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   25771篇
  免费   1260篇
  国内免费   1124篇
系统科学   1926篇
丛书文集   714篇
教育与普及   304篇
理论与方法论   358篇
现状及发展   1031篇
研究方法   53篇
综合类   23763篇
自然研究   6篇
  2024年   54篇
  2023年   171篇
  2022年   288篇
  2021年   348篇
  2020年   247篇
  2019年   177篇
  2018年   889篇
  2017年   993篇
  2016年   685篇
  2015年   551篇
  2014年   751篇
  2013年   837篇
  2012年   1164篇
  2011年   1981篇
  2010年   1755篇
  2009年   1588篇
  2008年   1748篇
  2007年   2049篇
  2006年   1144篇
  2005年   1030篇
  2004年   992篇
  2003年   901篇
  2002年   888篇
  2001年   797篇
  2000年   719篇
  1999年   839篇
  1998年   551篇
  1997年   643篇
  1996年   528篇
  1995年   515篇
  1994年   469篇
  1993年   373篇
  1992年   325篇
  1991年   292篇
  1990年   249篇
  1989年   225篇
  1988年   170篇
  1987年   120篇
  1986年   65篇
  1985年   27篇
  1984年   7篇
  1983年   2篇
  1982年   1篇
  1981年   1篇
  1980年   2篇
  1969年   1篇
  1967年   1篇
  1955年   2篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
As was mentioned by Nicolas Lori in his (Found Sci, 2010) commentary, the definition of Information in Physics is something about which not all authors agreed. According to physicists like me Information decreases when Entropy increases (so entropy would be a negative measure of information), while many physicists, seemingly the majority of them, are convinced of the contrary (even in the camp of Quantum Information Theoreticians). In this reply I reproduce, and make more precise, some of my arguments, that appeared here and there in my (2010) paper, in order to clarify the presentation of my personal point of view on the subject.  相似文献   
932.
This paper aims to contribute to the literature on the explanatory power of behavior models with heterogeneous agents. The authors present a new nonlinear structural stock market model which is a nonlinear deterministic process buffeted by dynamic noise. An exogenous noise is introduced to the model with the assumption of IID normal innovations of the fundamental value in order to investigate how noisy dynamics interacts with deterministic process. The market is composed of two typical trader types: the rational fundamentalists and the boundedly rational traders governed by greed and fear. The interaction between noise and deterministic element determines the evolution process of the system as key parameters are changed. The authors find the model is able to generate time series that exhibit dynamical and statistical properties closely resembling those of the S&P500 index, such as volatility clustering, fat tails (leptokurtosis), autocorrelation in square and absolute return, larger amplitude, crashes and bubbles. The authors also investigate the nonlinear dependence structure in our data. The results indicate that the GARCH-type model cannot completely account for all nonlinearity in our simulated market, which is thus consistent with the results from real markets. It seems that the nonlinear structural model is more powerful to give a satisfied explanation to market behavior than the traditional stochastic approach.  相似文献   
933.
复合主题的文献在进行信息标引的时候,按照不同的标引次序所组成的类目体系是不相同的,因而也就造成检索结果的不同。通过分析采用不同引用次序对建立类目体系的影响,从而得出根据不同的需要采用相应的引用次序。  相似文献   
934.
阳光体育是处于初级阶段的,是顺应时代要求的,是对学校体育的一种提升,有其科学的内涵和健康的本质。目前,阳光体育正以鲜明的政治要求,继续加大专项资金投入。我们应放眼国内外,探索开创阳光体育市场,不断积累文化底蕴,打造阳光体育文化品牌,勇于实践,塑造中流砥柱的典型阳光体育活动。  相似文献   
935.
