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   482853篇
  免费   5119篇
  国内免费   1821篇
化学   261115篇
晶体学   6948篇
力学   21211篇
综合类   14篇
数学   55354篇
物理学   145151篇
  2020年   3538篇
  2019年   3895篇
  2018年   4516篇
  2017年   4446篇
  2016年   7495篇
  2015年   5286篇
  2014年   7632篇
  2013年   21908篇
  2012年   16414篇
  2011年   20476篇
  2010年   13462篇
  2009年   13502篇
  2008年   18598篇
  2007年   18740篇
  2006年   17479篇
  2005年   16226篇
  2004年   14559篇
  2003年   12804篇
  2002年   12676篇
  2001年   14219篇
  2000年   10915篇
  1999年   8542篇
  1998年   7078篇
  1997年   7067篇
  1996年   6638篇
  1995年   6220篇
  1994年   5921篇
  1993年   5795篇
  1992年   6404篇
  1991年   6514篇
  1990年   5989篇
  1989年   5952篇
  1988年   5977篇
  1987年   5744篇
  1986年   5477篇
  1985年   7671篇
  1984年   7816篇
  1983年   6470篇
  1982年   7040篇
  1981年   6727篇
  1980年   6554篇
  1979年   6721篇
  1978年   7029篇
  1977年   6792篇
  1976年   6942篇
  1975年   6470篇
  1974年   6571篇
  1973年   6856篇
  1972年   4304篇
  1971年   3416篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
In this paper a new state called odd-excited binomial state (OEBS) is introduced. It interpolates between the odd number state and the odd-excited coherent state. We discuss some statistical properties, such as the Glauber second-order correlation function and squeezing phenomenon (normal and amplitude-squared squeezing) for this state. The quasiprobability distribution functions (Husimi Q-function and Wigner W-function) are also examined.  相似文献   
32.
33.
A new geometrical method to determine the surface damage threshold for ultrashort pulses is presented. It consists in the formation of a surface damage profile by a movement of the sample across the laser beam focus. A single measurement of the maximum transversal dimension of this damage profile, which depends solely on the laser beam power, is used to calculate the local damage threshold intensity. It is also theoretically shown that the damage in the transverse dimension can be controlled under the diffraction-limited spot.  相似文献   
34.
R. J. Hinde 《Few-Body Systems》2006,38(2-4):187-191
We compute the vibrational coupling between two H2 molecules from ab initio quantum chemical calculations of the H2-H2 potential carried out at the full configuration interaction level of theory using the atom-centered aug-cc-pVTZ basis set for hydrogen. We compare the full configuration interaction results with those obtained using two variants of coupled cluster theory and find that a fully iterative treatment of connected triples may be required to estimate the H2-H2 vibrational coupling accurately using coupled cluster theory.  相似文献   
35.
A simple scheme is developed for treatment of vertical bed topography in shallow water flows. The effect of the vertical step on flows is modelled with the shallow water equations including local energy loss terms. The bed elevation is denoted with zb for the left and zb+ for the right values at each grid point, hence exactly representing a discontinuity in the bed topography. The surface gradient method (SGM) is generalized to reconstruct water depths at cell interfaces involving a vertical step so that the fluxes at the cell interfaces can accurately be calculated with a Riemann solver. The scheme is verified by predicting a surge crossing a step, a tidal flow over a step and dam‐break flows on wet/dry beds. The results have shown good agreements compared with analytical solutions and available experimental data. The scheme is efficient, robust, and may be used for practical flow calculations. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   
36.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 6, pp. 970–975, June, 1991.  相似文献   
37.
In a twisted nematic liquid crystal cell we report and study the self-transparency effect induced by the creation of an isotropic droplet. A simple theoretical model is presented and good agreement with experimental data is found. In a two beams configuration transistor-like behavior is observed and theoretically discussed.  相似文献   
38.
In the following we find all of the three-dimensional flat spacetimes which are static and rotationally symmetric.  相似文献   
39.
A method using third order moments for estimating the regression coefficients as well as the latent state scores of the reduced-rank regression model when the latent variable(s) are non-normally distributed is presented in this paper. It is shown that the factor analysis type indeterminacy of the regression coefficient matrices is eliminated. A real life example of the proposed method is presented. Differences of this solution with the reduced-rank regression eigen solution are discussed.  相似文献   
40.
In developing travel demand models it is generally assumed that the base-year data used in developing the parameters, as well as the forecasted data to be used as independent variables for the design year, are of acceptable quality. The purpose of this paper is to present the application of error propagation theory in assesing the predictive quality of one type of travel demand forecasting model (multinomial logit models) and to demonstrate how error considerations can be used as a tool for identifying the optimal model. The general conclusions of this study are that: (1) it is indeed possible to quantify errors in dependent variables in logit models as a consequence of errors in independent variables; and (2) error consideration can be used as a tool for identifying the optimal model from a set of candidate models. Further research is recommended to develop better insights into the phenomenon of error propagation so that the consideration of errors can be a factor in decisions on model selection.  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