2945篇
  2009年   2600篇
  2008年   1670篇
  2007年   1594篇
  2006年   1423篇
  2005年   5094篇
  2004年   4482篇
  2003年   2732篇
  2002年   909篇
  2001年   626篇
  2000年   311篇
  1999年   388篇
  1998年   273篇
  1997年   195篇
  1996年   186篇
  1995年   154篇
  1994年   142篇
  1993年   112篇
  1992年   255篇
  1991年   214篇
  1990年   198篇
  1989年   148篇
  1988年   147篇
  1987年   87篇
  1986年   72篇
  1985年   75篇
  1984年   51篇
  1983年   54篇
  1981年   39篇
  1979年   45篇
  1976年   83篇
  1975年   42篇
  1974年   49篇
  1973年   53篇
  1972年   42篇
  1969年   38篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
We study a class of shape optimization problems for semi-linear elliptic equations with Dirichlet boundary conditions in smooth domains in ℝ2. A part of the boundary of the domain is variable as the graph of a smooth function. The problem is equivalently reformulated on a fixed domain. Continuity of the solution to the state equation with respect to domain variations is shown. This is used to obtain differentiability in the general case, and moreover a useful formula for the gradient of the cost functional in the case where the principal part of the differential operator is the Laplacian. Online publication 23 January 2004.  相似文献   
22.
23.
Imaging of latent fingerprints using time-resolved (TR) method offers a broader platform to eliminate the unwanted background emission. In this paper, a novel TR imaging technique is demonstrated and implemented, which facilitates the detection of latent fingerprints with nanosecond resolution. Simulated experiments were carried out with two overlapping fingerprints treated with two fluorescent powders having different lifetimes in nanosecond range. The dependence of the fluorescence emission intensity in nanosecond resolution of TR imaging is also revealed.  相似文献   
24.
Abnormalities in the cochlear function usually cause broadening of the auditory filters which reduces the speech intelligibility. An attempt to apply a spectral enhancement algorithm has been undertaken to improve the identification of Polish vowels by subjects with cochlear-based hearing-impairment. The identification scores of natural (unprocessed) vowels and spectrally enhanced (processed) vowels has been measured for hearing-impaired subjects. It has been found that spectral enhancement improves vowel scores by about 10% for those subjects, however, a wide variation in individual performance among subjects has been observed. The overall vowels identification scores obtained were 85% for natural vowels and 96% for spectrally enhanced vowels.  相似文献   
25.
多目标半定规划的互补弱鞍点和G-鞍点最优性条件   总被引:1,自引:0,他引:1  
对于含矩阵函数半定约束和多个目标函数的多目标半定规划问题,给出Lagrange函数在弱有效意义下的互补弱鞍点和Geofrrion恰当有效意义下的G-鞍点的定义及其等价定义.然后,在较弱的凸性条件下,利用含矩阵和向量约束的择一性定理,建立多目标半定规划的互补弱鞍点和G-鞍点充分必要条件.  相似文献   
26.
The Kα to Lα and Kβ to Lβ intensity ratios of seven elements have been measured following photoionization at 59.5 keV by using a Si (Li) detector (FWHM=155 eV at 5.96 keV). The intensity ratios were determined by measuring K and L x-rays emitted from a standard target of a given element. The theoretical values of the Kα to Lα and Kβ to Lβ intensity ratios were calculated using theoretically tabulated values of shell/subshell photoionization cross sections, fluorescence yields, Coster-Kronig transition probabilities, and radiative decay rates for ηKLi≠ and ηKLi=0. The measured values are in good agreement with theoretical results. From Optika i Spektroskopiya, Vol. 97, No. 2, 2004, pp. 186–189. Original English Text Copyright ? 2004 by A. Kü?ük?nder, S?ğüt, E. Kü?ük?nder, Büyükkasap. This article was submitted by the authors in English.  相似文献   
27.
A series of thin films of copper phthalocyanine derivatives with bromine and alkoxyl substituents [CuPc(OC8H17)4Brm, m=0,1,2,4] were prepared on single-crystal silicon substrates by spin coating. The ellipsometric spectra of the films have been studied on a rotating analyzer–polarizer type of scanning ellipsometer. The optical, dielectric constants and absorption coefficients of the films in 500–800 nm wavelength region were obtained. The results show that the complex refractive index, dielectric constants and absorption coefficient of these thin films are influenced regularly by bromine substituent on conjugated macrocycle. It was found that there are approximately linear relationships between the resonance absorption wavelengths of the films and the average number of bromine atoms substituted on the phthalocyanine ring.  相似文献   
28.
Parameters of Gaussian multivariate models are often estimated using the maximum likelihood approach. In spite of its merits, this methodology is not practical when the sample size is very large, as, for example, in the case of massive georeferenced data sets. In this paper, we study the asymptotic properties of the estimators that minimize three alternatives to the likelihood function, designed to increase the computational efficiency. This is achieved by applying the information sandwich technique to expansions of the pseudo-likelihood functions as quadratic forms of independent normal random variables. Theoretical calculations are given for a first-order autoregressive time series and then extended to a two-dimensional autoregressive process on a lattice. We compare the efficiency of the three estimators to that of the maximum likelihood estimator as well as among themselves, using numerical calculations of the theoretical results and simulations.  相似文献   
29.
In this paper, by using elementary analysis, we establish some new Lyapunov-type inequalities for nonlinear systems of differential equations, special cases of which contain the well-known equations such as Emden-Fowler-type and half-linear equations. The inequalities obtained here can be used as handy tools in the study of qualitative behaviour of solutions of the associated equations.  相似文献   
30.
The crystal structure of 1-[3-(3,4-dimethoxyphenyl)-2-propenoyl]pyrrolidine (C15H19NO3) (I) has been determined by X-ray analysis. It crystallizes orthorhombic space group Pbca with a = 24.295(3), b = 15.086(3), c = 7.552(3)A, V = 2768(1)A3, Z = 8, Dcalc = 1.254 g/cm3, mu = (Mo K(alpha)) = 0.87 cm(-1). The title compound has analgesic activity of cycloaliphatic amine part. The molecule is deviated from planar configuration.  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