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   36544篇
  免费   5325篇
  国内免费   2598篇
化学   4991篇
晶体学   100篇
力学   4939篇
综合类   491篇
数学   10697篇
物理学   12159篇
无线电   11090篇
  2024年   100篇
  2023年   399篇
  2022年   938篇
  2021年   1028篇
  2020年   1256篇
  2019年   985篇
  2018年   976篇
  2017年   1362篇
  2016年   1669篇
  2015年   1423篇
  2014年   2247篇
  2013年   2775篇
  2012年   2333篇
  2011年   2621篇
  2010年   2116篇
  2009年   2441篇
  2008年   2343篇
  2007年   2322篇
  2006年   1950篇
  2005年   1752篇
  2004年   1583篇
  2003年   1345篇
  2002年   1233篇
  2001年   951篇
  2000年   921篇
  1999年   763篇
  1998年   682篇
  1997年   509篇
  1996年   442篇
  1995年   428篇
  1994年   375篇
  1993年   310篇
  1992年   278篇
  1991年   204篇
  1990年   186篇
  1989年   138篇
  1988年   131篇
  1987年   131篇
  1986年   109篇
  1985年   129篇
  1984年   135篇
  1983年   72篇
  1982年   97篇
  1981年   66篇
  1980年   29篇
  1979年   39篇
  1978年   30篇
  1977年   28篇
  1976年   15篇
  1973年   22篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
191.
This paper presents a discrete age structured SI model of tracheal mite (Acarapis woodi (Rennie)) infestation of honey bees (Apis mellifera L.) at the colony level. The model describes the elevated honey bee attrition during winter and predicts that winter brood rearing should promote colony survival by decreasing the colony's infestation and increasing the colony's population. The model also predicts that swarming and the artificial removal of part of the colony's forager honey bees should reduce the colony's infestation level.  相似文献   
192.
Based on the stress transport model, a rate-dependent algebraic expression for the Reynolds stress tensor is developed. It is shown that the new model includes the normal stress effects and exhibits viscoelastic behavior. Furthermore, it is compatible with recently developed improved models of turbulence. The model is also consistent with the limiting behavior of turbulence in the inertial sublayer and is capable of predicting secondary flows in noncircular ducts. The TEACH code is modified according to the requirements of the rate-dependent model and is used to predict turbulent flow fields in a channel and behind a backward-facing step. The predicted results are compared with the available experimental data and those obtained from the standard k-ε and algebraic stress models. It is shown that the predictions of the new model are in better agreements with the experimental data.  相似文献   
193.
1 IntroductionWiththerapiddevelopmentofInternetandmobilecommunications,theresearchonwirelessmobilenetworkshasbecomethenewhotspotinin formationtechnologyfield .Asweknow ,mobileterminals (mobilephone,forexample)arethetooltohelppeopletoaccessanyinformationtheyneedbyanymediaatanytimeandanywhere .However,itissometimesdifficultformobileterminalstoplaysucharolebecauseofthelimitationssuchaslimitedca pacity ,lowpower,andinconvenienceinoperating .So,peoplehopetofindanotherwaytoaccessnet workinformatio…  相似文献   
194.
环境因子的定义及研究现状   总被引:12,自引:0,他引:12  
从普遍接受的三个基本假设出发,讨论环境因子的定义和失效机理不变的约束条件,重点综述环境因子的研究现状和常用方法,并探讨引入反应论模型解决环境因子预测问题的可能途径。  相似文献   
195.
196.
Summary The interceluster organization of a two-dimensional colloidal system undergoing diffusion-limited cluster-cluster aggregation is considered. During this process the system evolves into a stationary, scaling state. In this state the organization arises from effective intercluster repulsion due to the mutually exclusive depletion zones surrounding each cluster. It is shown that in the scaling regime the relevant size of the depletion zones is determined by the separation between clusters, rather than the cluster size. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.  相似文献   
197.
The reference [4] proved the consistency of S1 and S2 among Lewis' five strict implication systems in the modal logic by using the method of the Boolean-valued model. But, in this method, the consistency of S3, S4 and S5 in Lewis' five strict implication systems is not decided. This paper makes use of the properties: (1) the equivalence of the modal systems S3 and P3, S4 and P4; (2) the modal systems P3 and P4 all contained the modal axiom T(□p → p); (3) the modal axiom T is correspondence to the reflexive property in VB. Hence, the paper proves: (a) ‖As31‖ = 1; (b) ‖AS41‖ = 1; (c) ‖AS5l‖ = 1 in the model (where B is a complete Boolean algebra, R is reflexive property in VB). Therefore, the paper finally proves that the Boolean-valued model VB of the ZFC axiom system in set theory is also a Boolean-valued model of Lewis' the strict implication system S3, S4 and S5.  相似文献   
198.
青岛港货运吞吐量的时间序列模型   总被引:2,自引:0,他引:2  
运用时间序列分析方法对时间序列建立ARMA,ARIMA模型.搜集了青岛港1999年1月~2003年5月的货运吞吐量数据,对进行分析,建立了青岛港货运吞吐量的模型.通过预留的部分数据对模型进行检验,并对模型的残差进行检验,得出模型比较合理.  相似文献   
199.
The measure of uncertainty is adopted as a measure of information. The measures of fuzziness are known as fuzzy information measures. The measure of a quantity of fuzzy information gained from a fuzzy set or fuzzy system is known as fuzzy entropy. Fuzzy entropy has been focused and studied by many researchers in various fields. In this paper, firstly, the axiomatic definition of fuzzy entropy is discussed. Then, neural networks model of fuzzy entropy is proposed, based on the computing capability of neural networks. In the end, two examples are discussed to show the efficiency of the model.  相似文献   
200.
本文通过模拟研究,讨论了最大似然方法和Bayes方法在分析结构方程模型中的相似点和不同之处。  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