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   20103篇
  免费   609篇
  国内免费   462篇
化学   646篇
晶体学   17篇
力学   47篇
综合类   105篇
数学   1873篇
物理学   487篇
综合类   17999篇
  2024年   29篇
  2023年   64篇
  2022年   158篇
  2021年   177篇
  2020年   177篇
  2019年   142篇
  2018年   123篇
  2017年   170篇
  2016年   224篇
  2015年   427篇
  2014年   1021篇
  2013年   776篇
  2012年   1102篇
  2011年   1495篇
  2010年   1308篇
  2009年   1342篇
  2008年   1578篇
  2007年   2186篇
  2006年   1721篇
  2005年   1577篇
  2004年   1211篇
  2003年   1047篇
  2002年   666篇
  2001年   551篇
  2000年   355篇
  1999年   282篇
  1998年   237篇
  1997年   186篇
  1996年   161篇
  1995年   106篇
  1994年   124篇
  1993年   93篇
  1992年   84篇
  1991年   62篇
  1990年   51篇
  1989年   39篇
  1988年   49篇
  1987年   30篇
  1986年   20篇
  1985年   8篇
  1984年   3篇
  1983年   2篇
  1982年   3篇
  1981年   3篇
  1980年   1篇
  1979年   2篇
  1977年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
政府协调下群体性突发事件的演化博弈分析及应用   总被引:1,自引:0,他引:1  
在群体性突发事件应急处置过程中,政府处置不当会使得事态扩大,甚至由非利益直接相关者转变为矛盾冲突方。本文将政府部门视为社会矛盾的调解者,运用演化博弈理论分析了政府作为利益协调者在群体性突发事件中的重要作用,并结合对公平因素的考虑,详细分析了一些主要参数对决策行为的影响。结果表明,政府提早介入、公平的利益再分配以及适当的惩罚措施有助于减少群体性突发事件。最后,通过分析案例“重庆市出租车罢工事件”的发展演化历程及政府的协调作用,说明了本文模型的适用性。  相似文献   
942.
This is a report of a study on the evolution of manufacturing technology policy during the deployment of domestic advanced manufacturing systems in thirty-four plants and two panels of data collection separated by one year. Changing firm environment was significantly correlated with pioneering product introduction business strategy (p<0.05). More importantly, it was found that manufacturing technology policy is significantly (p<0.05) associated with pioneering business strategy. Further, findings indicate that fine-tuning or modest adjustment in this policy (versus doing nothing or drastic change) was significantly (p<0.05) associated with the maximum levels of reported utilization of these new systems in a subsample of second panel, complete data cases (n=21). This curvilinear relationship between the absolute value of changes in technology policy and performance measure did not hold for the percentage of target cycle time achieved nor uptime, although results concerning performance are considered preliminary at the time of this writing. Advertising this processing technology tends to be inversely related to the radicalness of the technology incorporated into the system (p=0.076) during the deployment period tracked thus far. That is, firms installing more radical systems tend to become very cautious about sharing information about the project once installation begins.  相似文献   
943.
Consideration is given to continuous-time, parameter-dependent optimal control problems with state-variable jump discontinuities atN variable interior times. A maximum principle involving known costate jump conditions is stated and is proved by transforming the problem into a standard Mayer control problem. An illustrative example for fisheries management is included.This work was partially supported by a grant from Control Data. The authors are grateful to Professor T. L. Vincent for drawing their attention to Refs. 4–6 listed below.  相似文献   
944.
This paper presents an adaptive memory-based method for solving the Capacitated Vehicle Routing Problem (CVRP), called BoneRoute. The CVRP deals with the problem of finding the optimal sequence of deliveries conducted by a fleet of homogeneous vehicles, based at one depot, to serve a set of customers. The computational performance of the BoneRoute was found to be very efficient, producing high quality solutions over two sets of well known case studies examined.  相似文献   
945.
