全文获取类型
收费全文 | 117062篇 |
免费 | 8169篇 |
国内免费 | 15463篇 |
专业分类
化学 | 52085篇 |
晶体学 | 2582篇 |
力学 | 9964篇 |
综合类 | 1689篇 |
数学 | 38762篇 |
物理学 | 35612篇 |
出版年
2024年 | 110篇 |
2023年 | 380篇 |
2022年 | 1529篇 |
2021年 | 1430篇 |
2020年 | 1393篇 |
2019年 | 1422篇 |
2018年 | 11772篇 |
2017年 | 12263篇 |
2016年 | 7442篇 |
2015年 | 2939篇 |
2014年 | 2794篇 |
2013年 | 3594篇 |
2012年 | 6908篇 |
2011年 | 13931篇 |
2010年 | 9173篇 |
2009年 | 9678篇 |
2008年 | 10669篇 |
2007年 | 12332篇 |
2006年 | 3854篇 |
2005年 | 4364篇 |
2004年 | 3926篇 |
2003年 | 3600篇 |
2002年 | 2758篇 |
2001年 | 2021篇 |
2000年 | 2284篇 |
1999年 | 1233篇 |
1998年 | 723篇 |
1997年 | 556篇 |
1996年 | 578篇 |
1995年 | 446篇 |
1994年 | 404篇 |
1993年 | 435篇 |
1992年 | 346篇 |
1991年 | 303篇 |
1990年 | 280篇 |
1989年 | 275篇 |
1988年 | 269篇 |
1987年 | 237篇 |
1986年 | 193篇 |
1985年 | 135篇 |
1984年 | 151篇 |
1983年 | 130篇 |
1982年 | 99篇 |
1981年 | 90篇 |
1980年 | 96篇 |
1979年 | 90篇 |
1978年 | 52篇 |
1914年 | 45篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
Data Envelopment Analysis (DEA) offers a piece-wise linear approximation of the production frontier. The approximation tends to be poor if the true frontier is not concave, eg in case of economies of scale or of specialisation. To improve the flexibility of the DEA frontier and to gain in empirical fit, we propose to extend DEA towards a more general piece-wise quadratic approximation, called Quadratic Data Envelopment Analysis (QDEA). We show that QDEA gives statistically consistent estimates for all production frontiers with bounded Hessian eigenvalues. Our Monte-Carlo simulations suggest that QDEA can substantially improve efficiency estimation in finite samples relative to standard DEA models. 相似文献
23.
24.
25.
26.
27.
Graham Smith 《The Journal of the Operational Research Society》1989,40(9):761-770
Courses which teach discrete-event simulation are based on many different simulation languages. The requirements for a language to support teaching simulation are discussed. In particular, it is recommended that such languages separate into distinct modules those aspects of simulation which are taught as separate topics. Implementation of the separation is discussed. The SEESIM language, developed as a teaching aid, is described, and examples of its use are given. Straightforward use of SEESIM can be learned quickly, yet the language provides facilities for a staged introduction to advanced concepts of simulation. 相似文献
28.
LIU Jixue ZHANG Sanguo & CHEN Xiru Department of Statistics Finance University of Science Technology of China Hefei China Department of Mathematics Graduate School of Chinese Academy of Sciences Beijing China 《中国科学A辑(英文版)》2006,49(6):752-769
This paper studies the linear EV model when replicate observations are made only on independent variables. We construct the estimates of regression coefficients and prove the consistency and asymptotic normality under some proper conditions. Results obtained reveal the difference between the case where the independent and dependent variables are observed repeatedly and simultaneously and the case studied in this article. 相似文献
29.
A pressure sensitive paint (PSP) measurement has been known as a pressure field measurement technique based on the oxygen quenching phenomenon of luminescence of specific luminophores. A PSP measurement was applied for pressure field measurement in a low-solidity circular cascade diffuser of a single-stage transonic centrifugal compressor with 5 in pressure ratio for HFC134a gas. The oxygen concentration was about 500 ppm. Ru (bath-phen) was adsorbed on a silica-gel thinlayer chromatography sheet, and the sheet was pasted onto the side-wall between the cascade vanes. A drastic change in luminescent intensity was recognized during a surge condition. Also the pressure variations based on luminescent intensity agreed well with the pressure fluctuations measured using a semiconductor pressure sensor with high-frequency-response. It was shown that a PSP measurement worked well to investigate the unsteady pressure fields in a circular cascade diffuser of a transonic centrifugal compressor. Moreover, the time response of PSP becomes clear as a problem to be overcome for the present. 相似文献
30.
Theoretical and experimental investigations on the performance of micro-perforated -panel absorbers are reviewed in this paper. By reviewing recent research work, this paper reveals a relationship between the maximum absorption coefficient and the limit of the absorption frequency bandwidth. It has been demonstrated that the absorption frequency bandwidth can be extended up to 3 or 4 octaves as the diameters of the micro-holes decrease. This has become possible with the development of the technologies for manufacturing micro-perforated panels, such as laser drilling, powder metallurgy, welded meshing and electro-etching to form micrometer order holes. In this paper, absorption characteristics of such absorbers in random fields and in high sound intensity are discussed both theoretically and experimentally. A new absorbing structure based on micro-perforated-panel absorbers demonstrate experimentally high sound absorption capability. This review shows that the micro-perforated-panel absorber has potentials to be one of ideal absorbing materials in the 21st century. 相似文献