全文获取类型
收费全文 | 7786篇 |
免费 | 712篇 |
国内免费 | 923篇 |
专业分类
化学 | 1666篇 |
晶体学 | 29篇 |
力学 | 337篇 |
综合类 | 177篇 |
数学 | 5964篇 |
物理学 | 1248篇 |
出版年
2024年 | 12篇 |
2023年 | 71篇 |
2022年 | 160篇 |
2021年 | 177篇 |
2020年 | 204篇 |
2019年 | 233篇 |
2018年 | 235篇 |
2017年 | 232篇 |
2016年 | 299篇 |
2015年 | 203篇 |
2014年 | 385篇 |
2013年 | 614篇 |
2012年 | 336篇 |
2011年 | 466篇 |
2010年 | 405篇 |
2009年 | 485篇 |
2008年 | 574篇 |
2007年 | 498篇 |
2006年 | 496篇 |
2005年 | 388篇 |
2004年 | 353篇 |
2003年 | 378篇 |
2002年 | 363篇 |
2001年 | 246篇 |
2000年 | 272篇 |
1999年 | 228篇 |
1998年 | 199篇 |
1997年 | 189篇 |
1996年 | 146篇 |
1995年 | 99篇 |
1994年 | 82篇 |
1993年 | 63篇 |
1992年 | 45篇 |
1991年 | 37篇 |
1990年 | 36篇 |
1989年 | 22篇 |
1988年 | 30篇 |
1987年 | 11篇 |
1986年 | 16篇 |
1985年 | 15篇 |
1984年 | 23篇 |
1983年 | 8篇 |
1982年 | 15篇 |
1981年 | 19篇 |
1980年 | 15篇 |
1979年 | 5篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1936年 | 5篇 |
排序方式: 共有9421条查询结果,搜索用时 46 毫秒
1.
The growth-fragmentation equation describes a system of growing and dividing particles, and arises in models of cell division, protein polymerisation and even telecommunications protocols. Several important questions about the equation concern the asymptotic behaviour of solutions at large times: at what rate do they converge to zero or infinity, and what does the asymptotic profile of the solutions look like? Does the rescaled solution converge to its asymptotic profile at an exponential speed? These questions have traditionally been studied using analytic techniques such as entropy methods or splitting of operators. In this work, we present a probabilistic approach: we use a Feynman–Kac formula to relate the solution of the growth-fragmentation equation to the semigroup of a Markov process, and characterise the rate of decay or growth in terms of this process. We then identify the Malthus exponent and the asymptotic profile in terms of a related Markov process, and give a spectral interpretation in terms of the growth-fragmentation operator and its dual. 相似文献
2.
The satisfiability (SAT) problem is a core problem in computer science. Existing studies have shown that most industrial SAT instances can be effectively solved by modern SAT solvers while random SAT instances cannot. It is believed that the structural characteristics of different SAT formula classes are the reasons behind this difference. In this paper, we study the structural properties of propositional formulas in conjunctive normal form (CNF) by the principle of structural entropy of formulas. First, we used structural entropy to measure the complex structure of a formula and found that the difficulty solving the formula is related to the structural entropy of the formula. The smaller the compressing information of a formula, the more difficult it is to solve the formula. Secondly, we proposed a -approximation strategy to approximate the structural entropy of large formulas. The experimental results showed that the proposed strategy can effectively approximate the structural entropy of the original formula and that the approximation ratio is more than 92%. Finally, we analyzed the structural properties of a formula in the solution process and found that a local search solver tends to select variables in different communities to perform the next round of searches during a search and that the structural entropy of a variable affects the probability of the variable being flipped. By using these conclusions, we also proposed an initial candidate solution generation strategy for a local search for SAT, and the experimental results showed that this strategy effectively improves the performance of the solvers CCAsat and Sparrow2011 when incorporated into these two solvers. 相似文献
3.
4.
5.
Bijian Lan Chunming Liu Xiang Yin Hua Zhang Wei Xu Zhongyi Hua 《Frontiers of Chemistry in China》2006,1(3):296-299
Organic materials of D-π-A type MR-X (MR-1: p-dimethylaminophenylethenetrica-rbonitrile and MR-2: p-diphenylaminophenylethene tricarbonitrile) were designed and synthesized. The device with a sandwich structure shows good
rectificative phenomena. The highest rectification ratio 10000 was achieved in device Cu/MR-1/Ag, and about 100 in other device
M/MR-X/M (M: Cu, Ag). It has been found that rectificative phenomena exist only in the atmosphere-liquid interface region
by means of liquid adsorption, and electric field could help form the oriented molecular film.
__________
Translated from Journal of Fudan University (Natural Science), 2005, 44(4) (in Chinese) 相似文献
6.
7.
在1100℃制备了钙钛矿型非计量系列物Sr_(1-x)Bi_xFeO_(3-y),与文献相比,明显具有温度低、时间短、能耗少的特点。用化学分析方法测得其化学式及Fe离子的平均价态,用XRD、交流阻抗、IR、Mossbauer谱等方法研究了其结构与性质,该类化合物具有Pm3m对称性和半导体性质,铁离子会产生电荷歧化现象。 相似文献
8.
Mark L. Agranovsky E. K. Narayanan 《Proceedings of the American Mathematical Society》2006,134(7):2117-2123
A test for a function to be a solution of an elliptic PDE is given in terms of extensions, as solutions, from the boundaries inside the domains belonging to an isotopic family. It generalizes a result of Ehrenpreis for spheres moved along a straight line.
9.
浅析二阶齐次线性变系数微分方程的一个可积类型 总被引:1,自引:0,他引:1
姬志飞 《应用数学与计算数学学报》2006,20(1):125-128
本文讨论了二阶齐次线性变系数微分方程的特殊形式,给出了这种微分方程的一个可积类型. 相似文献
10.
本文讨论中立型大系统,获得了这类问题稳定性的一些充分判据,作为特殊情况,还得到了与[1-11]中相应的一些不同的结果.这些结果并不象文献[1]和[2]那样,要求a_(ii)的上界q_(ii)小于零. 相似文献