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   15370篇
  免费   1140篇
  国内免费   775篇
化学   1486篇
晶体学   22篇
力学   953篇
综合类   216篇
数学   12463篇
物理学   2145篇
  2023年   84篇
  2022年   136篇
  2021年   209篇
  2020年   231篇
  2019年   267篇
  2018年   271篇
  2017年   376篇
  2016年   408篇
  2015年   293篇
  2014年   667篇
  2013年   978篇
  2012年   835篇
  2011年   718篇
  2010年   699篇
  2009年   908篇
  2008年   993篇
  2007年   1036篇
  2006年   812篇
  2005年   748篇
  2004年   618篇
  2003年   552篇
  2002年   583篇
  2001年   522篇
  2000年   486篇
  1999年   431篇
  1998年   358篇
  1997年   324篇
  1996年   322篇
  1995年   305篇
  1994年   223篇
  1993年   207篇
  1992年   204篇
  1991年   180篇
  1990年   139篇
  1989年   110篇
  1988年   114篇
  1987年   91篇
  1986年   107篇
  1985年   140篇
  1984年   110篇
  1983年   38篇
  1982年   70篇
  1981年   57篇
  1980年   51篇
  1979年   50篇
  1978年   58篇
  1977年   61篇
  1976年   35篇
  1975年   15篇
  1974年   16篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
为提高电商仓库的拣货作业效率,本文提出了基于聚类和动态规划的组合路径策略,实现了生成路径消耗时间和路径长度之间的平衡,并将这一策略成功地应用到多区型仓库。该策略分四步:首先,根据待拣储位分布特征,运用聚类分析法对其进行分类;然后,以各类的首末储位作为节点,运用动态规划法对已得分类进行排序,得到相应的类序;其次,得到各类内部路径;最后,依次拣取待拣商品,并返回出发点完成拣货作业。在提出新的路径策略后,通过仿真方法将新策略与三种传统路径策略(穿越策略、最大间隙策略和混合策略)和一种优质算法(蚁群算法)进行了对比分析,结果表明:该策略具备良好的适用性和实用性。  相似文献   
42.
In this work, the reactivities of acetonyl and benzoyl radicals in aromatic substitution and addition reactions have been compared in an experimental and computational study. The results show that acetonyl is more electrophilic than benzoyl, which is rather nucleophilic. A Hammett plot analysis of the addition reactions of the two radicals to substituted styrenes clearly support the nucleophilicity of benzoyl, but in the case of acetonyl, no satisfactory linear correlation with a single substituent-related parameter was found. Computational calculations helped to rationalize this effect, and a good linear correlation was found with a combination of polar parameters (σ+) and the radical stabilization energies of the formed intermediates. Based on the calculated philicity indices for benzoyl and acetonyl, a quantitative comparison of these two radicals with many other reported radicals is possible, which may help to predict the reactivities of other aromatic radical substitution reactions.  相似文献   
43.
We establish that solutions to the Cauchy–Dirichlet problem
?tu?div(Dξf(x,Du))=0
for functionals f:Ω×RN×n[0,) of linear growth can be obtained as limits of solutions to flows with p-growth in the limit p1. The result can be interpreted on the one hand as a stability result. On the other hand it provides an existence result for general flows with linear growth.  相似文献   
44.
A linear optical unambiguous discrimination of hyperentangled Bell states is proposed for two‐photon systems entangled in both the polarization and momentum degrees of freedom (DOFs) assisted by time bin. This unambiguous discrimination scheme can completely identify 16 orthogonal hyperentangled Bell states using only linear optical elements, where the function of the auxiliary entangled Bell state is replaced by time bin. Moreover, the possibility of extending this scheme for distinguishing hyperentangled Bell states in n DOFs is discussed, and it shows that 2 n + k + 1 hyperentangled Bell states in n ( n 2 ) DOFs can be distinguished with k ( k < n ) auxiliary entangled states of additional DOFs by introducing a time delay, which decreases the auxiliary entanglement resource required for unambiguous discrimination of hyperentangled Bell state. Therefore, this scheme provides a new way for distinguishing hyperentangled states with current technology, which will extend the application of discrimination of hyperentangled states via linear optics to other quantum information protocols besides hyperdense coding schemes in the future.  相似文献   
45.
Natural resources are not infinitely resilient and should not be modeled as being such. Finitely resilient resources feature tipping points and history dependence. This paper provides a didactical discussion of mathematical methods that are needed to understand the optimal management of such resources: viscosity solutions of Hamilton–Jacobi–Bellman equations, the costate equation and the associated canonical equations, exact root counting, and geometrical methods to analyze the geometry of the invariant manifolds of the canonical equations. Recommendations for Resource Managers
  • Management of natural resources has to take into account the possible breakdown of resilience and induced regime shifts.
  • Depending on the characteristics of the resource and on its present and future economic importance, either for all initial states the same kind of management policy is optimal, or the type of the optimal management policy depends on the initial state.
  • Modeling should reflect the finiteness of the data.
  相似文献   
46.
针对设计的一种场畸变气体开关,研究中间电极材料分别为不锈钢和黄铜条件下的烧蚀特性,结合开关寿命期间静态与触发特性的变化规律,获得决定开关寿命的关键因素,为三电极场畸变气体开关的性能优化提供理论支撑。研究结果表明,采用不锈钢和黄铜作为中间电极的烧蚀区域以及表面粗糙度均随着放电次数增加而增大,黄铜电极烧蚀较为严重且表面有明显的烧蚀圆斑,不锈钢电极则具有更高的表面粗糙度,阴阳极表面烧蚀存在明显差异,随着放电次数的增加,击穿点向电极边缘区域集中,影响开关的沿面绝缘特性,是导致开关寿命终结的主要原因。  相似文献   
47.
48.
49.
Many sports leagues first announce the games to be played in each round and then determine their matchdays as the season progresses. This study focuses on the fairness criterion of minimizing the total rest difference among opposing teams to find the matchdays for an announced schedule. We show that the problem is decomposable into optimizing the rounds separately. We also provide a polynomial-time exact algorithm for canonical schedules.  相似文献   
50.
This paper studies risk in a stochastic auction which facilitates the integration of renewable generation in electricity markets. We model market participants who are risk averse and reflect their risk aversion through coherent risk measures. We uncover a closed form characterization of a risk-averse generator’s optimal pre-commitment behaviour for a given real-time policy, both with and without risk trading.  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