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   4946篇
  免费   205篇
  国内免费   158篇
化学   137篇
晶体学   2篇
力学   47篇
综合类   37篇
数学   990篇
物理学   331篇
综合类   3765篇
  2025年   9篇
  2024年   48篇
  2023年   43篇
  2022年   79篇
  2021年   97篇
  2020年   89篇
  2019年   76篇
  2018年   59篇
  2017年   70篇
  2016年   80篇
  2015年   99篇
  2014年   212篇
  2013年   242篇
  2012年   257篇
  2011年   264篇
  2010年   246篇
  2009年   238篇
  2008年   303篇
  2007年   449篇
  2006年   375篇
  2005年   377篇
  2004年   315篇
  2003年   283篇
  2002年   229篇
  2001年   150篇
  2000年   151篇
  1999年   109篇
  1998年   77篇
  1997年   67篇
  1996年   58篇
  1995年   41篇
  1994年   38篇
  1993年   18篇
  1992年   13篇
  1991年   9篇
  1990年   8篇
  1989年   10篇
  1988年   2篇
  1987年   4篇
  1986年   4篇
  1985年   3篇
  1984年   4篇
  1982年   1篇
  1979年   2篇
  1974年   1篇
排序方式: 共有5309条查询结果,搜索用时 0 毫秒
81.
本文以数码插图为主要对象,探讨了数字化技术对插图艺术的影响、人们新的审美观念的形成以及数码插图的市场前景。  相似文献   
82.
高职高专市场营销专业实践教学改革新模式——以赛带训   总被引:1,自引:0,他引:1  
传统的市场营销专业课堂理论教学无法满足学生的发展需要,已有的实践教学模式成本较大,运作困难。因此,必须转变观念,更新方法,探索新的模式来推进市场营销专业的教学改革和发展。以赛带训模式很好地利用了各种比赛的平台,调动学生学习的积极性,训练学生的各种实践技能,且便于推广和操作。它是对传统教学的改革和创新,是符合新时代学生培养目标要求的。  相似文献   
83.
A three-dimensional bio-reactor model of exploitative competition of two predator organisms with inhibition responses for the same renewable organism with reproductive properties is considered. By constructing a modified Lyapunov function and using LaSalle’s invariant principle, it is shown that the lower “break-even” concentration predator organism survives in the competition.   相似文献   
84.
Recent studies show that a negative shock in stock prices will generate more volatility than a positive shock of similar magnitude. The aim of this paper is to appraise the hypothesis under which the conditional mean and the conditional variance of stock returns are asymmetric functions of past information. We compare the results for the Portuguese Stock Market Index PSI 20 with six other Stock Market Indices, namely the SP 500, FTSE 100, DAX 30, CAC 40, ASE 20, and IBEX 35. In order to assess asymmetric volatility we use autoregressive conditional heteroskedasticity specifications known as TARCH and EGARCH. We also test for asymmetry after controlling for the effect of macroeconomic factors on stock market returns using TAR and M-TAR specifications within a VAR framework. Our results show that the conditional variance is an asymmetric function of past innovations raising proportionately more during market declines, a phenomenon known as the leverage effect. However, when we control for the effect of changes in macroeconomic variables, we find no significant evidence of asymmetric behaviour of the stock market returns. There are some signs that the Portuguese Stock Market tends to show somewhat less market efficiency than other markets since the effect of the shocks appear to take a longer time to dissipate.  相似文献   
85.
Universal features in stock markets and their derivative markets are studied by means of probability distributions in internal rates of return on buy and sell transaction pairs. Unlike the stylized facts in normalized log returns, the probability distributions for such single asset encounters incorporate the time factor by means of the internal rate of return, defined as the continuous compound interest. Resulting stylized facts are shown in the probability distributions derived from the daily series of TOPIX, S & P 500 and FTSE 100 index close values. The application of the above analysis to minute-tick data of NIKKEI 225 and its futures market, respectively, reveals an interesting difference in the behavior of the two probability distributions, in case a threshold on the minimal duration of the long position is imposed. It is therefore suggested that the probability distributions of the internal rates of return could be used for causality mining between the underlying and derivative stock markets. The highly specific discrete spectrum, which results from noise trader strategies as opposed to the smooth distributions observed for fundamentalist strategies in single encounter transactions may be useful in deducing the type of investment strategy from trading revenues of small portfolio investors.  相似文献   
86.
This paper seeks to solve the difficult nonlinear problem in financial markets on the complex system theory and the nonlinear dynamics principle, with the data-model-concept-practice issue-oriented reconstruction of the phase space by the high frequency trade data. In theory, we have achieved the differentiable manifold geometry configuration, discovered the Yang-Mills functional in financial markets, obtained a meaningful conserved quantity through corresponding space-time non-Abel localization gauge symmetry transformation, and derived the financial solitons, which shows that there is a strict symmetry between manifold fiber bundle and guage field in financial markets. In practical applications of financial markets, we have repeatedly carried out experimental tests in a fluctuant evolvement, directly simulating and validating the existence of solitons by researching the price fluctuations (society phenomena) using the same methods and criterion as in natural science and in actual trade to test the stock Guangzhou Proprietary and the futures Fuel Oil in China. The results demonstrate that the financial solitons discovered indicates that there is a kind of new substance and form of energy existing in financial trade markets, which likely indicates a new science paradigm in the economy and society domains beyond physics.   相似文献   
87.
In this paper, we present a theoretical model for the gain competition in dual-wavelength dye lasers. For this purpose the rate equations under the steady-state conditions have been used. The degree of the gain competition is shown to depend on the polarization of the two laser beams. The dependence of the gain competition on various working parameters such as pumping power, dye concentration and the reorientation rate of the molecules or solvent viscosity is studied for two different cases of parallel and orthogonal polarizations. The results of our model show dramatically different behaviors in the two proposed cases.  相似文献   
88.
89.
Abstract

Leaf carbon isotope ratios (δ13C), an indicator of long-term intercellular carbon dioxide concentration, and also stem and root carbon isotope ratios were measured on the obligate CAM species Crassula argentea cultivated in pure and mixed cultures with the succulent C3 Peperomia obtusifolia in open-air conditions under two different levels of nitrogen and water supply.

As expected, a diminished water supply and a relatively dry and hot summer climate cause a shift of δ13C values to a less strong 13C discrimination (less negative δ13C values). A diminished nitrogen supply causes a shift of the δ13C values in direction of a higher 13C discrimination (more negative δ13C values), particularly in the leaves. Competition causes also an increased 13C discrimination, especially valid for shoot axes.

The shift of 13C/12C isotope ratios in case of nitrogen deficiency is discussed to be a result of a decreased PEPCase activity in the night.  相似文献   
90.
浅谈大学生物理实验竞赛的组织及意义   总被引:2,自引:0,他引:2  
主要对首届辽宁省大学生物理实验竞赛的组织设计、特点及意义等方面进行了总结和探讨,认为组织大学生物理实验竞赛对激发大学生对物理实验的兴趣与潜能、提高物理实验教学质量具有重要意义。  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