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   244090篇
  免费   3154篇
  国内免费   1475篇
化学   137928篇
晶体学   3414篇
力学   9486篇
综合类   62篇
数学   28569篇
物理学   69260篇
  2020年   1940篇
  2019年   2147篇
  2018年   2686篇
  2017年   2720篇
  2016年   4043篇
  2015年   2729篇
  2014年   3957篇
  2013年   10093篇
  2012年   8727篇
  2011年   10701篇
  2010年   7345篇
  2009年   7011篇
  2008年   9980篇
  2007年   10140篇
  2006年   9611篇
  2005年   8936篇
  2004年   7963篇
  2003年   6959篇
  2002年   6874篇
  2001年   7016篇
  2000年   5326篇
  1999年   3893篇
  1998年   3295篇
  1997年   3291篇
  1996年   3364篇
  1995年   2878篇
  1994年   2967篇
  1993年   2839篇
  1992年   3064篇
  1991年   3105篇
  1990年   2910篇
  1989年   2838篇
  1988年   2799篇
  1987年   2719篇
  1986年   2738篇
  1985年   3648篇
  1984年   3735篇
  1983年   3080篇
  1982年   3419篇
  1981年   3177篇
  1980年   2907篇
  1979年   3090篇
  1978年   3310篇
  1977年   3329篇
  1976年   3312篇
  1975年   3035篇
  1974年   3139篇
  1973年   3175篇
  1972年   2449篇
  1971年   1953篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
61.
A general nonlinear programming problem with interval functions is considered. Two reductions of this problem to the deterministic nonlinear programming problem are proposed, and illustrative examples are discussed.  相似文献   
62.
This paper develops a modified quasi-Newton method for structured unconstrained optimization with partial information on the Hessian, based on a better approximation to the Hessian in current search direction. The new approximation is decided by both function values and gradients at the last two iterations unlike the original one which only uses the gradients at the last two iterations. The modified method owns local and superlinear convergence. Numerical experiments show that the proposed method is encouraging comparing with the methods proposed in [4] for structured unconstrained optimization Presented at the 6th International Conference on Optimization: Techniques and Applications, Ballarat, Australia, December 9–11, 2004  相似文献   
63.
We investigate the conjecture that every circulant graph X admits a k‐isofactorization for every k dividing |E(X)|. We obtain partial results with an emphasis on small values of k. © 2006 Wiley Periodicals, Inc. J Combin Designs 14: 406–414, 2006  相似文献   
64.
Confocal laser scanning microscopy (CLSM) was used to examine molecules related to the cholinergic neurotransmission system and detected at all the larval stages of Paracentrotus lividus, by histochemical and immunohistochemical methods. CLSM, providing spatial resolution of the cells located both at the larval surface and at depth, allows a complete mapping in a three-dimensional volumetric frame. At early larval stages acetylcholinesterase- as well as choline acetyltransferase-like molecules were found mainly in the gut wall cells, and along the ciliary bands of the arms, together with muscarinic acetylcholine receptors. At perimetamorphic stages, cholinergic molecules were present in the ciliate strands along the arms, in the larval body and in the rudiment. At metamorphosis, positivity to cholinergic molecules translocated to the juvenile, where a high frequency of mAChR- and ChAT-like positive cells was found.  相似文献   
65.
The temperature dependence of the expansion ?C of trans conformers of the carbon skeleton of macromolecules in crystallized and amorphous poly(ethylene terephthalate) (PET) due to zero-point and thermal atomic vibrations is investigated by IR spectroscopy. It is found that the thermal expansion coefficient βC jumpwise increases at the characteristic temperatures T t and T b. This increase is associated with the crossover from the quantum to classical statistics of torsional and bending vibration modes. The quantum and classical contributions to the expansion ?C are determined for each mode. The quantum and classical contributions of the torsional vibrational mode in the amorphous polymer are approximately 1.5 times larger than those in the crystallized polymer. This effect is caused by an increase in the anharmonicity of torsional vibrations in the amorphous polymer.  相似文献   
66.
Magnetization and remagnetization processes in a close-packed nanodispersed barium hexaferrite powder sample in the magnetically stable state were analyzed. Reversibility effects were discussed in terms of interparticle interaction. Judging from the magnetization curve and the parameters characterizing remagnetization irreversibility, the sample under study is a model system of small Stoner-Wohlfarth particles.  相似文献   
67.
The equilibrium geometry and hyperfine interaction constants of the nearest and next-to-nearest neighbor atoms are calculated for a negatively charged silicon vacancy in the high-spin state in cubic SiC. The calculations are performed within the cluster approach in terms of the density-functional theory (DFT). It is shown that the results of calculations with the use of a 70-atom cluster are in good agreement with experimental data. A detailed consideration is given to spin polarization in the electron subsystem and the applicability of a simple LCAO model that is commonly used in the interpretation of the electron paramagnetic resonance data for semiconductors. The spin density distribution for the defect under investigation is analyzed in terms of localized orbitals.  相似文献   
68.
We consider Markov processes built from pasting together pieces of strong Markov processes which are killed at a position dependent rate and connected via a transition kernel. We give necessary and sufficient conditions for local absolute continuity of probability laws for such processes on a suitable path space and derive an explicit formula for the corresponding likelihood ratio process. The main tool is the consideration of the process between successive jumps – what we call ‘elementary experiments’ – and criteria for absolute continuity of laws of the process there. We apply our results to systems of branching diffusions with interactions and immigrations. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
69.
70.
We discuss a mass change that has its origin in the action of forces on an object. This phenomenon, well-known in the context of quantum field theory (mass renormalization), can be discussed systematically in both classical and quantum mechanics in a framework given by Stückelberg. We employ this framework to resolve an interesting conflict of opinions between Schopenhauer and Hegel in the mid-19th century. We show that Hegel, Kant, and Schopenhauer demonstrated remarkable prescience in their views as seen from a modern perspective.  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