全文获取类型
收费全文 | 17697篇 |
免费 | 3065篇 |
国内免费 | 4511篇 |
专业分类
化学 | 13307篇 |
晶体学 | 519篇 |
力学 | 958篇 |
综合类 | 423篇 |
数学 | 2652篇 |
物理学 | 7414篇 |
出版年
2024年 | 30篇 |
2023年 | 163篇 |
2022年 | 541篇 |
2021年 | 521篇 |
2020年 | 592篇 |
2019年 | 603篇 |
2018年 | 602篇 |
2017年 | 715篇 |
2016年 | 736篇 |
2015年 | 859篇 |
2014年 | 1114篇 |
2013年 | 1412篇 |
2012年 | 1407篇 |
2011年 | 1519篇 |
2010年 | 1335篇 |
2009年 | 1423篇 |
2008年 | 1569篇 |
2007年 | 1328篇 |
2006年 | 1342篇 |
2005年 | 1070篇 |
2004年 | 900篇 |
2003年 | 785篇 |
2002年 | 881篇 |
2001年 | 790篇 |
2000年 | 681篇 |
1999年 | 416篇 |
1998年 | 245篇 |
1997年 | 190篇 |
1996年 | 188篇 |
1995年 | 137篇 |
1994年 | 173篇 |
1993年 | 131篇 |
1992年 | 123篇 |
1991年 | 117篇 |
1990年 | 99篇 |
1989年 | 69篇 |
1988年 | 68篇 |
1987年 | 47篇 |
1986年 | 46篇 |
1985年 | 47篇 |
1984年 | 39篇 |
1983年 | 32篇 |
1982年 | 27篇 |
1981年 | 30篇 |
1980年 | 22篇 |
1979年 | 21篇 |
1978年 | 15篇 |
1976年 | 10篇 |
1975年 | 7篇 |
1971年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
31.
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
32.
ZhongJunMA XianLI YangLU ChengWANG QiTaiZHENG 《中国化学快报》2003,14(6):594-596
A novel cycloartane,named sphaerophysone A,9,19-cycloart-7β,24β,25-triol-1-en-3-one,was isolated from the ethanol extract of Sphaerophysa salsula DC.The structure was elucidated on the basis of spectral evidences and confirmed by X-ray analysis,the stereochemistry of the compound was also defined by X-ray analysis. 相似文献
33.
34.
35.
36.
37.
38.
Ching Sum Hui Lillian Sze Man Lam Chun Yin Wai Kin Chan Aleksandra B. Djurii 《Journal of polymer science. Part A, Polymer chemistry》2003,41(11):1708-1715
Polyurethanes incorporated with rhenium diimine complexes were synthesized. The polymers exhibited interesting morphologies and solution properties. Results from gel permeation chromatography suggested the formation of polymer aggregates in solutions. The polymers could act as probes for metal ions. The addition of metal ions to polymer solutions led to significant changes in the electronic absorption properties of the polymer solutions. This was attributed to the interactions between the polyether moieties and metal ions. The metal complexes could also act as efficient photosensitizers. After doping with charge‐transport viologens, the photoconductivity of the polymers was greatly enhanced. The experimental quantum efficiency was simulated with Onsager's theory. The thermalization distances and the primary yields were typically 12–14 Å and 10?3, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1708–1715, 2003 相似文献
39.
Based on the algebraic entanglement measure proposed [G. Vidal et al., Phys. Rev. A 65 (2002) 032314],we study the entanglement evolution of both pure quantum states and mixed ones of 2-qutrit system in a symmetrybroken environment consisting of a fermionic bath. Entanglement of states will decrease or remain constant under the influence of environment, and the class of states which remain unchanged has been found out. 相似文献
40.
从电子上看康普顿效应 总被引:1,自引:0,他引:1
在康普顿效应中,散射光子与入射光子的频率不同,但在电子静止的参考系,可以证明光子的频率在碰撞前后相同。 相似文献