全文获取类型
收费全文 | 355189篇 |
免费 | 2277篇 |
国内免费 | 879篇 |
专业分类
化学 | 158385篇 |
晶体学 | 5430篇 |
力学 | 19710篇 |
综合类 | 11篇 |
数学 | 69012篇 |
物理学 | 105797篇 |
出版年
2020年 | 1725篇 |
2019年 | 1919篇 |
2018年 | 18723篇 |
2017年 | 19327篇 |
2016年 | 12048篇 |
2015年 | 3683篇 |
2014年 | 3930篇 |
2013年 | 9467篇 |
2012年 | 13343篇 |
2011年 | 27618篇 |
2010年 | 16709篇 |
2009年 | 17161篇 |
2008年 | 22134篇 |
2007年 | 27061篇 |
2006年 | 7071篇 |
2005年 | 13687篇 |
2004年 | 10048篇 |
2003年 | 9946篇 |
2002年 | 7061篇 |
2001年 | 6498篇 |
2000年 | 5171篇 |
1999年 | 3688篇 |
1998年 | 2958篇 |
1997年 | 2845篇 |
1996年 | 2882篇 |
1995年 | 2622篇 |
1994年 | 2472篇 |
1993年 | 2310篇 |
1992年 | 2932篇 |
1991年 | 2843篇 |
1990年 | 2778篇 |
1989年 | 2810篇 |
1988年 | 2843篇 |
1987年 | 2844篇 |
1986年 | 2708篇 |
1985年 | 3423篇 |
1984年 | 3441篇 |
1983年 | 2727篇 |
1982年 | 2828篇 |
1981年 | 2880篇 |
1980年 | 2657篇 |
1979年 | 3005篇 |
1978年 | 3001篇 |
1977年 | 3026篇 |
1976年 | 2887篇 |
1975年 | 2616篇 |
1974年 | 2565篇 |
1973年 | 2565篇 |
1972年 | 1755篇 |
1968年 | 1808篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
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
22.
K. A. Bronnikov 《Annalen der Physik》1991,503(8):527-534
A multidimensional generalization of the Reissner-Nordström solution of general relativity is obtained for the case of n Ricci-flat internal spaces. A two-parameter family of black-hole solutions for an arbitrary dimensionality D is selected. Nontrivial black holes with D > 4 are shown to exist only with a nonzero electric charge. Observational consequences are discussed, in particular, a violation of Coulomb's law. 相似文献
23.
24.
Yu. K. Voron'ko A. B. Kudryavtsev V. V. Osiko E. V. Sorokin 《Journal of Applied Spectroscopy》1991,55(4):953-957
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 55, No. 4, pp. 535–540, October, 1991. 相似文献
25.
26.
27.
The quantitative rainbow schlieren deflectometry (RSD) technique was applied to measure temperature and oxygen concentration in an axisymmetric hydrogen gas-jet diffusion flame burning in quiescent air at fuel jet exit Reynolds number of 70. Schlieren measurements were compared with conventional measurements using a thermocouple and a gas-sampling probe. Good agreement between the two measurement techniques was achieved on the fuel-lean side of the flame. 相似文献
28.
David K Bisset 《国际流体数值方法杂志》2002,39(10):961-977
Turbulent flow simulation methods based on finite differences are attractive for their simplicity, flexibility and efficiency, but not always for accuracy or stability. This paper demonstrates that a good compromise is possible with the advected grid explicit (AGE) method. Starting from the same initial field as a previous spectral DNS, AGE method simulations of a planar turbulent wake were carried out as DNS, and then at three levels of reduced resolution. The latter cases were in a sense large‐eddy simulations (LES), although no specific sub‐grid‐scale model was used. Results for the two DNS methods, including variances and power spectra, were very similar, but the AGE simulation required much less computational effort. Small‐scale information was lost in the reduced resolution runs, but large‐scale mean and instantaneous properties were reproduced quite well, with further large reductions in computational effort. Quality of results becomes more sensitive to the value chosen for one of the AGE method parameters as resolution is reduced, from which it is inferred that the numerical stability procedure controlled by the parameter is acting in part as a sub‐grid‐scale model. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
29.
30.
A firm's logistics cost, including shipping and inventory-carrying, is a substantial percentage of its sales. Nevertheless, typical inventory-control methods ignore or insufficiently represent the shipping cost. This paper describes a recursive algorithm that determines the reorder cycle-time that minimizes total logistics cost. It allows for a realistic accounting of shipping cost, which is modelled here as a function of shipping distance and weight. The algorithm uses a relaxation procedure to identify a suitable initial approximation to the optimal order cycle-time and then, through a series of recursive steps, moves to the optimal result. We demonstrate the algorithm with a single item, with a group of items that share a common order cycle, and with multi-items when item demands are random variables. Experience with this algorithm indicates that it converges to the optimal result in a very few steps. 相似文献