全文获取类型
收费全文 | 343421篇 |
免费 | 2731篇 |
国内免费 | 1005篇 |
专业分类
化学 | 165630篇 |
晶体学 | 4909篇 |
力学 | 18491篇 |
综合类 | 9篇 |
数学 | 60673篇 |
物理学 | 97445篇 |
出版年
2019年 | 2307篇 |
2018年 | 13252篇 |
2017年 | 13095篇 |
2016年 | 10370篇 |
2015年 | 3492篇 |
2014年 | 4574篇 |
2013年 | 11017篇 |
2012年 | 11693篇 |
2011年 | 19958篇 |
2010年 | 12543篇 |
2009年 | 12734篇 |
2008年 | 15683篇 |
2007年 | 17922篇 |
2006年 | 9138篇 |
2005年 | 9310篇 |
2004年 | 8925篇 |
2003年 | 8662篇 |
2002年 | 7624篇 |
2001年 | 7561篇 |
2000年 | 5941篇 |
1999年 | 4514篇 |
1998年 | 4055篇 |
1997年 | 3987篇 |
1996年 | 3808篇 |
1995年 | 3464篇 |
1994年 | 3581篇 |
1993年 | 3306篇 |
1992年 | 3732篇 |
1991年 | 3743篇 |
1990年 | 3657篇 |
1989年 | 3530篇 |
1988年 | 3640篇 |
1987年 | 3535篇 |
1986年 | 3350篇 |
1985年 | 4427篇 |
1984年 | 4700篇 |
1983年 | 3922篇 |
1982年 | 4069篇 |
1981年 | 3982篇 |
1980年 | 3965篇 |
1979年 | 3939篇 |
1978年 | 4244篇 |
1977年 | 4142篇 |
1976年 | 4199篇 |
1975年 | 3881篇 |
1974年 | 3927篇 |
1973年 | 4088篇 |
1972年 | 2871篇 |
1971年 | 2391篇 |
1970年 | 2188篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
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
202.
Zygmunt Bak 《Czechoslovak Journal of Physics》2002,52(1):A25-A28
We consider RKKY interaction in a quasi 2D system with nonparabolic dispersion. In our paper we calculate the RKKY range function assuming the in-layer confinement via effective dimensionality approach. We show, that indirect magnetic exchange in our system can be modelled by the effective spectral dimension which equals one. 相似文献
203.
Recursive algorithms for the computation of standard deviation and average deviation are derived and their applications in data acquisition are discussed. The relative speeds and accuracies of the two algorithms are compared for synthetic data. The performance of recursive estimation under shot and proportional noise limitations is also described. As an example of the utility of these algorithms, absorbance data with constant confidence intervals are collected regardless of incident and transmitted intensities. The desired precision is specified prior to data acquisition and used to control signal-averaging of the data in real time. 相似文献
204.
M E Lutman S Gatehouse A G Worthington 《The Journal of the Acoustical Society of America》1991,89(1):320-328
Frequency resolution ability was measured using a psychoacoustical tuning curve (PTC) or a notch-noise technique in two population samples. The first sample incorporated 1764 subjects with various degrees of sensorineural hearing impairment and ranging in age from 17-80 years. The second sample included 240 subjects aged between 50 and 75 years, carefully balanced in terms of impairment and age to avoid confounding between these two variables. In both samples, frequency resolution ability declined with increasing hearing threshold level (HTL), as measured by either method. In a subsample tested with both methods, the correlation between the two was only modest. After accounting for HTL, there was a minor dependence of frequency resolution on age, older subjects having poorer frequency resolution once HTL had been accounted for. No addition to the explained variance was achieved by taking sex, occupational group, or audiogram slope into account. Despite the documented reproducibility of the measures, much of the variance in the frequency resolution measurements remained unrelated to HTL or age. 相似文献
205.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 5, pp. 815–819, May, 1991. 相似文献
206.
207.
A. L. Apanasenko A. V. Kuznichenko Yu. B. Govyadovskii V. G. Yakunin 《Journal of Applied Spectroscopy》1991,54(3):271-276
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 3, pp. 438–444, March, 1991. 相似文献
208.
Aidan Mooney John G. Keating Daniel M. Heffernan 《Chaos, solitons, and fractals》2006,30(5):1088-1097
A digital watermark is a visible, or preferably invisible, identification code that is permanently embedded in digital media, to prove owner authentication and provide protection for documents. Given the interest in watermark generation using chaotic functions a detailed study of one chaotic function for this purpose is performed. In this paper, we present an approach for the generation of watermarks using the logistic map. Using this function, in conjunction with seed management, it is possible to generate chaotic sequences that may be used to create highpass or lowpass digital watermarks. In this paper we provide a detailed study on the generation of optically detectable watermarks and we provide some guidelines on successful chaotic watermark generation using the logistic map, and show using a recently published scheme, how care must be taken in the selection of the function seed. 相似文献
209.
V. G. Osmolovski 《Journal of Mathematical Sciences》2006,132(3):304-312
Dependence of the phase transition temperature on the domain size is investigated for a double-well quadratic potential. It
is shown that for a domain whose boundary is subjected to a hydrostatical pressure, the temperature of phase transitions is
independent of the domain and the surface tension coefficient and depends exclusively on the properties of the elastic media.
If the displacement field vanishes on the boundary, then for sufficiently small domains, the temperature also does not depend
on the surface tension and domain size and is determined by properties of the elastic media only. Bibliography: 8 titles.
__________
Translated from Zapiski Nauchnykh Seminarov POMI, Vol. 310, 2004, pp. 98–113. 相似文献
210.
G. G. Kozlov 《Optics and Spectroscopy》2006,101(1):151-156
It is shown that scattering of a plane monochromatic wave by a finite two-dimensional crystal (quantum well or Langmuir-Blodgett film) in the region of exciton resonance shows, along with the peak of specular reflection, a peak of comparable magnitude corresponding to backward scattering propagating toward the incident beam (antispecular reflection). The effect is related to reflection of the exciton by the boundary of the two-dimensional crystal. The solution of the scattering problem for a simple model system with a Frenkel exciton, demonstrating this phenomenon, is given. 相似文献