全文获取类型
收费全文 | 22180篇 |
免费 | 2718篇 |
国内免费 | 2190篇 |
专业分类
化学 | 2885篇 |
晶体学 | 112篇 |
力学 | 565篇 |
综合类 | 207篇 |
数学 | 1510篇 |
物理学 | 7082篇 |
无线电 | 14727篇 |
出版年
2024年 | 109篇 |
2023年 | 284篇 |
2022年 | 565篇 |
2021年 | 776篇 |
2020年 | 702篇 |
2019年 | 592篇 |
2018年 | 544篇 |
2017年 | 812篇 |
2016年 | 937篇 |
2015年 | 1128篇 |
2014年 | 1729篇 |
2013年 | 1743篇 |
2012年 | 1660篇 |
2011年 | 1581篇 |
2010年 | 1133篇 |
2009年 | 1304篇 |
2008年 | 1544篇 |
2007年 | 1596篇 |
2006年 | 1312篇 |
2005年 | 1071篇 |
2004年 | 925篇 |
2003年 | 830篇 |
2002年 | 672篇 |
2001年 | 556篇 |
2000年 | 483篇 |
1999年 | 386篇 |
1998年 | 320篇 |
1997年 | 312篇 |
1996年 | 235篇 |
1995年 | 222篇 |
1994年 | 169篇 |
1993年 | 164篇 |
1992年 | 131篇 |
1991年 | 114篇 |
1990年 | 57篇 |
1989年 | 58篇 |
1988年 | 94篇 |
1987年 | 45篇 |
1986年 | 23篇 |
1985年 | 34篇 |
1984年 | 25篇 |
1983年 | 18篇 |
1982年 | 11篇 |
1981年 | 25篇 |
1980年 | 12篇 |
1979年 | 12篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1975年 | 4篇 |
1959年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
在部分相干理论的基础上,分别用表观传递函数和能量分布作评价指标,计算了部分相干光学系统的最佳象面位置,并讨论了其与相干度、空间频率及象差的关系。 相似文献
102.
Raymond H. Chan Michael K. Ng Robert J. Plemmons 《Numerical Linear Algebra with Applications》1996,3(1):45-64
In this paper, we propose a method to generalize Strang's circulant preconditioner for arbitrary n-by-n matrices An. The th column of our circulant preconditioner Sn is equal to the th column of the given matrix An. Thus if An is a square Toeplitz matrix, then Sn is just the Strang circulant preconditioner. When Sn is not Hermitian, our circulant preconditioner can be defined as . This construction is similar to the forward-backward projection method used in constructing preconditioners for tomographic inversion problems in medical imaging. We show that if the matrix An has decaying coefficients away from the main diagonal, then is a good preconditioner for An. Comparisons of our preconditioner with other circulant-based preconditioners are carried out for some 1-D Toeplitz least squares problems: min ∥ b - Ax∥2. Preliminary numerical results show that our preconditioner performs quite well, in comparison to other circulant preconditioners. Promising test results are also reported for a 2-D deconvolution problem arising in ground-based atmospheric imaging. 相似文献
103.
Extended well-posedness of optimization problems 总被引:8,自引:0,他引:8
T. Zolezzi 《Journal of Optimization Theory and Applications》1996,91(1):257-266
The well-posedness concept introduced in Ref. 1 for global optimization problems with a unique solution is generalized here to problems with many minimizers, under the name of extended well-posedness. It is shown that this new property can be characterized by metric criteria, which parallel to some extent those known about generalized Tikhonov well-posedness.This work was partially supported by MURST, Fondi 40%, Rome, Italy. 相似文献
104.
Magnetic resonance imaging (MRI) segmentation is a fundamental and significant task since it can guide subsequent clinic diagnosis and treatment. However, images are often corrupted by defects such as low-contrast, noise, intensity inhomogeneity, and so on. Therefore, a weighted level set model (WLSM) is proposed in this study to segment inhomogeneous intensity MRI destroyed by noise and weak boundaries. First, in order to segment the intertwined regions of brain tissue accurately, a weighted neighborhood information measure scheme based on local multi information and kernel function is designed. Then, the membership function of fuzzy c-means clustering is used as the spatial constraint of level set model to overcome the sensitivity of level set to initialization, and the evolution of level set function can be adaptively changed according to different tissue information. Finally, the distance regularization term in level set function is replaced by a double potential function to ensure the stability of the energy function in the evolution process. Both real and synthetic MRI images can show the effectiveness and performance of WLSM. In addition, compared with several state-of-the-art models, segmentation accuracy and Jaccard similarity coefficient obtained by WLSM are increased by 0.0586, 0.0362 and 0.1087, 0.0703, respectively. 相似文献
105.
