全文获取类型
收费全文 | 42315篇 |
免费 | 6931篇 |
国内免费 | 4733篇 |
专业分类
化学 | 29959篇 |
晶体学 | 539篇 |
力学 | 2580篇 |
综合类 | 424篇 |
数学 | 4656篇 |
物理学 | 15821篇 |
出版年
2024年 | 78篇 |
2023年 | 891篇 |
2022年 | 999篇 |
2021年 | 1348篇 |
2020年 | 1723篇 |
2019年 | 1598篇 |
2018年 | 1320篇 |
2017年 | 1274篇 |
2016年 | 1935篇 |
2015年 | 1826篇 |
2014年 | 2225篇 |
2013年 | 2906篇 |
2012年 | 3747篇 |
2011年 | 3839篇 |
2010年 | 2590篇 |
2009年 | 2418篇 |
2008年 | 2743篇 |
2007年 | 2600篇 |
2006年 | 2397篇 |
2005年 | 2016篇 |
2004年 | 1535篇 |
2003年 | 1302篇 |
2002年 | 1148篇 |
2001年 | 900篇 |
2000年 | 889篇 |
1999年 | 1032篇 |
1998年 | 825篇 |
1997年 | 780篇 |
1996年 | 733篇 |
1995年 | 671篇 |
1994年 | 650篇 |
1993年 | 567篇 |
1992年 | 458篇 |
1991年 | 425篇 |
1990年 | 407篇 |
1989年 | 271篇 |
1988年 | 204篇 |
1987年 | 172篇 |
1986年 | 153篇 |
1985年 | 108篇 |
1984年 | 80篇 |
1983年 | 62篇 |
1982年 | 60篇 |
1981年 | 38篇 |
1980年 | 20篇 |
1979年 | 4篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1957年 | 8篇 |
1936年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
81.
82.
Existence and Uniqueness of Endemic States for the Age-structured MSEIR Epidemic Model 总被引:1,自引:0,他引:1
Geni Gupur 《应用数学学报(英文版)》2002,18(3):441-454
The existence and uniqueness of positive steady states for the age-structured MSEIR epidemic model with age-dependent transmission coefficient is considered. Threshold results for the existence of endemic states are established; under certain conditions, uniqueness is also shown. 相似文献
83.
84.
85.
86.
根据Co P非晶态合金结构的短程有序和结构中可能存在P -P相互作用的实验事实 ,选择了单磷原子簇模型ConP(n =1~ 5 )和双磷原子簇模型ConP2 (n =1~ 4 ) ,用密度泛函理论方法对其进行计算 .结果表明 ,在单磷Co2 P( 2 ) 、Co3 P( 1) 及Co4P( 2 ) 模型体系中 ,Co原子供给P原子电子 ,与电负性规则一致 ,同时Co和P之间具较强化学作用 ,可以形成稳定的原子簇 ;而在双磷和单磷原子簇Co5P( 1) 模型体系中 ,形成的原子簇不稳定 ,采用单磷Co2 P( 2 ) 、Co3 P( 1) 及Co4P( 2 ) 模型能较好地反映Co P非晶态合金的结构特点 . 相似文献
87.
It is widely accepted that next-generation networks will provide guaranteed services, in contrast to the “best effort” approach today. We study and analyze queueing policies for network switches that support the QoS (Quality of Service) feature. One realization of the QoS feature is that packets are not necessarily all equal, with some having higher priorities than the others. We model this situation by assigning an intrinsic value to each packet. In this paper we are concerned with three different queueing policies: the nonpreemptive model, the FIFO preemptive model, and the bounded delay model. We concentrate on the situation where the incoming traffic overloads the queue, resulting in packet loss. The objective is to maximize the total value of packets transmitted by the queueing policy. The difficulty lies in the unpredictable nature of the future packet arrivals. We analyze the performance of the online queueing policies via competitive analysis, providing upper and lower bounds for the competitive ratios. We develop practical yet sophisticated online algorithms (queueing policies) for the three queueing models. The algorithms in many cases have provably optimal worst-case bounds. For the nonpreemptive model, we devise an optimal online algorithm for the common 2-value model. We provide a tight logarithmic bound for the general nonpreemptive model. For the FIFO preemptive model, we improve the general lower bound to 1.414, while showing a tight bound of 1.434 for the special case of queue size 2. We prove that the bounded delay model with uniform delay 2 is equivalent to a modified FIFO preemptive model with queue size 2. We then give improved upper and lower bounds on the 2-uniform bounded delay model. We also show an improved lower bound of 1.618 for the 2-variable bounded delay model, matching the previously known upper bound. 相似文献
88.
89.
This paper is devoted to a detailed theoretical study of an ion pair SN2 reaction LiNCO+CH3F in the gas phase and in solution at the level of MP2(full)/6-31+G**//HF/6-31+G**. Two possible reaction mechanisms, inversion and retention, are discussed. There are eight possible reaction pathways. The inversion mechanism is more favorable no matter in the gas phase or in solution based on analyses of the transition structures. Methyl isocyanate should form preferentially in the gas phase and more stable methyl cyanate is the main product in solution. The retardation of the reaction in solvents was attributed to the difference in solvation in the separated reactants and in the transition state. 相似文献
90.