全文获取类型
收费全文 | 284859篇 |
免费 | 4145篇 |
国内免费 | 907篇 |
专业分类
化学 | 156113篇 |
晶体学 | 3907篇 |
力学 | 10600篇 |
综合类 | 3篇 |
数学 | 35509篇 |
物理学 | 83779篇 |
出版年
2021年 | 2048篇 |
2020年 | 2411篇 |
2019年 | 2647篇 |
2018年 | 3331篇 |
2017年 | 3198篇 |
2016年 | 5359篇 |
2015年 | 3873篇 |
2014年 | 5141篇 |
2013年 | 13937篇 |
2012年 | 10701篇 |
2011年 | 12437篇 |
2010年 | 8227篇 |
2009年 | 7890篇 |
2008年 | 11096篇 |
2007年 | 11088篇 |
2006年 | 10107篇 |
2005年 | 9637篇 |
2004年 | 8500篇 |
2003年 | 7475篇 |
2002年 | 7247篇 |
2001年 | 7963篇 |
2000年 | 6170篇 |
1999年 | 4806篇 |
1998年 | 3995篇 |
1997年 | 3965篇 |
1996年 | 3834篇 |
1995年 | 3511篇 |
1994年 | 3410篇 |
1993年 | 3406篇 |
1992年 | 3731篇 |
1991年 | 3698篇 |
1990年 | 3569篇 |
1989年 | 3436篇 |
1988年 | 3510篇 |
1987年 | 3332篇 |
1986年 | 3219篇 |
1985年 | 4392篇 |
1984年 | 4529篇 |
1983年 | 3704篇 |
1982年 | 4057篇 |
1981年 | 3969篇 |
1980年 | 3777篇 |
1979年 | 3806篇 |
1978年 | 4022篇 |
1977年 | 3847篇 |
1976年 | 4071篇 |
1975年 | 3609篇 |
1974年 | 3750篇 |
1973年 | 4064篇 |
1972年 | 2462篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
161.
In developing travel demand models it is generally assumed that the base-year data used in developing the parameters, as well as the forecasted data to be used as independent variables for the design year, are of acceptable quality. The purpose of this paper is to present the application of error propagation theory in assesing the predictive quality of one type of travel demand forecasting model (multinomial logit models) and to demonstrate how error considerations can be used as a tool for identifying the optimal model. The general conclusions of this study are that: (1) it is indeed possible to quantify errors in dependent variables in logit models as a consequence of errors in independent variables; and (2) error consideration can be used as a tool for identifying the optimal model from a set of candidate models. Further research is recommended to develop better insights into the phenomenon of error propagation so that the consideration of errors can be a factor in decisions on model selection. 相似文献
162.
Michael R. Caputo 《Natural Resource Modeling》1989,3(2):241-259
This paper analyzes the extent to which standard dynamic renewable resource models possess refutable implications. Both the steady state comparative static and local comparative dynamic properties of the standard model are studied. A unified framework is developed which enables one to analyze the qualitative properties of any standard renewable resource model. This is achieved by explicitly linking the local stability, steady state comparative static, and local comparative dynamic properties of the model. 相似文献
163.
R. Kaufman 《Israel Journal of Mathematics》1989,67(1):117-122
A closed setE is constructed so thatA∼(E) is an inseparable Banach space but its maximal ideal space isE.
Supported in part by the National Science Foundation. 相似文献
164.
M. R. Booty 《Studies in Applied Mathematics》1989,80(3):203-228
A model equation describing the configuration of a simple plasma maintained by external radiation is studied. A branch of steady solutions of the equation was found by Eckhaus et al. to terminate at a finite critical value of the power of the external source, and this is attributed to the discontinuous nature of a nonlinear term in the governing equation. On introducing a small parameter to render the term continuous, a second branch of solutions is constructed in a neighborhood of the termination point of the original branch. This suggests that the termination point is formed as the limit of a subcritical fold in the surface of the steady solution branch. 相似文献
165.
166.
P. David J. Hartfiel H. Janszen R. von Mutius J. Arvieux L. Farvacque B. Berthier B. Bonin J. C. Lugol 《Zeitschrift für Physik A Hadrons and Nuclei》1987,326(4):367-372
The excitation function of the fission probability P E E x) for238U has been measured in the reaction238U(α, α′ f) at 480 MeV bombarding energy. The reaction mechanism of this reaction is discussed for excitation energies belowB nf , the threshold for second chance fission, and aboveB nf up toE x =37 MeV. In comparing with results from fission induced by photons and by particle transfer reactions the (α, α′f) reaction gives too low values for the fission probabilityP f at excitation energies well aboveB nE . The role of the quasi-elastic knock-out process in this reaction is discussed. 相似文献
167.
168.
The tape stripping technique is an experimental method frequently used for reconstruction of the in-depth distribution of various topically administered substances within the horny layer of human skin, e.g., compounds contained in sunscreens. Titanium dioxide (TiO2) nanoparticles (25–200 nm in diameter) are one such compound. Optical techniques which apply blue light are found to be suitable for reconstruction. However, the presence of particles affects the light propagation within the skin and therefore causes incorrect determination of strip thickness, leading to an improper reconstructed distribution of nanoparticle concentration revealed from the experimental data. This study evaluates the errors emerging from the use of blue (400 nm) and NIR (800 nm) radiation and finds the use of longer wavelength light more advantageous. Particles of different diameters are considered, and it is revealed that the application of small particles (25–60 nm) results in the lowest rate of error. 相似文献
169.
Addition of aqueous hydrogen peroxide significantly accelerates the substitution reactions of hydroxide salts with haloarenes bearing electron withdrawing substituents. A similar effect is observed in the reactions of hydroxide salts with halogenated heteroarenes. Reactions are carried out in water or water-THF at ambient temperature or at 50-60 °C. 相似文献
170.
We study the asymptotic properties of the Bayes estimator in models differentiable in quadratic mean (DQM) in the case of
independent and identically distributed observations. The goal is to define weak assumptions on the model under which this
estimator is asymptotically efficient, regular, and asymptotically of minimal risk. The results of the paper are applied to
models based on a mixture distribution, the Cauchy distribution with location and scale parameter, and the Weibull distribution.
Bibliography: 10 titles.
Published in Zapiski Nauchnykh Seminarov POMI, Vol. 328, 2005, pp. 114–146. 相似文献