Francesco Finazzi 《商业与工业应用随机模型》2012,28(5):448-466
A new Bayesian approach is presented for extracting 2D object boundaries with measures of uncertainty. The boundaries are described by minimal closed sequences of segments and arcs, called mixed polygons. The sequence is minimal in the sense that it is able to describe all the geometrical properties of the boundary without being redundant. Based on geometrical measures evaluated on the object boundary model, a prior distribution is introduced in order to favor a mixed polygon with good geometrical properties, avoiding short sides, collinearity between segments, and so on. The estimation process is based on a two‐stage procedure that combines reversible‐jump MCMC (RJMCMC) and classic MCMC methods. The RJMCMC method is viewed as a model selection technique, and it is used to estimate the correct number of sides of the mixed polygon. The MCMC algorithm provides a sample of mixed polygons through which to evaluate the mixed polygon that best approximates the object boundary and its geometrical uncertainty. A convergence criterion for the RJMCMC method is provided. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
106.
The thermal dehydration of ZnK4(P3O9)2 · 6H2O was studied in the range 25–500°C by thermogravimetric analysis (TGA and DSC) and X‐ray diffraction. We found, based on the TGA and DSC scans, the dehydration of this salt takes place in three stages with a loss of the six water molecules. The infrared and Raman spectra of ZnK4(P3O9)2 · 6H2O have been recorded and interpreted using a factor group analysis. The internal modes are assigned in terms of POP and PO2 structural units using experimental and theoretical IR and Raman frequencies. 相似文献
107.
Liansheng Sui Meiting Xin Ailing Tian Haiyan Jin 《Optics and Lasers in Engineering》2013,51(12):1297-1309
A single-channel color image encryption is proposed based on a phase retrieve algorithm and a two-coupled logistic map. Firstly, a gray scale image is constituted with three channels of the color image, and then permuted by a sequence of chaotic pairs generated by the two-coupled logistic map. Secondly, the permutation image is decomposed into three new components, where each component is encoded into a phase-only function in the fractional Fourier domain with a phase retrieve algorithm that is proposed based on the iterative fractional Fourier transform. Finally, an interim image is formed by the combination of these phase-only functions and encrypted into the final gray scale ciphertext with stationary white noise distribution by using chaotic diffusion, which has camouflage property to some extent. In the process of encryption and decryption, chaotic permutation and diffusion makes the resultant image nonlinear and disorder both in spatial domain and frequency domain, and the proposed phase iterative algorithm has faster convergent speed. Additionally, the encryption scheme enlarges the key space of the cryptosystem. Simulation results and security analysis verify the feasibility and effectiveness of this method. 相似文献
108.
Die Arbeit behandelt theoretisch das Auflösungsvermögen der Absorptionsmethode zur Analyse von Binārsystemen bei Anwendung von monoenergetischer Gamma- und Rötgenstrahlung. Es wird eine analytische Beziehung für die Funktion Δp abgeleitet, die die kleinstmöglichen Veränderung in der Zusammensetzung des Systems chgarakterisiert und die wir noch mittelts dieser Methode in Abhängigkeit von allen zu berückischtigenden Faktoren bestimmen könne. Aus den Extremwerten der Funktion Δp werden dann die optimalen Werte der Energine, der Schichtdicke, der Strahlungshäufigkeit und die Optimalwerte der restlichen Parameter so bestimmt, daβ das Auflösungsvermögen der Methode maximal ist. Die Theorie wird auf eine Ag-Cu-Legierung angewendet. 相似文献
109.
E. Barth I. Tugtekin H. Weidenbach U. Wachter J. Vogt P. Radermacher 《Isotopes in environmental and health studies》2013,49(1-2):209-213
Abstract Breath tests using 13C-labelled substrates require the measurement of 13CO2/12CO2 ratio in breath gas samples. Next to isotope ratio mass spectrometry (IRMS), which is very sensitive but also complex and expensive, alternatively isotope selective nondispersive infrared spectrometry (NDIRS) can be used to determine the 13CO2/12CO2 ratio in expired breath. In this study we compared NDIRS- with IRMS-results to investigate whether the less expensive and very simply to operate NDIRS works as reliable as IRMS. For this purpose we applicated 1-13C-Phenylalanine to patients with advanced liver cirrhosis and healthy volunteers and took duplicated breath samples for IRMS and NDIRS at selected time points. Our data show a good correlation between these two methods for a small number of samples as required for simple breath tests. Longer series, where repeated measurements are required on the NDIRS instrument lead to a decreasing correlation. This indicates the superiority of IRMS concerning 13CO2-kinetics over longer time periods. 相似文献
110.