全文获取类型
收费全文 | 1350篇 |
免费 | 40篇 |
国内免费 | 6篇 |
专业分类
化学 | 835篇 |
晶体学 | 6篇 |
力学 | 30篇 |
数学 | 199篇 |
物理学 | 165篇 |
无线电 | 161篇 |
出版年
2022年 | 22篇 |
2021年 | 19篇 |
2020年 | 29篇 |
2019年 | 31篇 |
2018年 | 15篇 |
2017年 | 17篇 |
2016年 | 23篇 |
2015年 | 23篇 |
2014年 | 44篇 |
2013年 | 48篇 |
2012年 | 82篇 |
2011年 | 92篇 |
2010年 | 49篇 |
2009年 | 51篇 |
2008年 | 63篇 |
2007年 | 94篇 |
2006年 | 91篇 |
2005年 | 69篇 |
2004年 | 67篇 |
2003年 | 65篇 |
2002年 | 61篇 |
2001年 | 20篇 |
2000年 | 25篇 |
1999年 | 18篇 |
1998年 | 8篇 |
1997年 | 15篇 |
1996年 | 14篇 |
1995年 | 22篇 |
1994年 | 25篇 |
1993年 | 12篇 |
1992年 | 17篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1989年 | 5篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 12篇 |
1982年 | 7篇 |
1981年 | 13篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 8篇 |
1974年 | 5篇 |
1973年 | 5篇 |
1969年 | 4篇 |
排序方式: 共有1396条查询结果,搜索用时 15 毫秒
931.
932.
Debra Ann Van Rie 《Algebra Universalis》1994,31(4):492-505
Presented by R. McKenzie. 相似文献
933.
A generalized singular value decomposition (GSVD) based algorithm is proposed for enhancing multimicrophone speech signals degraded by additive colored noise. This GSVD-based multimicrophone algorithm can be considered to be an extension of the single-microphone signal subspace algorithms for enhancing noisy speech signals and amounts to a specific optimal filtering problem when the desired response signal cannot be observed. The optimal filter can be written as a function of the generalized singular vectors and singular values of a speech and noise data matrix. A number of symmetry properties are derived for the single-microphone and multimicrophone optimal filter, which are valid for the white noise case as well as for the colored noise case. In addition, the averaging step of some single-microphone signal subspace algorithms is examined, leading to the conclusion that this averaging operation is unnecessary and even suboptimal. For simple situations, where we consider localized sources and no multipath propagation, the GSVD-based optimal filtering technique exhibits the spatial directivity pattern of a beamformer. When comparing the noise reduction performance for realistic situations, simulations show that the GSVD-based optimal filtering technique has a better performance than standard fixed and adaptive beamforming techniques for all reverberation times and that it is more robust to deviations from the nominal situation, as, e.g., encountered in uncalibrated microphone arrays. 相似文献
934.
A Fourier-transform integral can be transformed into an integral of a periodic function over a finite interval. This integral can be calculated efficiently by the trapezium rule, while the periodic function, which is expressed as an infinite series, can be evaluated using Euler's transformation. A FORTRAN program implementing this method is presented. 相似文献
935.
Amylose tributyrate has been prepared and fractionated; the dilute solution behaviour of the polymer has been studied in four solvents, ethyl acetate, methyl ethyl ketone, carbon tetrachloride and tetrahydrofuran. The Mark-Houwink relations have been established for each solvent and the exponent γ varies between 0·72 and 0·86, for different solvents. Perturbed dimensions have been measured for the polymer dissolved in two solvents and an estimate of the unperturbed dimensions has been made; the latter appear to be solvent dependent. Heterogeneity corrections were applied using data established by gel permeation chromatography measurements. Values of the effective bond length b of between 17 and 19·5 × 10?8 cm and the Kuhn statistical segment Am of 70 to 180 x 10?8 cm, indicate that amylose tributyrate behaves like a moderately stiff coil in good solvents, with a chain stiffness intermediate between the vinyl and cellulosic polymers. 相似文献
936.
Four schemes for constructing rotational-orbital basis sets for rotational close coupling calculations for atom-molecule. collisions are considered and are applied to calculate 0 → 1, 0 → 2, and 0 → 3 rotational excitation cross sections for He+ HF at energies 0.05 and 0.017 eV. Adiabatic basis sets are shown to converge faster than conventional basis sets in all cases; in some cases the difference is very dramatic. Further, l-dominant bases are shown to be useful for the 0 → 2 and 0 → 3 cross sections. 相似文献
937.
The crosslinking reaction in a two component methyl silicone rubber has been studied by thermomechanical analysis (TMA) and differential scanning calorimetry (DSC). The rubber was formed from two methyl silicone prepolymers; one containing reactive hydrogens every 50 to 100 groups and the other polymer containing pendant vinyl groups at the same frequency. In the presence of a platinum catalyst above 60°C crosslinking proceeds without a loss or gain in weight. The heat of reaction, energy of activation (calculated by two methods) crosslink density and elastic modulus (Young's) were studied as a function of prepolymer concentration, dilution and swelling. A preliminary value for the heat of reaction per mole of SiH and SiCHCH2 has been calculated. From crosslink density measurements both by hexane swelling and TMA and DSC heats of reaction a qualitative picture has been obtained of the role of entangled chains in producing effective crosslinks. 相似文献
938.
939.
This paper describes a method for obtaining a time continuous reduced order model (ROM) from a system of time continuous linear differential equations. These equations are first put into a time discrete form using a finite difference approximation. The unit sample responses of the discrete system are calculated for each system input and these provide the Markov parameters of the system. An eigenvalue realization algorithm (ERA) is used to construct a discrete ROM. This ROM is then used to obtain a continuous ROM of the original continuous system. The focus of this paper is on the application of this method to the calculation of unsteady flows using the linearized Euler equations on moving meshes for aerofoils undergoing heave or linearized pitch motions. Applying a standard cell‐centre spatial discretization and taking account of mesh movement a continuous system of differential equations is obtained which are continuous in time. These are put into discrete time form using an implicit finite difference approximation. Results are presented demonstrating the efficiency of the system reduction method for this system. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
940.
Ysebaert G. Pisoni F. Bonaventura M. Hug R. Moonen M. 《Signal Processing, IEEE Transactions on》2004,52(9):2612-2624
Asymmetric digital subscriber lines (ADSLs) employ discrete multitone modulation (DMT) as transmission format, where subcarriers are assigned to the up- and/or downstream transmission direction. To separate up- and downstream signals, the ADSL standard allows the use of echo cancellation resulting in improved bit rates, reach, and/or noise margins. In DMT-based modems, typically, the mixed time/frequency (MTF) domain echo canceling scheme, as proposed by Ho et al., is implemented. This technique estimates the echo filter in the frequency domain using the least mean square (LMS) algorithm with the transmitted echo symbols as update directions. Since not every tone of the transmitted echo signal will carry data, i.e., will be excited, the MTF adaptation process does not lead to a good estimate for the echo channel, unless extra power on unused echo tones is transmitted. However, transmitting extra power on such tones is often undesired. In this paper, we present an alternative echo canceling scheme referred to as the circulant decomposition canceler (CDC), which works without extra power requirements and with comparable complexity as the method of Ho et al. Similar to MTF echo canceling, the CDC scheme can easily be incorporated into a multirate environment with different transmit and receive rates and can also cheaply be combined with per-tone equalization and double talk cancellation to allow fast tracking and/or convergence in the presence of a far-end signal. 相似文献