全文获取类型
收费全文 | 37608篇 |
免费 | 6263篇 |
国内免费 | 4680篇 |
专业分类
化学 | 26839篇 |
晶体学 | 448篇 |
力学 | 2235篇 |
综合类 | 380篇 |
数学 | 4539篇 |
物理学 | 14110篇 |
出版年
2024年 | 60篇 |
2023年 | 760篇 |
2022年 | 851篇 |
2021年 | 1215篇 |
2020年 | 1557篇 |
2019年 | 1544篇 |
2018年 | 1237篇 |
2017年 | 1167篇 |
2016年 | 1747篇 |
2015年 | 1711篇 |
2014年 | 2092篇 |
2013年 | 2686篇 |
2012年 | 3326篇 |
2011年 | 3405篇 |
2010年 | 2366篇 |
2009年 | 2341篇 |
2008年 | 2527篇 |
2007年 | 2232篇 |
2006年 | 2120篇 |
2005年 | 1714篇 |
2004年 | 1409篇 |
2003年 | 1143篇 |
2002年 | 1117篇 |
2001年 | 895篇 |
2000年 | 829篇 |
1999年 | 909篇 |
1998年 | 742篇 |
1997年 | 644篇 |
1996年 | 637篇 |
1995年 | 548篇 |
1994年 | 495篇 |
1993年 | 470篇 |
1992年 | 362篇 |
1991年 | 322篇 |
1990年 | 312篇 |
1989年 | 233篇 |
1988年 | 187篇 |
1987年 | 120篇 |
1986年 | 132篇 |
1985年 | 111篇 |
1984年 | 64篇 |
1983年 | 57篇 |
1982年 | 61篇 |
1981年 | 31篇 |
1980年 | 11篇 |
1979年 | 11篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1972年 | 4篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
111.
112.
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
113.
114.
115.
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
116.
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
117.
118.
Xuding Zhu 《Journal of Graph Theory》2005,48(3):186-209
For 1 ≤ d ≤ k, let Kk/d be the graph with vertices 0, 1, …, k ? 1, in which i ~j if d ≤ |i ? j| ≤ k ? d. The circular chromatic number χc(G) of a graph G is the minimum of those k/d for which G admits a homomorphism to Kk/d. The circular clique number ωc(G) of G is the maximum of those k/d for which Kk/d admits a homomorphism to G. A graph G is circular perfect if for every induced subgraph H of G, we have χc(H) = ωc(H). In this paper, we prove that if G is circular perfect then for every vertex x of G, NG[x] is a perfect graph. Conversely, we prove that if for every vertex x of G, NG[x] is a perfect graph and G ? N[x] is a bipartite graph with no induced P5 (the path with five vertices), then G is a circular perfect graph. In a companion paper, we apply the main result of this paper to prove an analog of Haj?os theorem for circular chromatic number for k/d ≥ 3. Namely, we shall design a few graph operations and prove that for any k/d ≥ 3, starting from the graph Kk/d, one can construct all graphs of circular chromatic number at least k/d by repeatedly applying these graph operations. © 2005 Wiley Periodicals, Inc. J Graph Theory 48: 186–209, 2005 相似文献
119.
Ching‐Yuan Cheng Kuan‐Jiuh Lin 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(8):m363-m365
In the title compound, catena‐poly[lithium‐μ3‐ethylenediphosphonato], [Li(C2H7O6P2)]n, the supramolecular monoclinic (C2/c) structure consists of one‐dimensional lithium chains [Li⋯Li = 2.7036 (8) Å] that are embedded within ethylenediphosphonate anions linked by strong symmetric hydrogen bonds [O⋯O = 2.473 (3) Å]. The Li atoms and the H atom in the symmetric hydrogen bond reside on twofold rotation axes and there is an inversion center at the mid‐point of the C—C bond of the ethylenediphosphonate ligand. 相似文献
120.
物理模型教学与创新能力的培养 总被引:1,自引:0,他引:1
现代教学以能力培养为重点.学生获得研究问题、探索问题的能力是十分重要的,物理模型的教学过程对培养学生研究问题、探索问题、发展创造思维、培养辩证唯物主义思想方法都极为有利,因此,必须重视物理模型教学. 相似文献