首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   801篇
  免费   45篇
  国内免费   196篇
化学   116篇
晶体学   2篇
力学   11篇
综合类   7篇
数学   810篇
物理学   96篇
  2024年   2篇
  2023年   11篇
  2022年   5篇
  2021年   8篇
  2020年   21篇
  2019年   15篇
  2018年   17篇
  2017年   15篇
  2016年   34篇
  2015年   19篇
  2014年   21篇
  2013年   84篇
  2012年   17篇
  2011年   33篇
  2010年   30篇
  2009年   50篇
  2008年   76篇
  2007年   60篇
  2006年   74篇
  2005年   54篇
  2004年   58篇
  2003年   49篇
  2002年   49篇
  2001年   51篇
  2000年   35篇
  1999年   29篇
  1998年   35篇
  1997年   17篇
  1996年   24篇
  1995年   6篇
  1994年   2篇
  1993年   5篇
  1992年   7篇
  1991年   8篇
  1990年   2篇
  1989年   3篇
  1987年   2篇
  1986年   3篇
  1985年   5篇
  1984年   1篇
  1983年   2篇
  1982年   1篇
  1978年   1篇
  1976年   1篇
排序方式: 共有1042条查询结果,搜索用时 109 毫秒
11.
有序分组资料的线性趋势检验主要用于检验几个二项总体反应率的线性趋势;在医学与生物学中最典型的用途是研究剂量和反应的关系. 如果用Z检验来代替通常在本检验中所用的卡方检验,就可容易地导出本检验的确切概率计算方法,和其他2×C有序列联表有着相同的形式.本文给出了该检验的确切概率计算方法.  相似文献   
12.
本文把代数结构与分析体系结合起来,运用同调的方法,较系统地确定了A上C^*-模的部分理论,这里A为复数域C上的交换C^*-代数。即不仅定义了与C^*-模有关的某些新概念,而且还得到了有关C^*-模的若干结果。  相似文献   
13.
本文讨论与Hilbert C~*-模相关的Kac-系统在C~*-代数上的作用的性质,给出了两个Morita等价的C~*-代数(在Kac-系统的作用下得到的C~*-代数的余交叉积是Morita等价的)。从而,Baaj和Skandalis等人的结论是本文定理的特殊情况。  相似文献   
14.
张伦传 《数学进展》2002,31(3):275-278
给出了可补Hilbert C^*-模一系列等价刻画,获得了一类有界模映射的分解定理。  相似文献   
15.
We investigate certain classes of normal completely positive (CP) maps on the hyperfinite II1 factorA. Using the representation theory of a suitable irrational rotation algebra, we propose some computable invariants for such CP maps. Dedicated to Professor K B Sinha  相似文献   
16.
Mechanisms of the cycloaddition reaction between singlet difluoromethylene carbene and acetone have been investigated with the second‐order Møller–Plesset (MP2)/6‐31G* method, including geometry optimization and vibrational analysis. Energies for the involved stationary points on the potential energy surface (PES) are corrected by zero‐point energy (ZPE) and CCSD(T)/6‐31G* single‐point calculations. From the PES obtained with the CCSD(T)//MP2/6‐31G* method for the cycloaddition reaction between singlet difluoromethylene carbene and acetone, it can be predicted that path B of reactions 2 and 3 should be two competitive leading channels of the cycloaddition reaction between difluoromethylene carbene and acetone. The former consists of two steps: (i) the two reactants first form a four‐membered ring intermediate, INT2, which is a barrier‐free exothermic reaction of 97.8 kJ/mol; (ii) the intermediate INT2 isomerizes to a four‐membered product P2b via a transition state TS2b with an energy barrier of 24.9 kJ/mol, which results from the methyl group transfer. The latter proceeds in three steps: (i) the two reactants first form an intermediate, INT1c, through a barrier‐free exothermic reaction of 199.4 kJ/mol; (ii) the intermediate INT1c further reacts with acetone to form a polycyclic intermediate, INT3, which is also a barrier‐free exothermic reaction of 27.4 kJ/mol; and (iii) INT3 isomerizes to a polycyclic product P3 via a transition state TS3 with an energy barrier of 25.8 kJ/mol. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007  相似文献   
17.
Tensor products of Calgebras over an abelian Walgebra are studied. The minimal Cnorm on is shown to be just the quotient of the minimal Cnorm on if or is exact.

  相似文献   

18.
In this paper we introduce an algebraic concept of the product of Ockham algebras called the Braided product. We show that ifL i MS(i=1, 2, ,n) then the Braided product ofL i(i=1, 2, ,n) exists if and only ifL 1, ,L n have isomorphic skeletons.  相似文献   
19.
The present article is the first part of a series devoted to extending the Repeat Space Theory (RST) to apply to carbon nanotubes and related molecular networks. Four key problems are formulated whose affirmative solutions imply the formation of the initial investigative bridge between the research field of nanotubes and that of the additivity and other network problems studied and solved by using the RST. All of these four problems are solved affirmatively by using tools from the RST. The Piecewise Monotone Lemmas (PMLs) are cornerstones of the proof of the Fukui conjecture concerning the additivity problems of hydrocarbons. The solution of the fourth problem gives a generalized analytical formula of the pi-electron energy band curves of nanotube (a, b), with two new complex parameters c and d. These two parameters bring forth a broad class of analytic curves to which the PMLs and associated theoretical devices apply. Based on the above affirmative solutions of the problems, a central theorem in the RST, called the asymptotic linearity theorem (ALT) has been applied to nanotubes and monocyclic polyenes. Analytical formulae derived in this application of the ALT illuminate in a new global context (i) the conductivity of nanotubes and (ii) the aromaticity of monocyclic polyenes; moreover an analytical formula obtained by using the ALT provides a fresh insight into Hückel’s (4n+2) rule. The present article forms a foundation of the forthcoming articles in this series. The present series of articles is closely associated with the series of articles entitled ‘Proof of the Fukui conjecture via resolution of singularities and related methods’ published in the JOMC.  相似文献   
20.
Abstract

We previously designed and prepared the first molecules to exhibit observable CD spectra by n-σ* excitation, 2,6-dithiaspiro[3.3]heptane 2,6-dioxide. Spiro[3.3]heptane frameworks possess axial asymmetry due to puckering of 2 four-membered rings; the ring bonds are rich in p-character due to acute bond angles, which lowers the σ* energy levels. In contrast, the lone pairs are rich in s-character, which results in a good donor with conformational fixation. We expected that, instead of lone pairs as donating orbitals, the use of σ-electron-donating Si-Si bonds should result in UV absorption in the observable range (>180?nm), so that the Cotton effect could appear, at least partially, in that range. We designed 2,6-disilyl-2,6-disilaspiro[3.3]heptanes as models, and performed theoretical calculations to confirm our idea.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号