Signed graphs for portfolio analysis in risk management   总被引:1,自引:0,他引:1  
We introduce the notion of structural balance for signed graphsin the context of portfolio analysis. A portfolio of securitiescan be represented as a signed graph with the nodes denotingthe securities and the edges representing the correlation betweenthe securities. With signed graphs, the characteristics of aportfolio from a risk management perspective can be uncoveredfor analysis purposes. It is shown that a portfolio characterizedby a signed graph of positive and negative edges that is structurallybalanced is characteristically more predictable. Investors whoundertake a portfolio position with all positively correlatedsecurities do so with the intention to speculate on the upside(or downside). If the portfolio consists of negative edges andis balanced, then it is likely that the position has a hedgingdisposition within it. On the other hand, an unbalanced signedgraph is representative of an investment portfolio which ischaracteristically unpredictable.  相似文献   
946.
Online IPA Gradient Estimators in Stochastic Continuous Fluid Models   总被引:1,自引:0,他引:1  
This paper applies infinitesimal perturbation analysis (IPA) to loss-related and workload-related metrics in a class of stochastic flow models (SFM). It derives closed-form formulas for the gradient estimators of these metrics with respect to various parameters of interest, such as buffer size, service rate, and inflow rate. The IPA estimators derived are simple and fast to compute, and are further shown to be unbiased and nonparametric, in the sense that they can be computed directly from the observed data without any knowledge of the underlying probability law. These properties hold out the promise of utilizing IPA gradient estimates as ingredients of online management and control of telecommunications networks. While this paper considers single-node SFMs, the analysis method developed is amenable to extensions to networks of SFM nodes with more general topologies.  相似文献   
947.
This paper introduces a class of unit-linked annuities that extends existing annuities by allowing portfolio shocks to be gradually absorbed into the annuity payouts. Consequently, our new class enables insurers to offer an affordable and adequate annuity with a stable payout stream. We show how to price and adequately hedge the annuity payouts in a general financial environment. In particular, our model accounts for various stylized facts of stock returns such as asymmetry and heavy-tailedness. Furthermore, the generality of our framework makes it possible to explore the impact of a parameter misspecification on the annuity price and the hedging performance.  相似文献   
948.
针对古村落发展面临的现实和潜在的问题,以四明山腹地宁渡余姚柿林村为例,探讨了古村落在市场经济条件下的管理制度与保护机制.  相似文献   
949.
双目标突发事件应急救援前摄性调度优化   总被引:1,自引:0,他引:1  
本文以救援时间最短化与计划鲁棒性最大化为目标,研究突发事件应急救援前摄性调度优化问题。作者首先对所研究问题进行界定,其中计划鲁棒性定义为各活动开始时间可调整时差的总和,任务是在应急预算和救援期限的约束下,确定活动执行模式与开始时间以实现上述两个目标。随后,构建问题的0-1规划优化模型,鉴于其强NP-hard属性,设计禁忌搜索启发式算法。最后用一个算例对研究进行说明,得到如下结论:救援时间随应急预算的增加而缩短,随救援期限的放宽而延长;计划鲁棒性随应急预算的增加或救援期限的放宽而提高;当权重分配系数增大时,应急救援时间先保持不变而后缩短,计划鲁棒性则呈减小趋势。本文研究可为突发事件应急救援的组织与协调提供决策支持。  相似文献   
950.
In the literature, decision models and techniques for supplier selection do not often consider inventory management of the items being purchased as part of the analysis. In this article, two mixed integer nonlinear programming models are proposed to select the best set of suppliers and determine the proper allocation of order quantities while minimizing the annual ordering, inventory holding, and purchasing costs under suppliers’ capacity and quality constraints. The first model allows independent order quantities for each supplier while the second model restricts all order quantities to be of equal size, as it would be required in a multi-stage (supply chain) inventory model. Illustrative examples are used to highlight the advantages of the proposed models over a previous model introduced in the literature.  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