全文获取类型
收费全文 | 703篇 |
免费 | 6篇 |
国内免费 | 18篇 |
专业分类
化学 | 199篇 |
力学 | 57篇 |
数学 | 248篇 |
物理学 | 223篇 |
出版年
2020年 | 6篇 |
2016年 | 12篇 |
2015年 | 7篇 |
2013年 | 47篇 |
2012年 | 16篇 |
2011年 | 29篇 |
2010年 | 6篇 |
2009年 | 15篇 |
2008年 | 23篇 |
2007年 | 26篇 |
2006年 | 20篇 |
2005年 | 20篇 |
2004年 | 17篇 |
2003年 | 6篇 |
2002年 | 15篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 11篇 |
1998年 | 9篇 |
1997年 | 13篇 |
1996年 | 12篇 |
1995年 | 16篇 |
1994年 | 9篇 |
1993年 | 22篇 |
1992年 | 10篇 |
1991年 | 8篇 |
1990年 | 12篇 |
1989年 | 13篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 11篇 |
1981年 | 17篇 |
1980年 | 15篇 |
1979年 | 15篇 |
1978年 | 18篇 |
1977年 | 14篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1974年 | 12篇 |
1973年 | 19篇 |
1972年 | 9篇 |
1971年 | 5篇 |
1969年 | 6篇 |
1967年 | 6篇 |
1966年 | 6篇 |
1965年 | 5篇 |
排序方式: 共有727条查询结果,搜索用时 62 毫秒
1.
Nathaniel A. Friedman 《Israel Journal of Mathematics》1989,68(3):365-375
A weakly mixing transformationT and a sequence (d
n) are constructed such thatT is uniformly mixing on (d
n),T is uniformly sweeping out on ([αd
n]) for allα∈(0, 1), and for all rationalα∈(0, 1)T is not mixing on ([αd
n]). 相似文献
2.
This paper compares the attitudes about mathematics of students from traditionally taught calculus classes and those from a “reformed” calculus course. The paper is based on three studies, which together present a consistent picture of student attitudes about calculus reform. The reformed course appeared to violate students' deeply held beliefs about the nature of mathematics and how it should be learned. Although during their first months in the reformed course most students disliked it, their attitudes gradually changed. One and 2 years after, reform students felt significantly more than the traditionally taught students that they better understood how math was used and that they had been required to understand math rather than memorize formulas. 相似文献
3.
David P. Dobkin Steven J. Friedman Kenneth J. Supowit 《Discrete and Computational Geometry》1990,5(1):399-407
LetS be any set ofN points in the plane and let DT(S) be the graph of the Delaunay triangulation ofS. For all pointsa andb ofS, letd(a, b) be the Euclidean distance froma tob and let DT(a, b) be the length of the shortest path in DT(S) froma tob. We show that there is a constantc (≤((1+√5)/2) π≈5.08) independent ofS andN such that $$\frac{{DT(a,b)}}{{d(a,b)}}< c.$$ 相似文献
4.
5.
Although hyperconjugation involving alkyl groups to a carbocation is a well-established concept, the analogous charge delocalization generated in anions by electronegative substitution has long been a subject of controversy. We have investigated this phenomenon for the β-hydroxyethyl and β-trifluoroethyl anions using ab initio electronic structure calculations. Split valence basis sets augmented by diffuse functions were used with fully optimized geometries. Three dimensional molecular orbital plots clearly show an in-plane HOMO-LUMO mixing of a C-X σ antibonding orbital with the carbon lone pair which yields hyperconjugative π bonding in the anti conformation. Deformation density maps further demonstrate that this delocalization leads to the development of a π component as well as a previously unrecognized sigma enhancement. These results support and extend the work of Apeloig and that of Schleyer and Kos, but are in opposition to the induction hypothesis of Streitwieser and Holtz. The recent experimental determination of the acidity of (CF3)3CH by Tatlow and their interpretation in terms of fluorine hyperconjugation are also in accord with this work. 相似文献
6.
The dam problem with general geometry is considered. Fluid is drawn from the bottomS
1 at a ratek where 0 k N,
S
1
k M; the objective is to minimize the total pressure of the fluid in the dam. A bang-bang principle is established for any optimal controlk
0, that is,k
0 = 0 on a setA andk
0 =N on the complement setS
1
A. In the case of a rectangular dam the structure ofA is determined and the uniqueness of the minimizerk
0 is established.This work is partially supported by National Science Foundation Grants DMS-8501397 and DMS-8420896. 相似文献
7.
We report simultaneous observation of coherent resonance Raman scattering and incoherently broadened resonance fluorescence following excitation of cytochrome c in the α(0, 0) region. The influence of incoherent broadening processes on resonance Raman scattering experiments is considered. 相似文献
8.
Chen Wang Amit Fischer Avner Ehrlich Yaakov Nahmias Itamar Willner 《Chemical science》2020,11(17):4516
The enzymes glucose oxidase (GOx), acetylcholine esterase (AchE) and urease that drive biocatalytic transformations to alter pH, are integrated into pH-responsive DNA-based hydrogels. A two-enzyme-loaded hydrogel composed of GOx/urease or AchE/urease and a three-enzyme-loaded hydrogel composed of GOx/AchE/urease are presented. The biocatalytic transformations within the hydrogels lead to the dictated reconfiguration of nucleic acid bridges and the switchable control over the stiffness of the respective hydrogels. The switchable stiffness features are used to develop biocatalytically guided shape-memory and self-healing matrices. In addition, loading of GOx/insulin in a pH-responsive DNA-based hydrogel yields a glucose-triggered matrix for the controlled release of insulin, acting as an artificial pancreas. The release of insulin is controlled by the concentrations of glucose, hence, the biocatalytic insulin-loaded hydrogel provides an interesting sense-and-treat carrier for controlling diabetes.Biocatalytic control over the stiffness of pH-responsive hydrogels is applied to develop shape-memory, self-healing and controlled release matrices. 相似文献
9.
L. Margulies N. Friedman M. Sheves Y. Mazur M.E. Lippitsch M. Riegler F.R. Aussenegg 《Tetrahedron》1985,41(1):191-195
Linear dichroism spectra of several retinoids and related polyenes incorporated in stretched polyethylene films were determined. It is suggested that the retinoids are oriented with the plane of the ring parallel to the stretching direction of the film, the long polyene chain being displaced from that direction. 相似文献
10.
Michael Stavola J.M. Friedman R.A. Stepnoski Mark G. Sceats 《Chemical physics letters》1981,80(1):192-194
Cooperative vibronic spectra involving Gd3+ electronic transitions and the vibrational transitions of nearby water molecules are used to determine the stretching spectrum for isotopically dilute OH in a solution of GdCl3 in D2O. The OH stretching spectrum of water molecules in the first hydration sphere is shifted to lower energy than that of the bulk liquid. 相似文献