This paper considers the optimal investment strategy for an insurer under the criterion of mean-variance. The risk process is a compound Poisson process and the insurer can invest in a risk-free asset and multiple risky assets. This paper obtains the optimal investment policy using the stochastic linear quadratic (LQ) control theory with no-shorting constraint. Then the efficient strategy (optimal investment strategy) and efficient frontier are derived explicitly by a verification theorem with the viscosity solution of Hamilton-Jacobi-Bellman (HJB) equation.  相似文献   
936.
This paper considers a class of stochastic variational inequality problems. As proposed by Jiang and Xu (2008), by using the so-called regularized gap function, the authors formulate the problems as constrained optimization problems and then propose a sample average approximation method for solving the problems. Under some moderate conditions, the authors investigate the limiting behavior of the optimal values and the optimal solutions of the approximation problems. Finally, some numerical results are reported to show efficiency of the proposed method.  相似文献   
937.
The optimum design of equivalent accelerated life testing plan based on proportional hazards-proportional odds model using D-optimality is presented.The defined equivalent test plan is the test plan that has the same value of the determinant of Fisher information matrix.The equivalent test plan of step stress accelerated life testing(SSALT) to a baseline optimum constant stress accelerated life testing(CSALT) plan is obtained by adjusting the censoring time of SSALT and solving the optimization problem for each case to achieve the same value of the determinant of Fisher information matrix as in the baseline optimum CSALT plan.Numerical examples are given finally which demonstrate the equivalent SSALT plan to the baseline optimum CSALT plan reduces almost half of the test time while achieving approximately the same estimation errors of model parameters.  相似文献   
938.
In this paper, the dynamic behaviors of a discrete epidemic model with a nonlinear incidence rate obtained by Euler method are discussed, which can exhibit the periodic motions and chaotic behaviors under the suitable system parameter conditions. Codimension-two bifurcations of the discrete epidemic model, associated with 1:1 strong resonance, 1:2 strong resonance, 1:3 strong resonance and 1:4 strong resonance, are analyzed by using the bifurcation theorem and the normal form method of maps. Moreover, in order to eliminate the chaotic behavior of the discrete epidemic model, a tracking controller is designed such that the disease disappears gradually. Finally, numerical simulations are obtained by the phase portraits, the maximum Lyapunov exponents diagrams for two different varying parameters in 3-dimension space, the bifurcation diagrams, the computations of Lyapunov exponents and the dynamic response. They not only illustrate the validity of the proposed results, but also display the interesting and complex dynamical behaviors.  相似文献   
939.
Starting from the crisis in our communities—research has indicated it to be widely spread—the author explores the scientific tradition preceding the rise of modernism in order to draw out a more humane way of thinking that may help our contemporary societies. He discerns between two types of Renaissance humanism, one characterised by its cleverness, the other by its compassion and desire to restore dignity to the lives of people struggling to escape the clutches of medievalism. The father of the latter is Erasmus of Rotterdam and we follow the development of his conception of a social humanism as it branches out, through the work of his successors, into every scientific discipline, both natural and human. These disciplines are united by an embracing systemic idea of philosophy that unites the mind with the heart and which Erasmus called “Philosophia Christi”. We examine the two main pillars upon which this philosophy is built, love for our neighbour and education as the only legitimate instrument to change society, and the extraordinary impact it had on science and on communities living in the seventeenth century. We contrast this with the idea of power, the chosen instrument of modernity to transform society, and trace some of its tragic outcomes. We conclude by discussing the incorporation of an Erasmian type of social humanism into systems education and the future development of such programmes.  相似文献   
940.
One of the key challenges for electrochemical water splitting is the development of low-cost and efficient hydrogen evolution cathode. In this work, a self-supported Ni-P cathode was synthesized by a facile electrodeposition method. The composition and morphology were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. The Ni-P cathode performed low onset over-potential, good catalytic activity and long-term stability under neutral and alkaline conditions. The mechanism of Ni-P electrode for hydrogen production was discussed by electrochemical impedance spectroscopy. The excellent performance of Ni-P cathode was mainly attributed to the synergistic effect of phosphate anions and the self-supported feature.  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