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   282822篇
  免费   4229篇
  国内免费   2728篇
化学   131582篇
晶体学   4521篇
力学   14861篇
综合类   153篇
数学   45454篇
物理学   93208篇
  2021年   1835篇
  2020年   2030篇
  2019年   2269篇
  2018年   11213篇
  2017年   12093篇
  2016年   7928篇
  2015年   3542篇
  2014年   4197篇
  2013年   8949篇
  2012年   11332篇
  2011年   20431篇
  2010年   13023篇
  2009年   13106篇
  2008年   17928篇
  2007年   21184篇
  2006年   7071篇
  2005年   12733篇
  2004年   8986篇
  2003年   8380篇
  2002年   6342篇
  2001年   5755篇
  2000年   4490篇
  1999年   3332篇
  1998年   2784篇
  1997年   2628篇
  1996年   2619篇
  1995年   2396篇
  1994年   2330篇
  1993年   2238篇
  1992年   2477篇
  1991年   2421篇
  1990年   2351篇
  1989年   2279篇
  1988年   2279篇
  1987年   2250篇
  1986年   2136篇
  1985年   2767篇
  1984年   2815篇
  1983年   2494篇
  1982年   2605篇
  1981年   2406篇
  1980年   2381篇
  1979年   2540篇
  1978年   2578篇
  1977年   2419篇
  1976年   2509篇
  1975年   2414篇
  1974年   2379篇
  1973年   2594篇
  1972年   1665篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
袁斌贤 《数学研究》2002,35(3):241-248
令Ω1与Ω2与C^n中的两个有界齐性域,假设φ:1Ω→Ω2是一个全纯映射。在本中,我们研究相应的复合算子Cφ:β(Ω2)→β(Ω1)的有界性和紧性,特别地,我们讨论取B^n和U^n的情形。  相似文献   
52.
A Dirac picture perturbation theory is developed for the time evolution operator in classical dynamics in the spirit of the Schwinger–Feynman–Dyson perturbation expansion and detailed rules are derived for computations. Complexification formalisms are given for the time evolution operator suitable for phase space analyses, and then extended to a two-dimensional setting for a study of the geometrical Berry phase as an example. Finally a direct integration of Hamilton's equations is shown to lead naturally to a path integral expression, as a resolution of the identity, as applied to arbitrary functions of generalized coordinates and momenta.  相似文献   
53.
Cognitive style refers to differences that can be perceived in individuals due to differing cognitive structures. There has been a large amount of research on how cognitive style affects managerial decision-making, particularly in the context of management information systems, but little or no attempt to relate these findings to operational research. This paper reviews the findings of cognitive-style research from the perspective of operational research, and draws a number of useful inferences concerning the matching of OR aids to managers, the presentation of information and recommendations, the use and acceptance of OR, and the structure of OR groups. Further, it is argued that cognitive-style findings help to give a justification to the move towards ‘soft’ approaches to analysis.  相似文献   
54.
55.
Selected experiences in the control of contamination and the threat it poses to the quality of analytical date are discussed in the context of the whole analytical process from collection of marine environmental samples, through handling and radiochemical separation, to the final interpretation of results. Examples include a demonstration of the contamination introduced during sediment core sectioning, contamination of sea water by a ship's pumping system, and the effect of filtration on the apparent partitioning of radionuclides between solid and liquid phases of sea water.  相似文献   
56.
The objective of this research is to evaluate a new methodology, data envelopment analysis (DEA), as a tool for measuring and evaluating the operational efficiency of US Air Force organizations. This study involves the application of DEA to locate possible inefficiencies in the performance of US Air Force real-property maintenance activities. Dynamic behaviour, as exhibited via window analyses, is reviewed along with static counterparts. The testing was done in close coordination with Air Force officials, who reviewed the results for accuracy, validity and relevance. We conclude that this type of efficiency analysis does have value for the Air Force, where it can serve as a guide to auditors, budget programmers, managers and others in measuring, evaluating and enhancing operational efficiency.  相似文献   
57.
The acoustic field inside a shell excited by a spatially inhomogeneous harmonic pressure field is studied. The shell is assumed to have a finite length, a set of orthogonal stiffening ribs, two ends bounding the acoustic volume, and a sound-insulating structure, which includes layers of sound-insulating material, resonant elements, and an interior panel. The shell is considered to be orthotropic with boundary conditions corresponding to a free support. For the acoustic field in the closed volume, analytical expressions are derived with allowance for the elastoacoustic interaction of the shell with the sound-insulating layers and with the medium both inside the shell (with arbitrary impedance values at the ends) and outside it. These expressions are used to investigate the effect of different types of resonant systems on the sound field inside the shell.  相似文献   
58.
1H-Pyrazoles, triazoles, and imidazoles in reaction with ethoxycarbonylimine and arylsulfonylimines of chloral yield addition products, corresponding 1-(1-amidotrichloroethyl)azoles. Derivatives of 1-alkylpyrazoles and pyrazolones react with chloral 4-chlorophenylsulfonylimine to furnish products of C-amidotrichloroethylation into position 4 of the azole ring.  相似文献   
59.
The effect of hydrogen on the photoluminescence and planar conductivity of GaAs/InGaAs quantum-well heterostructures with an island Pd layer at the anodically oxidized surface was studied. Unlike continuous deposited Pd layers, island layers do not cause the formation of defects in the GaAs surface region and yet the Pd layer maintains high catalytic activity with respect to hydrogen. It is found that the thermal treatment of such a structure in a hydrogen atmosphere causes atomic-hydrogen passivation of the defects in quantum wells. Studies of the characteristics of planar photoresistors with an island Pd layer acting as hydrogen sensors show that their hydrogen detectivity is approximately two orders of magnitude higher than that of diode structures with continuous Pd layers.  相似文献   
60.
Regioselective demethylation of 3,4-dihydropapaveraldine (2a) at 7 and 3' positions affords a properly substituted diphenolic key intermidiate (2d) for the synthesis of reticuline and N-norreticuline.  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