全文获取类型
收费全文 | 882篇 |
免费 | 117篇 |
国内免费 | 73篇 |
专业分类
化学 | 166篇 |
晶体学 | 2篇 |
力学 | 33篇 |
综合类 | 20篇 |
数学 | 746篇 |
物理学 | 105篇 |
出版年
2024年 | 2篇 |
2023年 | 10篇 |
2022年 | 29篇 |
2021年 | 40篇 |
2020年 | 27篇 |
2019年 | 38篇 |
2018年 | 16篇 |
2017年 | 37篇 |
2016年 | 31篇 |
2015年 | 27篇 |
2014年 | 43篇 |
2013年 | 65篇 |
2012年 | 53篇 |
2011年 | 61篇 |
2010年 | 56篇 |
2009年 | 35篇 |
2008年 | 54篇 |
2007年 | 67篇 |
2006年 | 44篇 |
2005年 | 40篇 |
2004年 | 35篇 |
2003年 | 42篇 |
2002年 | 18篇 |
2001年 | 23篇 |
2000年 | 24篇 |
1999年 | 16篇 |
1998年 | 20篇 |
1997年 | 17篇 |
1996年 | 9篇 |
1995年 | 12篇 |
1994年 | 19篇 |
1993年 | 4篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有1072条查询结果,搜索用时 234 毫秒
991.
We develop a model of the behavior of a potential investor (under uncertainty and in a fiscal environment) who wishes to invest
into a project in the real sector of an economy and faces a timing problem. We find an optimal solution within this model
and examine the dependence of the tax revenue from the newly created firm on the depreciation policy. It is shown that there
exists a domain in the space of the parameters of the investment project where both the tax revenue and the incentives can
be increased by using the depreciation policy. 相似文献
992.
This article examines coordinated decisions in a decentralized supply chain that consists of one supplier and one retailer, and faces random demand of a single product with a short life cycle. We consider a setting where the retailer has accurate demand information while the supplier does not. Such a problem with asymmetric demand information can be viewed as an extension of the newsboy problem in which both the supplier and the retailer possess the same demand information. Combining the mechanism of sharing demand information and that of quantity discount and return policy enables us to develop three coordinated models in contrast with the basic and uncoordinated model. We are able to show the ordinal relationship among the retailer’s optimal order quantities in these four models under a general form of random demand, and compare the supply chain profits and conduct sensitivity analysis analytically in four models under uniform random demand. We also provide numerical results under normal random demand that bear a resemblance to those under uniform random demand. 相似文献
993.
994.
针对自动化车床工序最优检测和刀具更换问题进行了探讨.将定期检测和将刀具更换作用于同一工序流程,在只考虑刀具故障条件下,通过概率论和更新过程理论建立了以单位时间内期望费用为目标函数的数学模型,以检测间隔和刀具更换间隔为策略,确定最优的策略使得目标函数达到最小,并求出了经长期运行单位时间内期望费用的明显表达式.最后还对结果进行了讨论. 相似文献
995.
H.T. Feng R.F. Zhuo J.T. Chen D. Yan J.J. Feng H.J. Li S. Cheng P.X. Yan 《Physica E: Low-dimensional Systems and Nanostructures》2009,41(9):1640-1644
We report the synthesis and characterization Sb-doped SnO2 nanowires by chemical vapor deposition (CVD) at 850 °C. The as-synthesized-doped nanowires showed unique periodical structures in contrast to the traditional nanowires with smooth surfaces. The fascinating structures lead to a much higher surface to volume ratio and greater changes of depletion layer volume after gas absorption, and the sensitivity of gas sensing devices may be improved by using axial periodical nanostructures, instead of ordinary one. The photoluminescence of the Sb-doped SnO2 nanostructures were measured. The doping of Sb atoms brings two new emission peaks at 561 and 670 nm. 相似文献
996.
Quanxin Zhu 《Mathematical Methods of Operations Research》2007,66(2):299-313
In this paper, we study the average optimality for continuous-time controlled jump Markov processes in general state and action spaces. The criterion to be minimized is the average expected costs. Both the transition rates and the cost rates are allowed to be unbounded. We propose another set of conditions under which we first establish one average optimality inequality by using the well-known “vanishing discounting factor approach”. Then, when the cost (or reward)
rates are nonnegative (or nonpositive), from the average optimality inequality we prove the existence of an average optimal
stationary policy in all randomized history dependent policies by using the Dynkin formula and the Tauberian theorem. Finally, when the cost (or reward) rates have neither upper nor lower bounds, we also prove the existence of an average optimal policy in all (deterministic) stationary policies by constructing a “new”
cost (or reward) rate.
Research partially supported by the Natural Science Foundation of China (Grant No: 10626021) and the Natural Science Foundation
of Guangdong Province (Grant No: 06300957). 相似文献
997.
Korean mobile carriers have implemented various service plans to gain additional market share. One of them is incremental discount policy on the call rate in which subscriber pays less for calls longer than a prescribed length. In this paper, a mathematical model is developed to find an optimal discount policy with the objective of maximizing the carrier’s average expected revenue while satisfying a prescribed connection success rate. Through the analysis of the objective function we find a reduced solution space in which an optimal solution can be found by a search method. Through solving example problems the validity of the model is illustrated. 相似文献
998.
肖燕婷 《数学的实践与认识》2008,38(8):122-126
为检验周期异方差的季节单位根模型,基于加权最小二乘估计,提出检验统计量,并在原假设下得到检验统计量的极限分布,与各个季节的周期异方差无关,是标准Wiener过程的泛函.模拟结果和实例分析都表明了用该方法检验季节单位根模型的有效性. 相似文献
999.
1000.