全文获取类型
收费全文 | 1207篇 |
免费 | 26篇 |
国内免费 | 1篇 |
专业分类
化学 | 736篇 |
晶体学 | 8篇 |
力学 | 20篇 |
数学 | 105篇 |
物理学 | 365篇 |
出版年
2020年 | 10篇 |
2019年 | 10篇 |
2018年 | 7篇 |
2016年 | 13篇 |
2015年 | 13篇 |
2014年 | 15篇 |
2013年 | 53篇 |
2012年 | 42篇 |
2011年 | 61篇 |
2010年 | 39篇 |
2009年 | 22篇 |
2008年 | 47篇 |
2007年 | 49篇 |
2006年 | 50篇 |
2005年 | 47篇 |
2004年 | 66篇 |
2003年 | 39篇 |
2002年 | 42篇 |
2001年 | 39篇 |
2000年 | 43篇 |
1999年 | 24篇 |
1998年 | 22篇 |
1997年 | 10篇 |
1996年 | 17篇 |
1995年 | 19篇 |
1994年 | 18篇 |
1993年 | 37篇 |
1992年 | 21篇 |
1991年 | 15篇 |
1990年 | 12篇 |
1989年 | 11篇 |
1988年 | 16篇 |
1987年 | 7篇 |
1986年 | 17篇 |
1985年 | 17篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 14篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 16篇 |
1978年 | 17篇 |
1977年 | 11篇 |
1976年 | 22篇 |
1975年 | 15篇 |
1974年 | 10篇 |
1973年 | 10篇 |
1972年 | 8篇 |
1971年 | 10篇 |
1935年 | 6篇 |
排序方式: 共有1234条查询结果,搜索用时 31 毫秒
1.
2.
3.
4.
5.
The electro-optic response of ferroelectric smectic C* liquid crystals has been studied. Anomalous switching behaviour of such materials which possess a negative dielectric anisotropy has been reported. These materials show a minimum in response time at a sufficiently high field. We present results showing the dependency of this minimum upon spontaneous polarisation and the effect of AC bias. Calculations based upon the equation of motion of the director around the cone are presented which describe this effect and its dependence on the relative magnitudes of the spontaneous polarization and dielectric anisotropy of the material. Good agreement with the experimental results is found. 相似文献
6.
In a previous paper we gave a new formulation and derived the Euler equations and other necessary conditions to solve strong, pathwise, stochastic variational problems with trajectories driven by Brownian motion. Thus, unlike current methods which minimize the control over deterministic functionals (the expected value), we find the control which gives the critical point solution of random functionals of a Brownian path and then, if we choose, find the expected value.This increase in information is balanced by the fact that our methods are anticipative while current methods are not. However, our methods are more directly connected to the theory and meaningful examples of deterministic variational theory and provide better means of solution for free and constrained problems. In addition, examples indicate that there are methods to obtain nonanticipative solutions from our equations although the anticipative optimal cost function has smaller expected value.In this paper we give new, efficient numerical methods to find the solution of these problems in the quadratic case. Of interest is that our numerical solution has a maximal, a priori, pointwise error of O(h3/2) where h is the node size. We believe our results are unique for any theory of stochastic control and that our methods of proof involve new and sophisticated ideas for strong solutions which extend previous deterministic results by the first author where the error was O(h2).We note that, although our solutions are given in terms of stochastic differential equations, we are not using the now standard numerical methods for stochastic differential equations. Instead we find an approximation to the critical point solution of the variational problem using relations derived from setting to zero the directional derivative of the cost functional in the direction of simple test functions.Our results are even more significant than they first appear because we can reformulate stochastic control problems or constrained calculus of variations problems in the unconstrained, stochastic calculus of variations formulation of this paper. This will allow us to find efficient and accurate numerical solutions for general constrained, stochastic optimization problems. This is not yet being done, even in the deterministic case, except by the first author. 相似文献
7.
A new set of matrix addressing schemes for ferroelectric liquid crystal displays is reported. The schemes use the minimum in the response time-voltage characteristic found in certain mixtures and deliver improved operating speed and contrast ratio compared with previously reported schemes operating in this mode. 相似文献
8.
9.
Appleby I Boulton LT Cobley CJ Hill C Hughes ML de Koning PD Lennon IC Praquin C Ramsden JA Samuel HJ Willis N 《Organic letters》2005,7(10):1931-1934
Two methods to produce (2S)-5-amino-2-(1-n-propyl-1H-imidazol-4-ylmethyl)-pentanoic acid were investigated. Diastereoisomeric salt resolution, using the quinidine salt, gave the desired intermediate in 98% ee and 33% yield. Asymmetric hydrogenation of various substrates gave high conversions, with up to 83% ee. Integration of these two approaches via asymmetric hydrogenation of a quinidine salt substrate followed by crystallization provided the desired intermediate in 94% ee and 76% yield. 相似文献
10.
Chemometric techniques have been applied to FTIR and DSC data to correlate polymer composition. Since structural differences
in the polymers with only hydrocarbon structure, often cause subtle changes in spectra, the ability of chemometric techniques
is required to discern these differences. FTIR spectra and thermal fractionation using DSC were measured for 28 types of polyethylenes
(PE) varying in chain branching type, content and distribution. Unsupervised clustering methods such as principal component
analysis (PCA) and supervised discriminant analysis were used to classify the PEs according to their structural class. The
DSC data was the more successful in both classifying PEs according to their class.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献